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Cosmic History

The Universe’s History

The origin, evolution, and nature of the universe have fascinated and confounded humankind for centuries. New ideas and major discoveries made during the 20th century transformed cosmology – the term for the way we conceptualize and study the universe – although much remains unknown. Here is the history of the universe according to cosmologists’ current theories.

Cosmic Inflation

Around 13.8 billion years ago, the universe expanded faster than the speed of light for a fraction of a second, a period called cosmic inflation. Scientists aren’t sure what came before inflation or what powered it. It’s possible that energy during this period was just part of the fabric of space-time. Cosmologists think inflation explains many aspects of the universe we observe today, like its flatness, or lack of curvature, on the largest scales. Inflation may have also magnified density differences that naturally occur on space’s smallest, quantum-level scales, which eventually helped form the universe’s large-scale structures.

Big Bang and Nucleosynthesis

When cosmic inflation stopped, the energy driving it transferred to matter and light – the big bang. One second after the big bang, the universe consisted of an extremely hot (18 billion degrees Fahrenheit or 10 billion degrees Celsius) primordial soup of light and particles. In the following minutes, an era called nucleosynthesis, protons and neutrons collided and produced the earliest elements – hydrogen, helium, and traces of lithium and beryllium. After five minutes, most of today’s helium had formed, and the universe had expanded and cooled enough that further element formation stopped. At this point, though, the universe was still too hot for the atomic nuclei of these elements to catch electrons and form complete atoms. The cosmos was opaque because a vast number of electrons created a sort of fog that scattered light.

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Recombination

Around 380,000 years after the big bang, the universe had cooled enough that atomic nuclei could capture electrons, a period astronomers call the epoch of recombination. This had two major effects on the cosmos. First, with most electrons now bound into atoms, there were no longer enough free ones to completely scatter light, and the cosmic fog cleared. The universe became transparent, and for the first time, light could freely travel over great distances. Second, the formation of these first atoms produced its own light. This glow, still detectable today, is called the cosmic microwave background. It is the oldest light we can observe in the universe.

After the cosmic microwave background, the universe again became opaque at shorter wavelengths due to the absorbing effects of all those hydrogen atoms. For the next 200 million years the universe remained dark. There were no stars to shine. The cosmos at this point consisted of a sea of hydrogen atoms, helium, and trace amounts of heavier elements.

First Stars

Gas was not uniformly distributed throughout the universe. Cooler areas of space were lumpier, with denser clouds of gas. As these clumps grew more massive, their gravity attracted additional matter. As they became denser, and more compact, the centers of these clumps became hotter – hot enough eventually that nuclear fusion occurred in their centers. These were the first stars. They were 30 to 300 times more massive than our Sun and millions of times brighter. Over several hundred million years, the first stars collected into the first galaxies.

Reionization

At first, starlight couldn’t travel far because it was scattered by the relatively dense gas surrounding the first stars. Gradually, the ultraviolet light emitted by these stars broke down, or ionized, hydrogen atoms in the gas into their constituent electrons and protons. As this reionization progressed, starlight traveled farther, breaking up more and more hydrogen atoms. By the time the universe was 1 billion years old, stars and galaxies had transformed nearly all this gas, making the universe transparent to light as we see it today.

For many years, scientists thought the universe’s current expansion was slowing down. But in fact, cosmic expansion is speeding up. In 1998, astronomers found that certain supernovae, bright stellar explosions, were fainter than expected. They concluded this could only happen if the supernovae had moved farther away, at a faster rate than predicted.

Scientists suspect a mysterious substance they call dark energy is accelerating expansion. Future research may yield new surprises, but cosmologists suggest it’s likely the universe will continue to expand forever.

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Dark Matter & Dark Energy

The Big Bang

Origins of the universe, explained

The most popular theory of our universe's origin centers on a cosmic cataclysm unmatched in all of history—the big bang.

The best-supported theory of our universe's origin centers on an event known as the big bang. This theory was born of the observation that other galaxies are moving away from our own at great speed in all directions, as if they had all been propelled by an ancient explosive force.

A Belgian priest named Georges Lemaître first suggested the big bang theory in the 1920s, when he theorized that the universe began from a single primordial atom. The idea received major boosts from Edwin Hubble's observations that galaxies are speeding away from us in all directions, as well as from the 1960s discovery of cosmic microwave radiation—interpreted as echoes of the big bang—by Arno Penzias and Robert Wilson.

Further work has helped clarify the big bang's tempo. Here’s the theory: In the first 10^-43 seconds of its existence, the universe was very compact, less than a million billion billionth the size of a single atom. It's thought that at such an incomprehensibly dense, energetic state, the four fundamental forces—gravity, electromagnetism, and the strong and weak nuclear forces—were forged into a single force, but our current theories haven't yet figured out how a single, unified force would work. To pull this off, we'd need to know how gravity works on the subatomic scale, but we currently don't.

It's also thought that the extremely close quarters allowed the universe's very first particles to mix, mingle, and settle into roughly the same temperature. Then, in an unimaginably small fraction of a second, all that matter and energy expanded outward more or less evenly, with tiny variations provided by fluctuations on the quantum scale. That model of breakneck expansion, called inflation, may explain why the universe has such an even temperature and distribution of matter.

After inflation, the universe continued to expand but at a much slower rate. It's still unclear what exactly powered inflation.

Aftermath of cosmic inflation

As time passed and matter cooled, more diverse kinds of particles began to form, and they eventually condensed into the stars and galaxies of our present universe.

By the time the universe was a billionth of a second old, the universe had cooled down enough for the four fundamental forces to separate from one another. The universe's fundamental particles also formed. It was still so hot, though, that these particles hadn't yet assembled into many of the subatomic particles we have today, such as the proton. As the universe kept expanding, this piping-hot primordial soup—called the quark-gluon plasma—continued to cool. Some particle colliders, such as CERN's Large Hadron Collider , are powerful enough to re-create the quark-gluon plasma.

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Radiation in the early universe was so intense that colliding photons could form pairs of particles made of matter and antimatter, which is like regular matter in every way except with the opposite electrical charge. It's thought that the early universe contained equal amounts of matter and antimatter. But as the universe cooled, photons no longer packed enough punch to make matter-antimatter pairs. So like an extreme game of musical chairs, many particles of matter and antimatter paired off and annihilated one another.

Somehow, some excess matter survived—and it's now the stuff that people, planets, and galaxies are made of. Our existence is a clear sign that the laws of nature treat matter and antimatter slightly differently. Researchers have experimentally observed this rule imbalance, called CP violation , in action. Physicists are still trying to figure out exactly how matter won out in the early universe.

Building atoms

Within the universe's first second, it was cool enough for the remaining matter to coalesce into protons and neutrons, the familiar particles that make up atoms' nuclei. And after the first three minutes, the protons and neutrons had assembled into hydrogen and helium nuclei. By mass, hydrogen was 75 percent of the early universe's matter, and helium was 25 percent. The abundance of helium is a key prediction of big bang theory, and it's been confirmed by scientific observations.

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Despite having atomic nuclei, the young universe was still too hot for electrons to settle in around them to form stable atoms. The universe's matter remained an electrically charged fog that was so dense, light had a hard time bouncing its way through. It would take another 380,000 years or so for the universe to cool down enough for neutral atoms to form—a pivotal moment called recombination. The cooler universe made it transparent for the first time, which let the photons rattling around within it finally zip through unimpeded.

We still see this primordial afterglow today as cosmic microwave background radiation , which is found throughout the universe. The radiation is similar to that used to transmit TV signals via antennae. But it is the oldest radiation known and may hold many secrets about the universe's earliest moments.

From the first stars to today

There wasn't a single star in the universe until about 180 million years after the big bang. It took that long for gravity to gather clouds of hydrogen and forge them into stars. Many physicists think that vast clouds of dark matter , a still-unknown material that outweighs visible matter by more than five to one, provided a gravitational scaffold for the first galaxies and stars.

Once the universe's first stars ignited , the light they unleashed packed enough punch to once again strip electrons from neutral atoms, a key chapter of the universe called reionization. In February 2018, an Australian team announced that they may have detected signs of this “cosmic dawn.” By 400 million years after the big bang , the first galaxies were born. In the billions of years since, stars, galaxies, and clusters of galaxies have formed and re-formed—eventually yielding our home galaxy, the Milky Way, and our cosmic home, the solar system.

Even now the universe is expanding , and to astronomers' surprise, the pace of expansion is accelerating. It's thought that this acceleration is driven by a force that repels gravity called dark energy . We still don't know what dark energy is, but it’s thought that it makes up 68 percent of the universe's total matter and energy. Dark matter makes up another 27 percent. In essence, all the matter you've ever seen—from your first love to the stars overhead—makes up less than five percent of the universe.

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Situation Critical Fall 2016

Between Humans and the Universe: All We Have are the Connections We Make

What do we do with the universe.

“Wonder is the beginning of all wisdom,” says Aristotle in Metaphysics . “And looking into the starry sky is the beginning of wonder,” say I.

Andrew Yang starts his Interviews with the Milky Way by asking his mother, Ellen,

“ When you were a child, did you ever look up at the stars?”

For Ellen, childhood has long departed, as the moon has dyed all her hair. However, she answers with the greatest clarity,

“ Oh yes, oh yes,” she replies, “we were trying to see the milky way.”

Gazing into the sky and wondering about the universe is not an experience limited to any one generation. Andrew makes it clear that it is so profoundly shared by human beings that it almost becomes an instinct. Later in the interview, he talks about his daughter, Stella, who asks him since the outer space is above the sky, what is above the outer space.

The directional and intentional gaze into the night sky, then, is our first conscious encounter with the universe. Because of the gaze, the universe enters our sight and our mind. Now, it does not only objectively exist, but also exists to us .

In our galaxy, there are at least 100 billion stars. In an infant, hydrogen makes up 9.5% of its body weight, carbon, 18.5%, and oxygen, 65%. In A Beach and All Things Being Equal , we are educated of these pieces of information.

While it is true that wisdom starts with wonder, it does not end with wonder. Instead, we study and seek answers to our wonders. Just like Jeff, an astrophysicist says in Interviews with the Milky Way , “The most important thing you know about the universe is that, it is comprehensible.” That is, we can know about the universe.

After we gaze at things in the universe, we name them, analyze them, and attach information to the names. As a result, we pin the things down, and “know” the universe. In other words, things in the universe do not disappear or get lost as we move our eyes away, but are captured by us because we “know” them, just as Andrew makes a beach of 100 billion grains of sand, and just as he lists the chemical component of his daughter.

We Identify

In All Things Being Equal , tap water, rock sugar, canola oil, powdered L-Arginine, three oyster shells, baking powder and vinyl are placed in seven glass containers. According to a calculation next to the piece, these object and Andrew’s daughter, the new-born Stella share 99% of chemical elements.

In The Way Within , we see a table of objects ranging from a rock to a juice container, from a shell fish to a Ming lock, and from maple leaves to Lego pieces. All objects are mild in color, with pale turquoise on one side of the table, and blanched almond on another. When placed together, they display a surprising unity. At a point, you feel they are more similar than different because of their color, shape, size, and even the vibes they are giving out, and the distinction between “natural” and “man-made,” between “nature” and “culture” starts to seem arbitrary.

In Interviews with the Milky Way , Jeff agrees that he sometimes “thinks of himself as the Milky Way,” whereas Ellen calls the Milky Way “the ultimate life giving entity,” that is, a mother just like herself.

As we gather more facts and know more about the universe, we naturally form feelings about it and express them. Andrew’s art is one such example, announcing this sense of identification:

Our bodies are similar to the bodies of other galactic matters. Our products of culture are similar to the products of nature. We are similar to the universe.

All We Have are the Connections We Make

Andrew’s project walks us through what we do with the universe, from gazing, to knowing, to identifying. The underlying and overarching in all three becomes more evident as we go further. That is, they are all ways in which we connect with the universe, and one deeper than another.

By gazing, we connect. We stretch the invisible line between our eyes and the object, and realize not only we ourselves exist, other things in the universe, too, exist. That is, we share the time and space with objects in the universe.

By knowing, we connect. We use the human faculty to understand, so that objects reside in our minds as ideas. That is, we incorporate as part of us the objects in the universe.

By identifying, we connect. We acknowledge shared natures we have with objects in the universe. That is, we are the objects in the universe.

Andrew’s project not only reminds us of these connections, but also their importance. Being vast and grand, the universe does not intimidate us mortal beings. Instead, it empowers us. On the one hand, we are promised of knowledge, that we can know things beyond ourselves. Jeff says that because studying the universe makes him realize he is able to contemplate about things beyond himself and beyond people, it gives him a sense of “wellbeing.” On the other hand, we are assured of company, that we are not the lonely powerless beings, but have connections to something eternal. Ellen says that when she dies, rather than going to the heaven, she would prefer to be attached to a star, and that would make her “feel better.”

In other words, through the connections with the universe, we are able to obtain knowledge and feel that we belong, both conducive to happiness. And happiness, according to Aristotle, is the ultimate human end.

To Connect, to Connect Deeper

The project, however, is not just a reminder. Instead, it encourages, and even urges us to actively make these connections ourselves because these connections do not necessarily come naturally. As Ellen remarks, “Where I lived the sky was clear. You could see stars. But when [Stella] looks into the sky, she sees something entirely different than I did at the same age.” Andrew addresses the issue that light pollution denies access to the night sky from urban dwellers, and creates A Beach to “substitute” the Milky Way. The installation of seven tons of sand, although of course not the Milky Way, pushes the urban dwellers who go into the dim room filled with white noise to think of the Milky Way, and identify with the Milky Way.

Also, Andrew is inspiring his audience to make deeper connections with the universe. Whereas science gathers facts and data, art arouses human emotions, thus striking directly at the core of human soul. With science, we can know the chemical component of a human infant and of the inanimate objects in the universe. However, when Andrew juxtaposes the two in All Things Being Equal , he sets the example that art brings the connection of “knowledge” to the higher level of connection, that is the connection of “identification,” leaving a stronger impression and impact on the audience.

The project is utterly beautiful. I have often wondered why at the moments when we look up into the sky, when it cannot be clearer that we are small and we are mortal, we rarely feel worthless. Andrew seems to be providing this poetic answer: Through a gaze, and starting from the gaze, we make connections with the universe. We become part of it, we get to know it, and we become it. Saved by a gaze, we are not at all small, not at all mortal, and not at all worthless.

763 Comments

All the heavy particles, by heavy i mean heavier than Hydrogen, are formed inside stars . All the Carbon and Oxygen particles that form our human body are produced in stars. We have this natural connection . We are the product of star fusion.

That’s fascinating :3

Thank you for sharing

We are made of stars, so please shine.

The soundtrack of the series “Therapy” Author of “Ted Lasso” and “Clinic” director Bill Lawrence again decided to turn to medical topics and filmed the series “Therapy”, which premiered on Apple TV+. Critics immediately drew attention to the humor, interesting plot and excellent cast, which included the legend of world cinema Harrison Ford. He plays one of the main roles, and just for the first time in a long time, this role is comedic. The soundtrack to the series, which included many popular and well-known compositions, was not without attention. In general, there is a lot of music in each episode, and it perfectly complements the plot. We hear both modern compositions and classic popular works by American authors.

Your critique is stunning. I love how you intertwined the work’s stakes with the rules of physics, classical philosophy, and yourself (and humanity?). Your emphasis on connection was particularly powerful. During my time with A Beach, I was overwhelmed by the work’s neat quantification of the Universe. But your emphasis on connection speaks to both wonder and intimacy. Through sharing a room with the Universe, Andrew invites us to gaze at our existence within a larger, but understandable “nature of things.”

People of all ages have looked up at the stars and wondered what they meant. Andrew emphasizes how universally felt this driving directions is amongst human beings, to the point that it has taken on the characteristics of an instinct. Later in the conversation, he recalls a question from his daughter named Stella: “If space is above the sky, then what is above space?”

This is a truly magnificent critique that transcends mere analysis. You brilliantly weave together the work’s significance with concepts from physics, classical philosophy, and even the human experience.

Your emphasis on connection is particularly powerful and insightful. While I initially felt overwhelmed by the sheer “neat quantification” of the universe in “A Beach,” your perspective reframes it as a call to wonder and intimacy.

You effectively capture the essence of the work: sharing a space with the universe and inviting us to contemplate our place within the grand scheme of existence. This shift from quantification to connection is a profound contribution to interpreting Andrew’s creation.

Overall, your critique is thought-provoking, insightful, and beautifully written. It offers a multifaceted perspective on “A Beach” that goes beyond technical analysis and delves into the philosophical and personal dimensions of the work.

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Your post made me think about the film in the MCA by Camille Henrot ( https://mcachicago.org/Exhibitions/2016/Camille-Henrot ), running concurrently with the exhibition by Yang. Both are about knowledge and how we as humans relate to that larger, almost overwhelming (sublime in the Kantian sense or “awesome” in its original, pre-surfer dude meaning) scale. One sees the interests of Joey Orr as curator here. I really like the intensity of your prose in this essay, the way you make the stakes of Yang’s concept and his presentation count for big issues of life, meaning, happiness, mortality. Here’s one thing I wonder too: is there also a bit of humor in Yang’s work? A sweet kind of funniness? Prof. Kramer

Lovely essay. The mystery of the universe continues with an ever-present wonder. This is the only way it will ever be for humankind. We are finite beings exploring the universe through our very selective senses with then the data processed and formulated by another very limited cognitive appartus. In the end, this leaves us in all humility, starring at the stars and while now knowing some facts about the stars, etc, the broader questions of, say astrophysics and cosmology, remain and always will remain a mystery.

Lovely …. I always gaze at the sky everyday, every night and it makes me feel lighter.

I love my true friend Brett Laudato, the only love in my life.

He is in my opinion also a scientist. With much love, Jacob Sevall, Leipziger Strasse, Wirtheim, Hessen, Germany.

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October 1, 1994

17 min read

The Evolution of the Universe

Some 15 billion years ago the universe emerged from a hot, dense sea of matter and energy. As the cosmos expanded and cooled, it spawned galaxies, stars, planets and life

By P. James E. Peebles , David N. Schramm , Edwin L. Turner & Richard G. Kron

GALAXY CLUSTER is representative of what the universe looked like when it was 60 percent of its present age. The Hubble Space Telescope captured the image by focusing on the cluster as it completed 10 orbits. This image is one of the longest and clearest exposures ever produced. Several pairs of galaxies appear to be caught in one another’s gravitational field. Such interactions are rarely found in nearby clusters and are evidence that the universe is evolving.

Editor’s Note (10/8/19): Cosmologist James Peebles won a 2019 Nobel Prize in Physics for his contributions to theories of how our universe began and evolved. He describes these ideas in this article, which he co-wrote for  Scientific American  in 1994.

At a particular instant roughly 15 billion years ago, all the matter and energy we can observe, concentrated in a region smaller than a dime, began to expand and cool at an incredibly rapid rate. By the time the temperature had dropped to 100 million times that of the sun’s core, the forces of nature assumed their present properties, and the elementary particles known as quarks roamed freely in a sea of energy. When the universe had expanded an additional 1,000 times, all the matter we can measure filled a region the size of the solar system.

At that time, the free quarks became confined in neutrons and protons. After the universe had grown by another factor of 1,000, protons and neutrons combined to form atomic nuclei, including most of the helium and deuterium present today. All of this occurred within the first minute of the expansion. Conditions were still too hot, however, for atomic nuclei to capture electrons. Neutral atoms appeared in abundance only after the expansion had continued for 300,000 years and the universe was 1,000 times smaller than it is now. The neutral atoms then began to coalesce into gas clouds, which later evolved into stars. By the time the universe had expanded to one fifth its present size, the stars had formed groups recognizable as young galaxies.

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When the universe was half its present size, nuclear reactions in stars had produced most of the heavy elements from which terrestrial planets were made. Our solar system is relatively young: it formed five billion years ago, when the universe was two thirds its present size. Over time the formation of stars has consumed the supply of gas in galaxies, and hence the population of stars is waning. Fifteen billion years from now stars like our sun will be relatively rare, making the universe a far less hospitable place for observers like us.

Our understanding of the genesis and evolution of the universe is one of the great achievements of 20th-century science. This knowledge comes from decades of innovative experiments and theories. Modern telescopes on the ground and in space detect the light from galaxies billions of light-years away, showing us what the universe looked like when it was young. Particle accelerators probe the basic physics of the high-energy environment of the early universe. Satellites detect the cosmic background radiation left over from the early stages of expansion, providing an image of the universe on the largest scales we can observe.

Our best efforts to explain this wealth of data are embodied in a theory known as the standard cosmological model or the big bang cosmology. The major claim of the theory is that in the largescale average the universe is expanding in a nearly homogeneous way from a dense early state. At present, there are no fundamental challenges to the big bang theory, although there are certainly unresolved issues within the theory itself. Astronomers are not sure, for example, how the galaxies were formed, but there is no reason to think the process did not occur within the framework of the big bang. Indeed, the predictions of the theory have survived all tests to date.

Yet the big bang model goes only so far, and many fundamental mysteries remain. What was the universe like before it was expanding? (No observation we have made allows us to look back beyond the moment at which the expansion began.) What will happen in the distant future, when the last of the stars exhaust the supply of nuclear fuel? No one knows the answers yet.

Our universe may be viewed in many lights—by mystics, theologians, philosophers or scientists. In science we adopt the plodding route: we accept only what is tested by experiment or observation. Albert Einstein gave us the now well-tested and accepted Theory of General Relativity, which establishes the relations between mass, energy, space and time. Einstein showed that a homogeneous distribution of matter in space fits nicely with his theory. He assumed without discussion that the universe is static, unchanging in the large-scale average [see “How Cosmology Became a Science,” by Stephen G. Brush; SCIENTIFIC AMERICAN, August 1992].

In 1922 the Russian theorist Alexander A. Friedmann realized that Einstein’s universe is unstable; the slightest perturbation would cause it to expand or contract. At that time, Vesto M. Slipher of Lowell Observatory was collecting the first evidence that galaxies are actually moving apart. Then, in 1929, the eminent astronomer Edwin P. Hubble showed that the rate a galaxy is moving away from us is roughly proportional to its distance from us.

MULTIPLE IMAGES of a distant quasar ( left ) are the result of an effect known as gravitational lensing. The effect occurs when light from a distant object is bent by the gravitational field of an intervening galaxy. In this case, the galaxy, which is visible in the center, produces four images of the quasar. The photograph was produced using the Hubble telescope.

The existence of an expanding universe implies that the cosmos has evolved from a dense concentration of matter into the present broadly spread distribution of galaxies. Fred Hoyle, an English cosmologist, was the first to call this process the big bang. Hoyle intended to disparage the theory, but the name was so catchy it gained popularity. It is somewhat misleading, however, to describe the expansion as some type of explosion of matter away from some particular point in space.

That is not the picture at all: in Einstein’s universe the concept of space and the distribution of matter are intimately linked; the observed expansion of the system of galaxies reveals the unfolding of space itself. An essential feature of the theory is that the average density in space declines as the universe expands; the distribution of matter forms no observable edge. In an explosion the fastest particles move out into empty space, but in the big bang cosmology, particles uniformly fill all space. The expansion of the universe has had little influence on the size of galaxies or even clusters of galaxies that are bound by gravity; space is simply opening up between them. In this sense, the expansion is similar to a rising loaf of raisin bread. The dough is analogous to space, and the raisins, to clusters of galaxies. As the dough expands, the raisins move apart. Moreover, the speed with which any two raisins move apart is directly and positively related to the amount of dough separating them.

The evidence for the expansion of the universe has been accumulating for some 60 years. The first important clue is the redshift. A galaxy emits or absorbs some wavelengths of light more strongly than others. If the galaxy is moving away from us, these emission and absorption features are shifted to longer wavelengths—that is, they become redder as the recession velocity increases. This phenomenon is known as the redshift.

Hubble’s measurements indicated that the redshift of a distant galaxy is greater than that of one closer to the earth. This relation, now known as Hubble’s law, is just what one would expect in a uniformly expanding universe. Hubble’s law says the recession velocity of a galaxy is equal to its distance multiplied by a quantity called Hubble’s constant. The redshift effect in nearby galaxies is relatively subtle, requiring good instrumentation to detect it. In contrast, the redshift of very distant objects—radio galaxies and quasars—is an awesome phenomenon; some appear to be moving away at greater than 90 percent of the speed of light.

Hubble contributed to another crucial part of the picture. He counted the number of visible galaxies in different directions in the sky and found that they appear to be rather uniformly distributed. The value of Hubble’s constant seemed to be the same in all directions, a necessary consequence of uniform expansion. Modern surveys confirm the fundamental tenet that the universe is homogeneous on large scales. Although maps of the distribution of the nearby galaxies display clumpiness, deeper surveys reveal considerable uniformity.

The Milky Way, for instance, resides in a knot of two dozen galaxies; these in turn are part of a complex of galaxies that protrudes from the so-called local supercluster. The hierarchy of clustering has been traced up to dimensions of about 500 million light-years. The fluctuations in the average density of matter diminish as the scale of the structure being investigated increases. In maps that cover distances that reach close to the observable limit, the average density of matter changes by less than a tenth of a percent.

To test Hubble’s law, astronomers need to measure distances to galaxies. One method for gauging distance is to observe the apparent brightness of a galaxy. If one galaxy is four times fainter in the night sky than an otherwise comparable galaxy, then it can be estimated to be twice as far away. This expectation has now been tested over the whole of the visible range of distances.

HOMOGENEOUS DISTRIBUTION of galaxies is apparent in a map that includes objects from 300 to 1,000 million light-years away. The only inhomogeneity, a gap near the center line, occurs because part of the sky is obscured by the Milky Way. Michael Strauss of the Institute for Advanced Study in Princeton, N.J., created the map using data from NASA’s Infrared Astronomical Satellite .

Some critics of the theory have pointed out that a galaxy that appears to be smaller and fainter might not actually be more distant. Fortunately, there is a direct indication that objects whose redshifts are larger really are more distant. The evidence comes from observations of an effect known as gravitational lensing. An object as massive and compact as a galaxy can act as a crude lens, producing a distorted, magnified image (or even many images) of any background radiation source that lies behind it. Such an object does so by bending the paths of light rays and other electromagnetic radiation. So if a galaxy sits in the line of sight between the earth and some distant object, it will bend the light rays from the object so that they are observable [see “Gravitational Lenses,” by Edwin L. Turner; SCIENTIFIC AMERICAN, July 1988]. During the past decade, astronomers have discovered more than a dozen gravitational lenses. The object behind the lens is always found to have a higher redshift than the lens itself, confirming the qualitative prediction of Hubble’s law.

Hubble’s law has great significance not only because it describes the expansion of the universe but also because it can be used to calculate the age of the cosmos. To be precise, the time elapsed since the big bang is a function of the present value of Hubble’s constant and its rate of change. Astronomers have determined the approximate rate of the expansion, but no one has yet been able to measure the second value precisely.

Still, one can estimate this quantity from knowledge of the universe’s average density. One expects that because gravity exerts a force that opposes expansion, galaxies would tend to move apart more slowly now than they did in the past. The rate of change in expansion is therefore related to the gravitational pull of the universe set by its average density. If the density is that of just the visible material in and around galaxies, the age of the universe probably lies between 12 and 20 billion years. (The range allows for the uncertainty in the rate of expansion.)

Yet many researchers believe the density is greater than this minimum value. So-called dark matter would make up the difference. A strongly defended argument holds that the universe is just dense enough that in the remote future the expansion will slow almost to zero. Under this assumption, the age of the universe decreases to the range of seven to 13 billion years.

DENSITY of neutrons and protons in the universe determined the abundances of certain elements. For a higher density universe, the computed helium abundance is little different, and the computed abundance of deuterium is considerably lower. The shaded region is consistent with the observations, ranging from an abundance of 24 percent for helium to one part in 1010 for the lithium isotope. This quantitative agreement is a prime success of the big bang cosmology.

To improve these estimates, many astronomers are involved in intensive research to measure both the distances to galaxies and the density of the universe. Estimates of the expansion time provide an important test for the big bang model of the universe. If the theory is correct, everything in the visible universe should be younger than the expansion time computed from Hubble’s law.

These two timescales do appear to be in at least rough concordance. For example, the oldest stars in the disk of the Milky Way galaxy are about nine billion years old—an estimate derived from the rate of cooling of white dwarf stars. The stars in the halo of the Milky Way are somewhat older, about 15 billion years—a value derived from the rate of nuclear fuel consumption in the cores of these stars. The ages of the oldest known chemical elements are also approximately 15 billion years—a number that comes from radioactive dating techniques. Workers in laboratories have derived these age estimates from atomic and nuclear physics. It is noteworthy that their results agree, at least approximately, with the age that astronomers have derived by measuring cosmic expansion.

Another theory, the steady state theory, also succeeds in accounting for the expansion and homogeneity of the universe. In 1946 three physicists in England—Hoyle, Hermann Bondi and Thomas Gold—proposed such a cosmology. In their theory the universe is forever expanding, and matter is created spontaneously to fill the voids. As this material accumulates, they suggested, it forms new stars to replace the old. This steady state hypothesis predicts that ensembles of galaxies close to us should look statistically the same as those far away. The big bang cosmology makes a different prediction: if galaxies were all formed long ago, distant galaxies should look younger than those nearby because light from them requires a longer time to reach us. Such galaxies should contain more shortlived stars and more gas out of which future generations of stars will form.

The test is simple conceptually, but it took decades for astronomers to develop detectors sensitive enough to study distant galaxies in detail. When astronomers examine nearby galaxies that are powerful emitters of radio wavelengths, they see, at optical wavelengths, relatively round systems of stars. Distant radio galaxies, on the other hand, appear to have elongated and sometimes irregular structures. Moreover, in most distant radio galaxies, unlike the ones nearby, the distribution of light tends to be aligned with the pattern of the radio emission.

Likewise, when astronomers study the population of massive, dense clusters of galaxies, they find differences between those that are close and those far away. Distant clusters contain bluish galaxies that show evidence of ongoing star formation. Similar clusters that are nearby contain reddish galaxies in which active star formation ceased long ago. Observations made with the Hubble Space Telescope confirm that at least some of the enhanced star formation in these younger clusters may be the result of collisions between their member galaxies, a process that is much rarer in the present epoch.

DISTANT GALAXIES differ greatly from those nearby—an observation that shows that galaxies evolved from earlier, more irregular forms. Among galaxies that are bright at both optical ( blue ) and radio ( red ) wavelengths, the nearby galaxies tend to have smooth elliptical shapes at optical wavelengths and very elongated radio images. As redshift, and therefore distance, increases, galaxies have more irregular elongated forms that appear aligned at optical and radio wavelengths. The galaxy at the far right is seen as it was at 10 percent of the present age of the universe. The images were assembled by Pat McCarthy of the Carnegie Institute.

So if galaxies are all moving away from one another and are evolving from earlier forms, it seems logical that they were once crowded together in some dense sea of matter and energy. Indeed, in 1927, before much was known about distant galaxies, a Belgian cosmologist and priest, Georges Lemaître, proposed that the expansion of the universe might be traced to an exceedingly dense state he called the primeval “super-atom.” It might even be possible, he thought, to detect remnant radiation from the primeval atom. But what would this radiation signature look like?

When the universe was very young and hot, radiation could not travel very far without being absorbed and emitted by some particle. This continuous exchange of energy maintained a state of thermal equilibrium; any particular region was unlikely to be much hotter or cooler than the average. When matter and energy settle to such a state, the result is a so-called thermal spectrum, where the intensity of radiation at each wavelength is a definite function of the temperature. Hence, radiation originating in the hot big bang is recognizable by its spectrum.

In fact, this thermal cosmic background radiation has been detected. While working on the development of radar in the 1940s, Robert H. Dicke, then at the Massachusetts Institute of Technology, invented the microwave radiometer—a device capable of detecting low levels of radiation. In the 1960s Bell Laboratories used a radiometer in a telescope that would track the early communications satellites Echo-1 and Telstar. The engineer who built this instrument found that it was detecting unexpected radiation. Arno A. Penzias and Robert W. Wilson identified the signal as the cosmic background radiation. It is interesting that Penzias and Wilson were led to this idea by the news that Dicke had suggested that one ought to use a radiometer to search for the cosmic background.

Astronomers have studied this radiation in great detail using the Cosmic Background Explorer (COBE) satellite and a number of rocket-launched, balloon-borne and ground-based experiments. The cosmic background radiation has two distinctive properties. First, it is nearly the same in all directions. (As George F. Smoot of Lawrence Berkeley Laboratory and his team discovered in 1992, the variation is just one part per 100,000.) The interpretation is that the radiation uniformly fills space, as predicted in the big bang cosmology. Second, the spectrum is very close to that of an object in thermal equilibrium at 2.726 kelvins above absolute zero. To be sure, the cosmic background radiation was produced when the universe was far hotter than 2.726 degrees, yet researchers anticipated correctly that the apparent temperature of the radiation would be low. In the 1930s Richard C. Tolman of the California Institute of Technology showed that the temperature of the cosmic background would diminish because of the universe’s expansion.

The cosmic background radiation provides direct evidence that the universe did expand from a dense, hot state, for this is the condition needed to produce the radiation. In the dense, hot early universe thermonuclear reactions produced elements heavier than hydrogen, including deuterium, helium and lithium. It is striking that the computed mix of the light elements agrees with the observed abundances. That is, all evidence indicates that the light elements were produced in the hot, young universe, whereas the heavier elements appeared later, as products of the thermonuclear reactions that power stars.

The theory for the origin of the light elements emerged from the burst of research that followed the end of World War II. George Gamow and graduate student Ralph A. Alpher of George Washington University and Robert Herman of the Johns Hopkins University Applied Physics Laboratory and others used nuclear physics data from the war e›ort to predict what kind of nuclear processes might have occurred in the early universe and what elements might have been produced. Alpher and Herman also realized that a remnant of the original expansion would still be detectable in the existing universe.

Despite the fact that significant details of this pioneering work were in error, it forged a link between nuclear physics and cosmology. The workers demonstrated that the early universe could be viewed as a type of thermonuclear reactor. As a result, physicists have now precisely calculated the abundances of light elements produced in the big bang and how those quantities have changed because of subsequent events in the interstellar medium and nuclear processes in stars.

Our grasp of the conditions that prevailed in the early universe does not translate into a full understanding of how galaxies formed. Nevertheless, we do have quite a few pieces of the puzzle. Gravity causes the growth of density fluctuations in the distribution of matter, because it more strongly slows the expansion of denser regions, making them grow still denser. This process is observed in the growth of nearby clusters of galaxies, and the galaxies themselves were probably assembled by the same process on a smaller scale.

The growth of structure in the early universe was prevented by radiation pressure, but that changed when the universe had expanded to about 0.1 percent of its present size. At that point, the temperature was about 3,000 kelvins, cool enough to allow the ions and electrons to combine to form neutral hydrogen and helium. The neutral matter was able to slip through the radiation and to form gas clouds that could collapse to star clusters. Observations show that by the time the universe was one fifth its present size, matter had gathered into gas clouds large enough to be called young galaxies.

A pressing challenge now is to reconcile the apparent uniformity of the early universe with the lumpy distribution of galaxies in the present universe. Astronomers know that the density of the early universe did not vary by much, because they observe only slight irregularities in the cosmic background radiation. So far it has been easy to develop theories that are consistent with the available measurements, but more critical tests are in progress. In particular, different theories for galaxy formation predict quite different fluctuations in the cosmic background radiation on angular scales less than about one degree. Measurements of such tiny fluctuations have not yet been done, but they might be accomplished in the generation of experiments now under way. It will be exciting to learn whether any of the theories of galaxy formation now under consideration survive these tests.

The present-day universe has provided ample opportunity for the development of life as we know it—there are some 100 billion billion stars similar to the sun in the part of the universe we can observe. The big bang cosmology implies, however, that life is possible only for a bounded span of time: the universe was too hot in the distant past, and it has limited resources for the future. Most galaxies are still producing new stars, but many others have already exhausted their supply of gas. Thirty billion years from now, galaxies will be much darker and filled with dead or dying stars, so there will be far fewer planets capable of supporting life as it now exists.

The universe may expand forever, in which case all the galaxies and stars will eventually grow dark and cold. The alternative to this big chill is a big crunch. If the mass of the universe is large enough, gravity will eventually reverse the expansion, and all matter and energy will be reunited. During the next decade, as researchers improve techniques for measuring the mass of the universe, we may learn whether the present expansion is headed toward a big chill or a big crunch.

In the near future, we expect new experiments to provide a better understanding of the big bang. As we improve measurements of the expansion rate and the ages of stars, we may be able to confirm that the stars are indeed younger than the expanding universe. The larger telescopes recently completed or under construction may allow us to see how the mass of the universe affects the curvature of spacetime, which in turn influences our observations of distant galaxies.

We will also continue to study issues that the big bang cosmology does not address. We do not know why there was a big bang or what may have existed before. We do not know whether our universe has siblings—other expanding regions well removed from what we can observe. We do not understand why the fundamental constants of nature have the values they do. Advances in particle physics suggest some interesting ways these questions might be answered; the challenge is to find experimental tests of the ideas.

In following the debate on such matters of cosmology, one should bear in mind that all physical theories are approximations of reality that can fail if pushed too far. Physical science advances by incorporating earlier theories that are experimentally supported into larger, more encompassing frameworks. The big bang theory is supported by a wealth of evidence: it explains the cosmic background radiation, the abundances of light elements and the Hubble expansion. Thus, any new cosmology surely will include the big bang picture. Whatever developments the coming decades may bring, cosmology has moved from a branch of philosophy to a physical science where hypotheses meet the test of observation and experiment.

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17 Origin of the Universe and Our Solar System

Learning Objectives

By the end of this chapter, students should be able to:

• Explain the formation of the universe and how we observe it.
• Understand the origin of our solar system .
• Describe how the objects in our solar system are identified, explored, and characterized.
• Describe the types of small bodies in our solar system, their locations, and how they formed.
• Describe the characteristics of the giant planets , terrestrial planets , and small bodies in the solar system.
• Explain what influences the temperature of a planet’s surface.
• Explain why there is geological activity on some planets and not on others.
• Describe different methods for dating planets and the age of the solar system.
• Describe how the characteristics of extrasolar systems help us to model our own solar system.

The universe began 13.77 billion years ago when energy, matter, and space expanded from a single point. Evidence for the big bang is the cosmic “afterglow” from when the universe was still very dense, and red-shifted light from distant galaxies, which tell us the universe is still expanding.

The big bang produced hydrogen, helium, and lithium, but heavier elements come from nuclear fusion reactions in stars. Large stars make elements such as silicon, iron, and magnesium, which are important in forming terrestrial planets. Large stars explode as supernovae and scatter the elements into space.

Planetary systems begin with the collapse of a cloud of gas and dust. Material drawn to the center forms a star, and the remainder forms a disk around the star. Material within the disk clumps together to form planets. In our solar system , rocky planets are closer to the Sun, and ice and gas giants are farther away. This is because temperatures near the Sun were too high for ice to form, but silicate minerals and metals could solidify.

Early Earth was heated by radioactive decay, collisions with bodies from space, and gravitational compression. Heating melted Earth, causing molten metal to sink to Earth’s center and form a core, and silicate melt to float to the surface and form the mantle and crust. A collision with a planet the size of Mars knocked debris into orbit around Earth, and the debris coalesced into the moon. Earth’s atmosphere is the result of volcanic degassing, contributions by comets and meteorites, and photosynthesis.

The search for exoplanets has identified 12 planets that are similar in size to Earth and within the habitable zone of their stars. These are thought to be rocky worlds like Earth, but the compositions of these planets are not known for certain.

17.1 The Big Bang

According to the big bang theory , the universe blinked violently into existence 13.77 billion years ago. The big bang is often described as an explosion, but imagining it as an enormous fireball isn’t accurate. The big bang involved a sudden expansion of matter, energy, and space from a single point. The kind of Hollywood explosion that might come to mind involves expansion of matter and energy within space, but during the big bang, space itself was created.

At the start of the big bang, the universe was too hot and dense to be anything but a sizzle of particles smaller than atoms, but as it expanded, it also cooled. Eventually some of the particles collided and stuck together. Those collisions produced hydrogen and helium, the most common elements in the universe, along with a small amount of lithium.

You may wonder how a universe can be created out of nothing, or how we can know that the big bang happened at all. Creating a universe out of nothing is mostly beyond the scope of this chapter, but there is a way to think about it. The particles that make up the universe have opposites that cancel each other out, similar to the way that we can add the numbers 1 and −1 to get zero (also known as “nothing”). As far as the math goes, having zero is exactly the same as having a 1 and a −1. It is also exactly the same as having a 2 and a −2, a 3 and a −3, two −1s and a 2, and so on. In other words,  nothing is really the potential for something if you divide it into its opposite parts. As for how we can know that the big bang happened at all, there are very good reasons to accept that it is indeed how our universe came to be.

17.1.1 Looking Back to the Early Stages of the Big Bang

The notion of seeing the past is often used metaphorically when we talk about ancient events, but in this case it is meant literally. In our everyday experience, when we watch an event take place, we perceive that we are watching it as it unfolds in real time. In fact, this isn’t true. To see the event, light from that event must travel to our eyes. Light travels very rapidly, but it does not travel instantly. If we were watching a digital clock 1 m away from us change from 11:59 a.m. to 12:00 p.m., we would actually see it turn to 12:00 p.m. three billionths of a second after it happened. This isn’t enough of a delay to cause us to be late for an appointment, but the universe is a very big place, and the “digital clock” in question is often much, much farther away. In fact, the universe is so big that it is convenient to describe distances in terms of light years , or the distance light travels in one year. What this means is that light from distant objects takes so long to get to us that we see those objects as they were at some considerable time in the past. For example, the star Proxima Centauri is 4.24 light years from the sun. If you viewed Proxima Centauri from Earth on January 1, 2015, you would actually see it as it appeared in early October 2010.

We now have tools that are powerful enough to look deep into space and see the arrival of light from early in the universe’s history. Astronomers can detect light from approximately 375,000 years after the big bang is thought to have occurred. Physicists tell us that if the big bang happened, then particles within the universe would still be very close together at this time. They would be so close that light wouldn’t be able to travel far without bumping into another particle and getting scattered in another direction. The effect would be to fill the sky with glowing fog, the “afterglow” from the formation of the universe.

In fact, this is exactly what we see when we look at light from 375,000 years after the big bang. The fog is referred to as the cosmic microwave background (or CMB), and it has been carefully mapped throughout the sky. The map displays the cosmic microwave background as temperature variations, but these variations translate to differences in the density of matter in the early universe. The red patches are the highest density regions and the blue patches are the lowest density. Higher density regions represent the eventual beginnings of stars and planets. The map has been likened to a baby picture of the universe.

17.1.2 The Big Bang is Still Happening, and We Can See the Universe Expanding

The expansion that started with the big bang never stopped. It continues today, and we can see it happening by observing galaxies that large clusters of billions of stars, called galaxies, are moving away from us. (The exception is the Andromeda galaxy with which we are on a collision course.) The astronomer Edwin Hubble came to this conclusion when he observed that the light from other galaxies was red-shifted. The red shift is a consequence of the Doppler effect. This refers to how we see waves when the object that is creating the waves is moving toward us or away from us.

Before we get to the Doppler effect as it pertains to the red shift, let’s see how it works on something more tangible. The swimming duckling is generating waves as it moves through the water. It is generating waves that move forward as well as back, but notice that the ripples ahead of the duckling are closer to each other than the ripples behind the duckling. The distance from one ripple to the next is called the wavelength . The wavelength is shorter in the direction that the duckling is moving, and longer as the duckling moves away.

When waves are in air as sound waves rather than in water as ripples, the different wavelengths manifest as sounds with different pitches—the short wavelengths have a higher pitch, and the long wavelengths have a lower pitch. This is why the pitch of a car’s engine changes as the car races past you.

If you are using an offline version of this text, access the quiz for section 17.1 via the QR code.

17.2 Overview of Our Planetary System [1]

The solar system consists of the Sun and many smaller objects: the planets, their moons and rings, and such “debris” as asteroids , comets , and dust. Decades of observation and spacecraft exploration have revealed that most of these objects formed together with the Sun about 4.5 billion years ago. They represent clumps of material that condensed from an enormous cloud of gas and dust. The central part of this cloud became the Sun, and a small fraction of the material in the outer parts eventually formed the other objects.

During the past 50 years, we have learned more about the solar system than anyone imagined before the space age. In addition to gathering information with powerful new telescopes, we have sent spacecraft directly to many members of the planetary system . (Planetary astronomy is the only branch of our science in which we can, at least vicariously, travel to the objects we want to study.) With evocative names such as  Voyager ,  Pioneer ,  Curiosity , and  Pathfinder , our robot explorers have flown past, orbited, or landed on every planet, returning images and data that have dazzled both astronomers and the public. In the process, we have also investigated two dwarf planets , hundreds of fascinating moons, four ring systems, a dozen asteroids, and several comets (smaller members of our solar system that we will discuss later).

Our probes have penetrated the atmosphere of Jupiter and landed on the surfaces of Venus, Mars, our  Moon , Saturn’s moon Titan, the asteroids Eros, Itokawa, Ryugu, and Bennu, and the Comet Churyumov-Gerasimenko (usually referred to as 67P). Humans have set foot on the Moon and returned samples of its surface soil for laboratory analysis. We have flown a helicopter drone on Mars. We have even discovered other places in our solar system that might be able to support some kind of life.

17.2.1 An Inventory

The Sun, a star that is brighter than about 80% of the stars in the Galaxy, is by far the most massive member of the solar system. It is an enormous ball about 1.4 million kilometers in diameter, with surface layers of incandescent gas and an interior temperature of millions of degrees. The Sun will be discussed in later chapters as our first, and best-studied, example of a star.

Table 17.1: Mass of members of the solar system. Note that the Sun is by far the most massive member of the solar system.

Most of the material of the planets in the solar system is actually concentrated in the largest one, Jupiter , which is more massive than all the rest of the planets combined. Astronomers were able to determine the masses of the planets centuries ago using Kepler’s laws of planetary motion and Newton’s law of gravity to measure the planets’ gravitational effects on one another or on moons that orbit them. Today, we make even more precise measurements of their masses by tracking their gravitational effects on the motion of spacecraft that pass near them.

Beside Earth, five other planets were known to the ancients—Mercury, Venus, Mars, Jupiter, and Saturn—and two were discovered after the invention of the telescope: Uranus and Neptune. The eight planets all revolve in the same direction around the Sun. They orbit in approximately the same plane, like cars traveling on concentric tracks on a giant, flat racecourse. Each planet stays in its own “traffic lane,” following a nearly circular orbit about the Sun and obeying the “traffic” laws discovered by Galileo, Kepler, and Newton. Besides these planets, we have also been discovering smaller worlds beyond Neptune that are called trans-Neptunian object s or TNOs. The first to be found, in 1930, was  Pluto , but others have been discovered during the twenty-first century. One of them,  Eris , is about the same size as Pluto and has at least one moon (Pluto has five known moons). The largest TNOs are also classed as dwarf planets ,  as is the largest asteroid,  Ceres . To date, more than 2600 of these TNOs have been discovered, and one, called Arrokoth, was explored by the New Horizons spacecraft.

Each of the planets and dwarf planets also rotates (spins) about an axis running through it, and in most cases the direction of rotation is the same as the direction of revolution about the Sun. The exceptions are  Venus , which rotates backward very slowly (that is, in a retrograde direction), and Uranus and  Pluto , which also have strange rotations, each spinning about an axis tipped nearly on its side. We do not yet know the spin orientations of Eris, Haumea, and Makemake.

The four planets closest to the Sun (Mercury through Mars) are called the inner or  terrestrial planets . Often, the  Moon is also discussed as a part of this group, bringing the total of terrestrial objects to five (we generally call Earth’s satellite “the Moon,” with a capital M, and the other satellites “moons,” with lowercase m’s). The terrestrial planets are relatively small worlds, composed primarily of rock and metal. All of them have solid surfaces that bear the records of their geological history in the forms of craters , mountains , and volcanoes .

The next four planets (Jupiter through Neptune) are much larger and are composed primarily of lighter ices, liquids, and gases. We call these four the Jovian planets (after “Jove,” another name for Jupiter in mythology) or giant planets —a name they richly deserve. About 1,300 Earths could fit inside Jupiter, for example. These planets do not have solid surfaces on which future explorers might land. They are more like vast, spherical oceans with much smaller, dense cores.

Near the outer edge of the system lies  Pluto , which was the first of the distant icy worlds to be discovered beyond Neptune (Pluto was visited by a spacecraft, the NASA New Horizons mission, in 2015).

Table 17.2: The planets.

The outermost part of the solar system is known as the Kuiper belt , which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud , a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well.

The IAU narrowed the definition of a planet to three criteria: 1) it must orbit a star (in our cosmic neighborhood, the Sun), 2) it must be big enough to have enough gravity to force it into a spherical shape, and 3) it must be big enough that its gravity cleared away any other objects of a similar size near its orbit around the Sun. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets.

17.2.2 Smaller Members of the Solar System

Most of the planets are accompanied by one or more moons ; only Mercury and Venus move through space alone. There are more than 210 known moons orbiting planets and dwarf planets, and undoubtedly many other small ones remain undiscovered. The largest of the moons are as big as small planets and just as interesting. In addition to our Moon, they include the four largest moons of Jupiter (called the Galilean moons, after their discoverer) and the largest moons of Saturn and Neptune (confusingly named Titan and Triton).

Each of the giant planets also has rings made up of countless small bodies ranging in size from mountains to mere grains of dust, all in orbit about the equator of the planet. The bright rings of Saturn are, by far, the easiest to see. They are among the most beautiful sights in the solar system.   But, all four ring systems are interesting to scientists because of their complicated forms, influenced by the pull of the moons that also orbit these giant planets.

The solar system has many other less-conspicuous members. Another group is the  asteroids , rocky bodies that orbit the Sun like miniature planets, mostly in the space between Mars and Jupiter (although some do cross the orbits of planets like Earth). Most asteroids are remnants of the initial population of the solar system that existed before the planets themselves formed. Some of the smallest moons of the planets, such as the moons of Mars, are very likely captured asteroids.

Another class of small bodies is composed mostly of ice, made of frozen gases such as water, carbon dioxide, and carbon monoxide; these objects are called  comets . Comets also are remnants from the formation of the solar system, but they were formed and continue (with rare exceptions) to orbit the Sun in distant, cooler regions—stored in a sort of cosmic deep freeze. This is also the realm of the larger icy worlds, called dwarf planets.

Finally, there are countless grains of broken rock, which we call cosmic dust, scattered throughout the solar system. When these particles enter Earth’s atmosphere (as millions do each day), they burn up, producing a brief flash of light in the night sky known as a meteor  (meteors are often referred to as shooting stars). Occasionally, some larger chunk of rocky or metallic material survives its passage through the atmosphere and lands on Earth. Any piece that strikes the ground is known as a  meteorite . You can see meteorites on display in many natural history museums and can sometimes even purchase pieces of them from gem and mineral dealers.

17.2.3 A Scale Model of the Solar System

Astronomy often deals with dimensions and distances that far exceed our ordinary experience. What does 1.4 billion kilometers—the distance from the Sun to Saturn—really mean to anyone? It can be helpful to visualize such large systems in terms of a scale model.

In our imaginations, let us build a scale model of the solar system, adopting a scale factor of 1 billion (10 9 )—that is, reducing the actual solar system by dividing every dimension by a factor of 10 9 . Earth, then, has a diameter of 1.3 centimeters, about the size of a grape. The Moon is a pea orbiting this at a distance of 40 centimeters, or a little more than a foot away. The Earth-Moon system fits into a standard backpack.

In this model, the Sun is nearly 1.5 meters in diameter, about the average height of an adult, and our Earth is at a distance of 150 meters—about one city block—from the Sun. Jupiter is five blocks away from the Sun, and its diameter is 15 centimeters, about the size of a very large grapefruit. Saturn is 10 blocks from the Sun; Uranus, 20 blocks; and Neptune, 30 blocks. Pluto, with a distance that varies quite a bit during its 249-year orbit, is currently just beyond 30 blocks and getting farther with time. Most of the moons of the outer solar system are the sizes of various kinds of seeds orbiting the grapefruit, oranges, and lemons that represent the outer planets.

In our scale model, a human is reduced to the dimensions of a single atom, and cars and spacecraft to the size of molecules. Sending the Voyager spacecraft to Neptune involves navigating a single molecule from the Earth–grape toward a lemon 5 kilometers away with an accuracy equivalent to the width of a thread in a spider’s web.

If that model represents the solar system, where would the nearest stars be? If we keep the same scale, the closest stars would be tens of thousands of kilometers away. If you built this scale model in the city where you live, you would have to place the representations of these stars on the other side of Earth or beyond.

By the way, model solar systems like the one we just presented have been built in cities throughout the world. In Sweden, for example, Stockholm’s huge Globe Arena has become a model for the Sun, and Pluto is represented by a 12-centimeter sculpture in the small town of Delsbo, 300 kilometers away. Another model solar system is in Washington on the Mall between the White House and Congress (perhaps proving they are worlds apart?).

If you are using an offline version of this text, access the quiz for section 17.2 via the QR code.

17.3 Composition and Structure of Planets [2]

The fact that there are two distinct kinds of planets—the rocky terrestrial planets and the gas-rich Jovian planets—leads us to believe that they formed under different conditions. Certainly their compositions are dominated by different elements . Let us look at each type in more detail.

17.3.1 The Giant Planets

The two largest planets,  Jupiter  and  Saturn , have nearly the same chemical makeup as the Sun; they are composed primarily of the two elements hydrogen and helium, with 75% of their mass being hydrogen and 25% helium. On Earth, both hydrogen and helium are gases, so Jupiter and Saturn are sometimes called gas planets. But, this name is misleading. Jupiter and Saturn are so large that the gas is compressed in their interior until the hydrogen becomes a liquid. Because the bulk of both planets consists of compressed, liquefied hydrogen, we should really call them liquid planets.

Under the force of gravity, the heavier elements sink toward the inner parts of a liquid or gaseous planet. Both Jupiter and Saturn, therefore, have cores composed of heavier rock, metal, and ice, but we cannot see these regions directly. In fact, when we look down from above, all we see is the atmosphere with its swirling clouds. We must infer the existence of the denser core inside these planets from studies of each planet’s gravity.

Uranus  and  Neptune are much smaller than Jupiter and Saturn, but each also has a core of rock, metal, and ice. Uranus and Neptune were less efficient at attracting hydrogen and helium gas, so they have much smaller atmospheres in proportion to their cores.

Chemically, each giant planet is dominated by hydrogen and its many compounds. Nearly all the oxygen present is combined chemically with hydrogen to form water (H 2 O). Chemists call such a hydrogen-dominated composition  reduced . Throughout the outer solar system, we find abundant water (mostly in the form of ice) and reducing chemistry.

17.3.2 The Terrestrial Planets

The terrestrial planets are quite different from the giants. In addition to being much smaller, they are composed primarily of rocks and metals. These, in turn, are made of elements that are less common in the universe as a whole. The most abundant rocks, called silicates , are made of silicon and oxygen, and the most common metal is iron. We can tell from their densities that  Mercury  has the greatest proportion of metals (which are denser) and the Moon has the lowest.  Earth ,  Venus , and  Mars  all have roughly similar bulk compositions: about one third of their mass consists of iron-nickel or iron-sulfur combinations; two thirds is made of silicates. Because these planets are largely composed of oxygen compounds (such as the silicate minerals of their crusts), their chemistry is said to be  oxidized .

When we look at the internal structure of each of the terrestrial planets, we find that the densest metals are in a central core, with the lighter silicates near the surface. If these planets were liquid, like the giant planets, we could understand this effect as the result the sinking of heavier elements due to the pull of gravity. This leads us to conclude that, although the terrestrial planets are solid today, at one time they must have been hot enough to melt.

Differentiation  is the process by which gravity helps separate a planet’s interior into layers of different compositions and densities. The heavier metals sink to form a core, while the lightest minerals float to the surface to form a crust. Later, when the planet cools, this layered structure is preserved. In order for a rocky planet to differentiate, it must be heated to the melting point of rocks, which is typically more than 1300 K.

17.3.3 Moons, Asteroids, and Comets

Chemically and structurally, Earth’s Moon is like the terrestrial planets, but most moons are in the outer solar system, and they have compositions similar to the cores of the giant planets around which they orbit. The three largest moons—Ganymede and Callisto in the Jovian system, and  Titan in the Saturnian system—are composed half of frozen water, and half of rocks and metals. Most of these moons differentiated during formation, and today they have cores of rock and metal, with upper layers and crusts of very cold and—thus very hard—ice.

Most of the asteroids and comets , as well as the smallest moons , were probably never heated to the melting point. However, some of the largest asteroids, such as  Vesta , appear to be differentiated; others are fragments from differentiated bodies. Many of the smaller objects seem to be fragments or rubble piles that are the result of collisions. Because most asteroids and comets retain their original composition, they represent relatively unmodified material dating back to the time of the formation of the solar system. In a sense, they act as chemical fossils, helping us to learn about a time long ago whose traces have been erased on larger worlds.

17.3.4 Temperatures: Going to Extremes

Generally speaking, the farther a planet or moon is from the Sun, the cooler its surface. The planets are heated by the radiant energy of the Sun, which gets weaker with the square of the distance. You know how rapidly the heating effect of a fireplace or an outdoor radiant heater diminishes as you walk away from it; the same effect applies to the Sun.  Mercury , the closest planet to the Sun, has a blistering surface temperature that ranges from 280–430 °C on its sunlit side, whereas the surface temperature on  Pluto is only about –220 °C, colder than liquid air.

Mathematically, the temperatures decrease approximately in proportion to the square root of the distance from the Sun. Pluto is about 30 AU at its closest to the Sun (or 100 times the distance of Mercury) and about 49 AU at its farthest from the Sun. Thus, Pluto’s temperature is less than that of Mercury by the square root of 100, or a factor of 10: from 500 K to 50 K.

In addition to its distance from the Sun, the surface temperature of a planet can be influenced strongly by its atmosphere . Without our atmospheric insulation (the greenhouse effect, which keeps the heat in), the oceans of Earth would be permanently frozen. Conversely, if Mars once had a larger atmosphere in the past, it could have supported a more temperate climate than it has today. Venus is an even more extreme example, where its thick atmosphere of carbon dioxide acts as insulation, reducing the escape of heat built up at the surface, resulting in temperatures greater than those on Mercury. Today, Earth is the only planet where surface temperatures generally lie between the freezing and boiling points of water. As far as we know, Earth is the only planet to support life.

17.3.5 Dating Planetary Surfaces [3]

How do we know the age of the surfaces we see on planets and moons? If a world has a surface (as opposed to being mostly gas and liquid), astronomers have developed some techniques for estimating how long ago that surface solidified. Note that the age of these surfaces is not necessarily the age of the planet as a whole. On geologically active objects (including Earth), vast outpourings of molten rock or the erosive effects of water and ice, which we call planet weathering , have erased evidence of earlier epochs and present us with only a relatively young surface for investigation.

One way to estimate the age of a surface is by counting the number of impact  craters . This technique works because the rate at which impacts have occurred in the solar system has been roughly constant for several billion years. Thus, in the absence of forces to eliminate craters, the number of craters is simply proportional to the length of time the surface has been exposed. This technique has been applied successfully to many solid planets and moons .

Bear in mind that crater counts can tell us only the time since the surface experienced a major change that could modify or erase preexisting craters. Estimating ages from crater counts is a little like walking along a sidewalk in a snowstorm after the snow has been falling steadily for a day or more. You may notice that in front of one house the snow is deep, while next door the sidewalk may be almost clear. Do you conclude that less snow has fallen in front of Ms. Jones’ house than Mr. Smith’s? More likely, you conclude that Jones has recently swept the walk clean and Smith has not. Similarly, the numbers of craters indicate how long it has been since a planetary surface was last “swept clean” by ongoing lava flows or by molten materials ejected when a large impact happened nearby.

Still, astronomers can use the numbers of craters on different parts of the same world to provide important clues about how regions on that world evolved. On a given planet or moon, the more heavily cratered terrain will generally be older (that is, more time will have elapsed there since something swept the region clean).

17.3.6 Radioactive Rocks

Another way to trace the history of a solid world is to measure the age of individual rocks. After samples were brought back from the Moon  by Apollo astronauts, the techniques that had been developed to date rocks on Earth were applied to rock samples from the Moon to establish a geological chronology for the Moon. Furthermore, a few samples of material from the Moon, Mars, and the large asteroid  Vesta have fallen to Earth as meteorites and can be examined directly.

Scientists measure the age of rocks using the properties of natural  radioactivity . Around the beginning of the twentieth century, physicists began to understand that some atomic nuclei are not stable but can split apart (decay) spontaneously into smaller nuclei. The process of radioactive decay involves the emission of particles such as electrons , or of radiation in the form of gamma rays .

For any one radioactive nucleus, it is not possible to predict when the decay process will happen. Such decay is random in nature, like the throw of dice: as gamblers have found all too often, it is impossible to say just when the dice will come up 7 or 11. But, for a very large number of dice tosses, we can calculate the odds that 7 or 11 will come up. Similarly, if we have a very large number of radioactive atoms of one type (say, uranium), there is a specific time period, called its  half-life , during which the chances are fifty-fifty that decay will occur for any of the nuclei.

A particular nucleus may last a shorter or longer time than its half-life, but in a large sample, almost exactly half of the nuclei will have decayed after a time equal to one half-life. Half of the remaining nuclei will have decayed after two half-lives pass, leaving only one half of a half—or one quarter—of the original sample.

If you had 1 gram of pure radioactive nuclei with a half-life of 100 years, then after 100 years you would have 1/2 gram; after 200 years, 1/4 gram; after 300 years, only 1/8 gram; and so forth. However, the material does not disappear. Instead, the radioactive atoms are replaced with their decay products. Sometimes the radioactive atoms are called  parents  and the decay products are called  daughter elements.

In this way, radioactive elements with half-lives we have determined can provide accurate nuclear clocks. By comparing how much of a radioactive parent element is left in a rock to how much of its daughter products have accumulated, we can learn how long the decay process has been going on and hence how long ago the rock formed. The following table summarizes the decay reactions used most often to date lunar and terrestrial rocks.

Table 17.3: Radioactive decay reaction used to date rocks. The number after each element is its atomic weight, equal to the number of protons plus neutrons in its nucleus. This specifies the isotope of the element, different isotopes of the same element differ in the number of neutrons.

When astronauts first flew to the Moon, one of their most important tasks was to bring back lunar rocks for radioactive age-dating. Until then, astronomers and geologists had no reliable way to measure the age of the lunar surface. Counting craters had let us calculate relative ages (for example, the heavily cratered lunar highlands were older than the dark lava plains), but scientists could not measure the actual age in years. Some thought that the ages were as young as those of Earth’s surface, which has been resurfaced by many geological events. For the Moon’s surface to be so young would imply active geology on our satellite. Only in 1969, when the first Apollo samples were dated, did we learn that the Moon is an ancient, geologically dead world. Using such dating techniques, we have been able to determine the ages of both Earth and the Moon: each was formed about 4.5 billion years ago (although, as we shall see, Earth probably formed earlier than the Moon).

We should also note that the decay of radioactive nuclei generally releases energy in the form of heat. Although the energy from a single nucleus is not very large (in human terms), the enormous numbers of radioactive nuclei in a planet or moon (especially early in its existence) can be a significant source of internal energy for that world. Geologists estimate that about half of Earth’s current internal heat budget comes from the decay of radioactive isotopes in its interior.

If you are using an offline version of this text, access the quiz for section 17.3 via the QR code.

17.4 Origin of the Solar System [4]

Much of astronomy is motivated by a desire to understand the origin of things: to find at least partial answers to age-old questions of where the universe , the Sun, Earth, and we ourselves came from. Each planet and moon is a fascinating place that may stimulate our imagination as we try to picture what it would be like to visit. Taken together, the members of the solar system preserve patterns that can tell us about the formation of the entire system. As we begin our exploration of the planets, we want to introduce our modern picture of how the solar system formed.

The recent discovery of thousands of planets in orbit around other stars has shown astronomers that many exoplanetary systems can be quite different from our own solar system. For example, it is common for these systems to include planets intermediate in size between our terrestrial and giant planets. These are often called  superearths . Some exoplanet systems even have giant planets close to the star, reversing the order we see in our system.

17.4.1 Looking for Patterns

One way to approach our question of origin is to look for regularities among the planets. We found, for example, that all the planets lie in nearly the same plane and revolve in the same direction around the Sun. The Sun also spins in the same direction about its own axis. Astronomers interpret this pattern as evidence that the Sun and planets formed together from a spinning cloud of gas and dust that we call the  solar nebula .

The composition of the planets gives another clue about origins. Spectroscopic analysis allows us to determine which elements are present in the Sun and the planets. The Sun has the same hydrogen-dominated composition as Jupiter and Saturn, and therefore appears to have been formed from the same reservoir of material. In comparison, the terrestrial planets and our Moon are relatively deficient in the light gases and the various ices that form from the common elements oxygen, carbon, and nitrogen. Instead, on Earth and its neighbors, we see mostly the rarer heavy elements such as iron and silicon. This pattern suggests that the processes that led to planet formation in the inner solar system must somehow have excluded much of the lighter materials that are common elsewhere. These lighter materials must have escaped, leaving a residue of heavy stuff.

The reason for this is not hard to guess, bearing in mind the heat of the Sun. The inner planets and most of the asteroids are made of rock and metal, which can survive heat, but they contain very little ice or gas, which evaporate when temperatures are high (to see what we mean, just compare how long a rock and an ice cube survive when they are placed in the sunlight). In the outer solar system, where it has always been cooler, the planets and their moons, as well as icy dwarf planets and comets , are composed mostly of ice and gas.

17.4.2 The Evidence from Far Away

A second approach to understanding the origins of the solar system is to look outward for evidence that other systems of planets are forming elsewhere. We cannot look back in time to the formation of our own system, but many stars in space are much younger than the Sun. In these systems, the processes of planet formation might still be accessible to direct observation. We observe that there are many other “ solar nebulas ” or  circumstellar disks —flattened, spinning clouds of gas and dust surrounding young stars. These disks resemble our own solar system’s initial stages of formation billions of years ago.

17.4.3 Building Planets

Circumstellar disks are a common occurrence around very young stars, suggesting that disks and stars form together. Astronomers can use theoretical calculations to see how solid bodies might form from the gas and dust in these disks as they cool. These models show that material begins to coalesce first by forming smaller objects, precursors of the planets, which we call  planetesimals .

Today’s fast computers can simulate the way millions of planetesimals, probably no larger than 100 kilometers in diameter, might gather together under their mutual gravity to form the planets we see today. We are beginning to understand that this process was a violent one, with planetesimals crashing into each other and sometimes even disrupting the growing planets themselves. As a consequence of those violent impacts (and the heat from radioactive elements in them), all the planets were heated until they were liquid and gas, and therefore differentiated, which helps explain their present internal structures.

The process of impacts and collisions in the early solar system was complex and, apparently, often random. The solar nebula model can explain many of the regularities we find in the solar system, but the random collisions of massive collections of planetesimals could be the reason for some exceptions to the “rules” of solar system behavior. For example, why do the planets Uranus and Pluto spin on their sides? Why does Venus spin slowly and in the opposite direction from the other planets? Why does the composition of the Moon resemble Earth in many ways and yet exhibit substantial differences? The answers to such questions probably lie in enormous collisions that took place in the solar system long before life on Earth began.

Today, some 4.5 billion years after its origin, the solar system is—thank goodness—a much less violent place. However, some planetesimals have continued to interact and collide, and their fragments move about the solar system as roving “transients” that can make trouble for the established members of the Sun’s family, such as our own Earth.

If you are using an offline version of this text, access the quiz for section 17.4 via the QR code.

Our solar system currently consists of the Sun, eight planets, five dwarf planets, nearly 200 known moons , and a host of smaller objects. The planets can be divided into two groups: the inner terrestrial planets and the outer giant planets. Smaller members of the solar system include asteroids (including the dwarf planet Ceres), which are rocky and metallic objects found mostly between Mars and Jupiter; comets , which are made mostly of frozen gases and generally orbit far from the Sun; and countless smaller grains of cosmic dust. When a meteor survives its passage through our atmosphere and falls to Earth, we call it a meteorite .

The ages of the surfaces of objects in the solar system can be estimated by counting craters: on a given world, a more heavily cratered region will generally be older than one that is less cratered. We can also use samples of rocks with radioactive elements in them to obtain the time since the layer in which the rock formed last solidified. The half-life of a radioactive element is the time it takes for half the sample to decay; we determine how many half-lives have passed by how much of a sample remains the radioactive element and how much has become the decay product. In this way, we have estimated the age of the Moon and Earth to be roughly 4.5 billion years.

Regularities among the planets have led astronomers to hypothesize that the Sun and the planets formed together in a giant, spinning cloud of gas and dust called the solar nebula . Astronomical observations show tantalizingly similar circumstellar disks around other stars. Within the solar nebula, material first coalesced into planetesimals ; many of these gathered together to make the planets and moons. The remainder can still be seen as comets and asteroids. Probably all planetary systems have formed in similar ways, but many exoplanet systems have evolved along quite different paths.

If you are using an offline version of this text, access the quiz for chapter 17 via the QR code.

Text References

Parts of this chapter are from OpenStax’s Astronomy (chapter 7) . 2016. CC BY 4.0 .

Chapter 17 Origin of Earth and the Solar System ( CC BY 4.0)  by Karla Panchuk was added from Earle, Steven (2019) Physical Geology, 2nd edition. BC Campus https://opentextbc.ca/physicalgeology2ed/chapter/22-1-starting-with-a-big-bang

Figure References

Figure 17.1: The big bang. NASA/WMAP Science Team. 2006. Public domain. https://en.wikipedia.org/wiki/File:CMB_Timeline300_no_WMAP.jpg

Figure 17.2: Cosmic microwave background (CMB) map of the sky, a baby picture of the universe. NASA / WMAP Science Team. 2012. Public domain. https://commons.wikimedia.org/wiki/File:Ilc_9yr_moll4096.png

Figure 17.3: Doppler effect. Charly Whisky. 2007. CC BY-SA 3.0 . https://commons.wikimedia.org/wiki/File%3ADopplerfrequenz.gif

Figure 17.4: Red shift in light from the supercluster BAS11 compared to the sun’s light. Kindred Grey. 2022. CC BY 4.0 . Includes Duck by parkjisun from Noun Project ( Noun Project license ).

Figure 17.5: Astronauts on the Moon. NASA Johnson Space Center; Restored by Bammesk. 1971. Public domain. https://en.wikipedia.org/wiki/File:AS15-88-11866_-_Apollo_15_flag,_rover,_LM,_Irwin_-_restoration1.jpg

Figure 17.6: Orbits of the planets. Arabik4892. 2022. CC BY-SA 4.0 . https://commons.wikimedia.org/wiki/File:Planet_Orbits.jpg

Figure 17.7: Surface of Mercury. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Mercury_Double-Ring_Impact_Basin.png

Figure 17.8: The four giant planets. Solar System Exploration, NASA. 2008. Public domain. https://commons.wikimedia.org/wiki/File:Gas_planet_size_comparisons.jpg

Figure 17.9: This intriguing image from the New Horizons spacecraft, taken when it flew by the dwarf planet Pluto in July 2015, shows some of its complex surface features. NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute. 2015. Public domain. https://en.wikipedia.org/wiki/File:Pluto-01_Stern_03_Pluto_Color_TXT.jpg

Figure 17.10: Saturn and its A, B, and C rings in visible and (inset) infrared light. NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic (ISS), NASA/JPL-Caltech/University of Arizona/CNRS/LPG-Nantes (VIMS). 2019. Public domain. https://commons.wikimedia.org/wiki/File:PIA23170-Saturn-Rings-IR-Map-20190613.jpg

Figure 17.11: Asteroid Eros. NASA/JPL/JHUAPL. 2000. Public domain. https://commons.wikimedia.org/wiki/File:Eros_-_PIA02923_(color).jpg

Figure 17.12: Comet Churyumov-Gerasimenko (67P). ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA. 2014. CC BY-SA 4.0 . https://commons.wikimedia.org/wiki/File:Comet_67P_True_color.jpg

Figure 17.13: Jupiter with its moon Europa on the left. NASA, ESA, STScI, A. Simon (Goddard Space Flight Center), and M.H. Wong (University of California, Berkeley) and the OPAL team. 2020. Public domain. https://commons.wikimedia.org/wiki/File:Jupiter_and_Europa_2020.tiff

Figure 17.14: Jupiter’s moon Ganymede. NOAA. 2009. Public domain. https://commons.wikimedia.org/wiki/File:Moon_Ganymede_by_NOAA.jpg

Figure 17.15: Our cratered Moon. NASA/Goddard/Arizona State University. 2011. Public domain. https://www.nasa.gov/mission_pages/LRO/news/lro-farside.html

Figure 17.16: Radioactive decay. Andrew Fraknoi, David Morrison, and Sidney Wolff. 2015. CC BY 4.0 . https://en.wikipedia.org/wiki/File:OSC_Astro_07_03_Decay_(1).jpg

Figure 17.17: NASA artist’s conception of various planet formation processes, including exocomets and other planetesimals, around Beta Pictoris, a very young type A V star. NASA/FUSE/Lynette Cook. 2007. Public domain. https://commons.wikimedia.org/wiki/File:NASA-ExocometsAroundBetaPictoris-ArtistView.jpg

Figure 17.18: Atlas of Planetary Nurseries. NASA/ESA and L. Ricci (ESO). 2009. CC BY 4.0 . https://esahubble.org/copyright/

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All of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy.

The generic term for a group of planets and other bodies circling a star is planetary system. Our planetary system is the only one officially called “solar system,” because our Sun is sometimes called Sol.

A large astronomical body that is neither a star nor a stellar remnant.

The measure of the vibrational (kinetic) energy of a substance.

Originating or existing outside the solar system.

The theory that the Universe started with a expansive explosion. Shortly after, elements were created (mostly hydrogen) and galaxies started to form.

A process inside stars where smaller atoms combine and form larger atoms.

The generic term for a group of planets and other bodies circling a star.

The process of atoms breaking down randomly and spontaneously.

Any planet beyond our solar system.

A group of all atoms with a specific number of protons, having specific, universal, and unique properties.

The distance that light can travel through space in a year. One light year is 9.4607 × 10^12 km.

Radiation left over from the an early stage in the development of the universe at the time when protons and neutrons were recombining to form atoms.

A gravitationally-bound system of stars and interstellar matter.

The increase in wavelength of light resulting from the fact that the source of the light is moving away from the observer.

The distance between any two repeating portions of a wave (e.g., two successive wave crests).

to move in a circular or curving course or orbit. Not to be confused with rotate, when something spins on an axis

An object that orbits a planet or something else that is not a star. Besides planets, moons can circle dwarf planets, large asteroids, and other bodies.

A small rocky body orbiting the sun.

a celestial object consisting of a nucleus of ice and dust and, when near the Sun, a “tail” of gas and dust particles pointing away from the Sun

A small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right.

The layers of gases surrounding a planet or other celestial body.

To move in a circular or curving course or orbit. Not to be confused with rotate, when something spins on an axis.

To spin on an axis. Not to be confused with revolve, when something moves in a circular or curving course or orbit.

A bowl-shaped depression, or hollowed-out area, produced by the impact of a meteorite, volcanic activity, or an explosion.

A landform that rises above its surrounding area.

Place where lava is erupted at the surface.

A circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun.

A spherical layer of icy objects surrounding our Sun; likely occupies space at a distance between about 2,000 and 100,000 astronomical units (AU) from the Sun.

The gases that are part of the Earth, which are mainly nitrogen and oxygen.

A small body of matter from outer space that enters the Earth's atmosphere, becoming incandescent as a result of friction and appearing as a streak of light.

A stoney and/or metallic object from our solar system which was never incorporated into a planet and has fallen onto Earth. Meteorite is used for the rock on Earth, meteoroid for the object in space, and meteor as the object travels in Earth's atmosphere.

The branch of science which deals with celestial objects, space, and the physical universe as a whole.

A straight line passing from side to side through the center of a body or figure, especially a circle or sphere.

Reduction involves a half-reaction in which a chemical species decreases its oxidation number, usually by gaining electrons.

Mineral group in which the silica tetrahedra, SiO4-4, is the building block.

A solid material that is typically hard, shiny, malleable, fusible, and ductile, with good electrical and thermal conductivity.

Oxidation is the loss of electrons or an increase in the oxidation state of a chemical or atoms within it.

In planetary science, differentiation is the process of separating out the different components within a planetary body as a consequence of their physical or chemical behavior (e.g. density and chemical affinities).

An AU (or astronomical unit) is the average distance from Earth to the Sun.

The kelvin, symbol K, is the SI base unit of temperature. Absolute zero is 0 K, the equivalent of −273.15°C.

Breaking down rocks into small pieces by chemical or mechanical means.

A stable subatomic particle with a charge of negative electricity, found in all atoms and acting as the primary carrier of electricity in solids.

A penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays.

A radioactive atom that can and will decay.

The atom that is made after a radioactive decay.

Of or pertaining to an exoplanet, a planet outside the solar system.

Rotating, flattened disk of gas and dust from which the solar system originated.

Turn from liquid into vapor.

A body that could or did come together with many others under gravitation to form a planet.

The calculated amount of time that half of the mass of an original (parent) radioactive isotope breaks down into a new (daughter) isotope.

Introduction to Earth Science Copyright © 2023 by Laura Neser is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Mathematics: The Beautiful Language of the Universe

Let us discuss the very nature of the cosmos. What you may find in this discussion is not what you expect. Going into a conversation about the universe as a whole, you would imagine a story full of wondrous events such as stellar collapse, galactic collisions, strange occurrences with particles, and even cataclysmic eruptions of energy. You may be expecting a story stretching the breadth of time as we understand it, starting from the Big Bang and landing you here, your eyes soaking in the photons being emitted from your screen. Of course, the story is grand. But there is an additional side to this amazing assortment of events that oftentimes is overlooked; that is until you truly attempt to understand what is going on. Behind all of those fantastic realizations, there is a mechanism at work that allows for us to discover all that you enjoy learning about. That mechanism is mathematics, and without it the universe would still be shrouded in darkness. In this article, I will attempt to persuade you that math isn’t some arbitrary and sometimes pointless mental task that society makes it out to be, and instead show you that it is a language we use to communicate with the stars.

We are currently bound to our solar system. This statement is actually better than it sounds, as being bound to our solar system is one major step up from being bound simply to our planet, as we were

before some very important minds elected to turn their geniuses toward the heavens. Before those like Galileo, who aimed his spyglass towards the sky, or Kepler discovering that planets move about the sun in ellipses, or Newton discovering a gravitational constant, mathematics was somewhat  limited, and our understanding of the universe rather ignorant. At its core, mathematics allows a species bound to its solar system to probe the depths of the cosmos from behind a desk. Now, in order to appreciate the wonder that is mathematics, we must first step back and briefly look at its beginnings and how it is integrally tied into our very existence.

Mathematics almost certainly came about from very early human tribes (predating Babylonian culture which is attributed to some of the first organized mathematics in recorded history), that may have used math as a way of keeping track of lunar or solar cycles, and keeping count of animals, food and/or people by leaders. It is as natural as when you are a young child and you can see that you have

one toy plus one other toy, meaning you have more than one toy. As you get older, you develop the ability to see that 1+1=2, and thus simple arithmetic seems to be interwoven into our very nature. Those that profess that they don’t have a mind for math are sadly mistaken because just as we all have a mind for breathing, or blinking, we all have this innate ability to understand arithmetic. Mathematics is both a natural occurrence and a human designed system. It would appear that nature grants us this ability to recognize patterns in the form of arithmetic, and then we systematically construct more complex mathematical systems that aren’t obvious in nature but let us further communicate with nature.

All this aside, mathematics developed alongside of human development, and carried on similarly with each culture that was developing it simultaneously. It’s a wonderful observation to see that cultures that had no contact with one another were developing similar mathematical constructs without conversing. However, it wasn’t until mankind decidedly turned their mathematical wonder towards the sky that math truly began to develop in an astonishing way. It is by no mere coincidence that our scientific revolution was spurred by the development of more advanced mathematics built not to tally sheep or people, but rather to further our understandings of our place within the universe. Once Galileo began measuring the rates at which objects fell in an attempt to show mathematically that the mass of an object had little to do with the speed in which it fell, mankind’s future would forever be altered.

This is where the cosmic perspective ties in to our want to further our mathematical knowledge. If it were not for math, we would still think we were on one of a few planets orbiting a star amidst the backdrop of seemingly motionless lights. This is a rather bleak outlook today compared to what we now know

about the awesomely large universe we reside in. This idea of the universe motivating us to understand more about mathematics can be inscribed in how Johannes Kepler used what he observed the planets doing, and then applied mathematics to it to develop a fairly accurate model (and method for predicting planetary motion) of the solar system. This is one of many demonstrations that illustrate the importance of mathematics within our history, especially within astronomy and physics.

The story of mathematics becomes even more amazing as we push forward to one of the most advanced thinkers humanity has ever known. Sir Isaac Newton, when pondering the motions of Halley’s Comet, came to the realization that the math that had been used thus far to describe physical motion of massive

bodies, simply would not suffice if we were to ever understand anything beyond that of our seemingly limited celestial nook. In a show of pure brilliance that lends validity to my earlier statement about how we can take what we naturally have and then construct a more complex system upon it, Newton developed the Calculus in which this way of approaching moving bodies, he was able to accurately model the motion of not only Halley’s comet, but also any other heavenly body that moved across the sky.

In one instant, our entire universe opened up before us, unlocking almost unlimited abilities for us to converse with the cosmos as never before. Newton also expanded upon what Kepler started. Newton recognized that Kepler’s mathematical equation for planetary motion, Kepler’s 3rd Law ( P 2 =A 3 ), was purely based on empirical observation, and was only meant to measure what we observed within our solar system. Newton’s mathematical brilliance was in realizing that this basic equation could be made universal by applying a gravitational constant to the equation, in which gave birth to perhaps one of the most important equations to ever be derived by mankind; Newton’s Version of Kepler’s Third Law.

What Newton realized was that when things move in non-linear ways, using basic Algebra would not produce the correct answer. Herein lays one of the main differences between Algebra and Calculus. Algebra allows one to find the slope (rate of change) of straight lines (constant rate of change), whereas Calculus allows one to find the slope of curved lines (variable rate of change). There are obviously many more applications of Calculus than just this, but I am merely illustrating a fundamental difference between the two in order to show you just how revolutionary this new concept was. All at once, the motions of planets and other objects that orbit the sun became more accurately measurable, and thus we gained the ability to understand the universe a little deeper. Referring back to Netwon’s Version of Kepler’s Third Law, we were now able to apply (and still do) this incredible physics equation to almost anything that is orbiting something else. From this equation, we can determine the mass of either of the objects, the distance apart they are from each other, the force of gravity that is exerted between the two, and other physical qualities built from these simple calculations.

With his understanding of mathematics, Newton was able to derive the aforementioned gravitational constant for all objects in the universe ( G = 6.672×10 -11 N m 2 kg -2 ). This constant allowed him to unify astronomy and physics which then permitted predictions about how things moved in the universe. We could now measure the masses of planets (and the sun) more accurately, simply according to Newtonian physics (aptly named to honor just how important Newton was within physics and mathematics). We could now apply this newfound language to the cosmos, and begin coercing it to divulge its secrets. This was a defining moment for humanity, in that all of those things that prohibited our understandings prior to this new form of math were now at our fingertips, ready to be discovered. This is the brilliance of understanding Calculus, in that you are speaking the language of the stars.

There perhaps is no better illustration of the power that mathematics awarded us then in the discovery of the planet Neptune. Up until its discovery in September of 1846, planets were discovered simply by observing certain “stars” that were moving against the backdrop of all the other stars in odd ways. The term planet is Greek for “wanderer”, in that these peculiar stars wandered across the sky in noticeable patterns at different times of the year. Once the telescope was first turned upwards towards the sky by Galileo, these wanderers resolved into other worlds that appeared to be like ours. If fact, some of these worlds appeared to be little solar systems themselves, as Galileo discovered when he began recording the moons of Jupiter as they orbited around it.

After Newton presented his physics equations to the world, mathematicians were ready and excited to begin applying them to what we had been keeping track of for years. It was as if we were thirsty for the knowledge, and finally someone turned on the faucet. We began measuring the motions of the planets and gaining more accurate models for how they behaved. We used these equations to approximate the mass of the Sun. We were able to make remarkable predictions that were validated time and again simply by observation. What we were doing was unprecedented, as we were using mathematics to make almost impossible to know predictions that you would think we could never make without actually going to these planets, and then using actual observation to prove the math correct. However, what we also did was begin to figure out some odd discrepancies with certain things. Uranus, for instance, was behaving not as it should according to Newton’s laws.

What makes the discovery of Neptune so wonderful was the manner in which it was discovered. What Newton had done was uncover a deeper language of the cosmos, in which the universe was able to reveal more to us. And this is exactly what happened when we applied this language to the orbit of Uranus. The manner in which Uranus orbited was curious and did not fit what it should have if it was the only planet that far out from the sun. Looking at the numbers, there had to be something else out there perturbing its orbit. Now, before Newton’s mathematical insights and laws, we would have had no reason to suspect anything was wrong in what we observed. Uranus orbited in the way Uranus orbited; it was just how it was. But, again revisiting that notion of mathematics being an ever increasing dialogue with the universe, once we asked the question in the right format, we realized that there really must be something else beyond what we couldn’t see. This is the beauty of mathematics writ large; an ongoing conversation with the universe in which more than we may expect is revealed.

It came to a French mathematician Urbain Le Verrier who sat down and painstakingly worked through the mathematical equations of the orbit of Uranus. What he was doing was using Newton’s mathematical equations backwards, realizing that there must be an object out there beyond the orbit of Uranus that was also orbiting the sun,

and then looking to apply the right mass and distance that this unseen object required for perturbing the orbit of Uranus in the way we were observing it was. This was phenomenal, as we were using parchment and ink to find a planet that nobody had ever actually observed. What he found was that an object, soon to be Neptune, had to be orbiting at a specific distance from the sun, with the specific mass that would cause the irregularities in the orbital path of Uranus. Confident of his mathematical calculations, he took his numbers to the New Berlin Observatory, where the astronomer Johann Gottfried Galle looked exactly where Verrier’s calculations told him to look, and there lay the 8th and final planet of our solar system, less than 1 degree off from where Verrier’s calculations said for him to look. What had just happened was an incredible confirmation of Newton’s gravitational theory and proved that his mathematics were correct.

These types of mathematical insights continued on long after Newton. Eventually, we began to learn much more about the universe with the advent of better technology (brought about by advances in mathematics). As we moved into the 20th century, quantum theory began to take shape, and we soon realized that Newtonian physics and mathematics seemed to hold no sway over what we observed on the quantum level. In another momentous event in human history, yet again brought forth by the advancement in mathematics, Albert Einstein unveiled his theories of General and Special Relativity, which was a new way to look not only at gravity, but

also on energy and the universe in general. What Einstein’s mathematics did was allow for us to yet again uncover an even deeper dialogue with the universe, in which we began to understand its origins.

Continuing this trend of advancing our understandings, what we have realized is that now there are two sects of physics that do not entirely align. Newtonian or “classical” physics, that works extraordinarily well with the very large (motions of planets, galaxies, etc…) and quantum physics that explains the extremely small (the interactions of sub-atomic particles, light, etc…). Currently, these two areas of physics are not in alignment, much like two different dialects of a language. They are similar and they both work, but they are not easily reconcilable with one another. One of the greatest challenges we face today is attempting to create a mathematical grand “theory of everything” which either unites the laws in the quantum world with that of the macroscopic world, or to work to explain everything solely in terms of quantum mechanics. This is no easy task, but we are striving forward nonetheless.

As you can see, mathematics is more than just a set of vague equations and complex rules that you are required to memorize. Mathematics is the language of the universe, and in learning this language, you are opening yourself up the core mechanisms by which the cosmos operates. It is the same as traveling to a new land, and slowly picking up on the native language so that you may begin to learn from them. This mathematical endeavor is what allows us, a species bound to our solar system, to explore the depths of the universe. As of now, there simply is no way for us to travel to the center of our galaxy and observe the supermassive black hole there to visually confirm its existence. There is no way for us to venture out into a Dark Nebula and watch in real time a star being born. Yet, through mathematics, we are able to understand how these things exist and work. When you set about to learn math, you are not only expanding your mind, but you are connecting with the universe on a fundamental level. You can, from your desk, explore the awesome physics at the event horizon of a black hole, or bear witness to the destructive fury behind a supernova. All of those things that I mentioned at the beginning of this article come into focus through mathematics. The grand story of the universe is written in mathematics, and our ability to translate those numbers into the events that we all love to learn about is nothing short of amazing. So remember, when you are presented with the opportunity to learn math, accept every bit of it because math connects us to the stars.

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18 Replies to “Mathematics: The Beautiful Language of the Universe”

I certainly agree that mathematics can be used to accurately describe many observations. However, there has been a colossal misuse of math today. It has been used to create reality instead of describe it. The best examples are the mathematical creation of dark matter and black holes. Neither of these concepts has ever been observed and they defy confirmation. They are merely mathematical constructs. What brought about the need for these constructs? The fact that astrophysicists and cosmologists have ruled that gravity must be the driving force for galaxy formation. When it was determined that gravity had insufficent forces to cause galaxy formation and maintain galactic structure the need for other forces (unseen and unknown, of course) was apparent. And oila, we have dark matter and black holes. Even with these add-ons intended to salvage gravity-based theories there are still inadequate forces. The use of math to salvage failed theories has led the field of cosmology down a dead end road. Only now with the newest technology and advances in radio telescopes do we see the real forces behind star and galaxy formation. Massive magnetic fields are now detected surrounding “black holes” which are now more correctly being referred to as galactic centers. It is becoming more likely that these centers are super dense plasmoids. There are galactic sized magnetic fields surrounding galaxies as well. Intergalactic electric currents (“rivers of hydrogen”) connecting stars and galaxies have been identified and it is likely that these currents are associated with equally vast magnetic fields. At Los Alamos National Lab, Anthony Pratt used plasma, magnetic fields and electric current to demonstrate spiral galaxy formation in his plasma physics lab. AND these lab induced “spiral galaxies” had the necessary velocities to maintain their shape. Black holes and dark matter were not required. These observations could be and were mathematically described. It seems that we may be leaving the age of Newton and entering the age of Faraday and Tesla. Progress can now be made in cosmology towards rational explanations of observations without the need for imaginary mathematical constructs. The use of the failed gravity based theories to explain the newest observations has resulted in “mysteries” and contradictions. When observed with consideration given to electromagnetic principles the mysteries disappear. I think the presentation by Donald Scott to NASA at the Goddard Colloquia on Engineering in 2009 is a good starting point for one to review the contributions that can be made to the field of cosmology by electrical engineers and plasma physicists. There are many that he references in this presentation that have done ground breaking work in the fields of electromagnetism and plasma physics that can be directly applied to the latest observations from radio telescopes. This work can clearly be used to explain “confounding mysteries” that exist in the field of cosmology. Without knowledge of a “Z pinch”, astrophysicists are left guessing about routine electrical events. Math can then be used to confirm these observations and electromagnetic based explanations. Science fiction can then be left to the novelists.

No. It’s not.

Nope, the electric universe still does not exist.

I recommend public lectures at Canadian Institute for Theoretical Astrophysics which are available on youtube here: http://www.youtube.com/user/citaseminars They have more than a dozen talks about astronomical magnetic fields. I think it has been a neglected topic because it has been hard to observe. But nowadays it is possible to map large scale magnetic field lines by observing the polarization of photons (something like that). But I never heard anyone of them talk about how that could somehow replace gravity or dark matter or affecting the rotation of galaxies. There is indeed an “electric universe” of sorts, but it does not conflict with established physics. Dark matter is carefully mapped. Black holes are observed.

Me and… me and my very, very small but tight-knit internet community knows more… more about outer space stuff than all the outer space science guys out there and… and… and I’m just a-gonna keep… keep a-coming back here and letting you all know over and over and over and over again because it shows the whole wide WORLD how much more smarter I am than… than all of you guys put toGETHer! ….Ya…! You’ll see…. You’ll ALL see someday. You WATCH!

Typical misconception. Dark matter was not just conjured up by some mathematician one day as a whim. It was postulated as a way of explaining observations, and only after all other hypotheses failed for one reason after another. That includes all weird appeals to magnetic or electric fields.

Scientists do not use mathematics to “create reality”, but to describe what they see.

I appreciate the awesome article. Thank you for your military also.

I second that.

Very clear and informing article. Congratulations to its author. But I cannot agree with readers comments based on the “Electric Universe” theory. That theory has no serious base and is not supported by correct math, not even by a thorough understanding of electromagnetic theory. Not to mention that the existence of singularities has nothing to do with the gravitational stability of galaxies.

Oh, they don’t care about being wrong. They just feel that they need something for themselves to believe in.

“Mathematics is the language of the universe”

This is actually not true.

The universe has a completely different language that mathematics. But mathematics is generic enough to be uses to describe and predict simplified models (e.g. 4 forces) of this universe. Mathematics can also be used to describe pure fictional worlds like used in gaming.

It is up to experiments to determine what mathematical formulas really exists in this universe and which are fictional.

But math is just logic. Physical reality is logical. Math looks generic because math in and of itself uses numbers and anonymous variables like “x”, precisely for the purpose of allowing maximum degrees of freedom. But once you use measured physical entities and use them mathematically, then math reveals what is true about the physical reality beyond what was measured. Even if you logically use physically impossible symbols, like negative numbers, during the calculation.

Great Article! And no buts about it.

This is an excellent article!

Galileo wrote, “La mathematca è l’ alfabeto nel quale DIO ha scritto l’ universo.” “Mathematics is the alphabet in which God wrote the universe. The alphabet, not the language.

Some scientists marvel at how mathematical the universe seems to be. But the nature of the universe does not necessarily flow from the mathematics. Mathematics is very flexible and the mathematics is chosen to fit the observations. Once a good fit is found it is then tested to see if it makes valid predictions. We must not assume the underlying nature of the universe is mathematical. Mathematics is just very powerful in representing it. The randomness of some quantum mechanical events may be the point where mathematics fails to mirror the real nature of the universe.

Maybe math might not be the problem but the user?

Reality is Reality twist it any way you want if it makes you happy but that does Not make it Real 🙂 Amen

Nice article, but once again as previously stated, without physically observing something in its true mult dimension universe we live in,the mathematical principles used to describe our cosmos are at the reins of the observer, thus allowing personal beliefs and persuasion. Truth be told that just because one mathematical model fits one answer doesnt mean that theres only one answer but rather there can still be multi answers of movement for that particular mathematical model. For example how and why do we teach and continue to believe facts that mathematical models used to describe movements in our galaxy and solar system in a 2 dimensional platform when we know we live in a multi dimension universe and galaxy. Just cause those mathematical principles fit those porpotions doesnt exclude milti answers in fact its much easier to fit a complex mathematical principle in a multi dimension into a simple 2 dimensional model than it is to do the opposite, which brings us to the reality that jyst like history, change is a constant and to solely rely on one apparatus like math to hold as absolute truth is just as ignorant as believing that the world is flat just cause its the simplest answer. We need to stop believing that theres only one way. Our current knowledge of the cosmos is still very immature and new discoveries everday are changing past theories that mathematical principles used worked but now have to be changed. As long as people want be self rightous and ego centric and so fixated on past theories thought to be true and excepted and take offense to new ways, then we are just Like the Roman Church that drove us into the dark ages and blocked our current knowledge from existed. Please people who consist of our scientific astronomy fans, stop repeating the mistakes of the past and stop being so fixated on your own views as right and CHANGE your mind to be accepting to change because thats the only way truths will be unveiled and we can progressive forward. I think its sad that a website named universetoday can be so self rightous on their views and be willingly to accept only mathematical principles and models as absolute truths kniwing that history as always proven this wrong.

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Home — Essay Samples — Science — Universe — The Beginning of the Universe

The Beginning of The Universe

• Categories: Creation Myth Universe

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Words: 1323 |

Published: Nov 16, 2018

Words: 1323 | Pages: 3 | 7 min read

Works Cited

• Greene, B. (2004). The Fabric of the Cosmos: Space, Time, and the Texture of Reality. Knopf.
• Guth, A. H. (1997). The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Perseus Books.
• Hawking, S. (1988). A Brief History of Time: From the Big Bang to Black Holes. Bantam Books.
• Krauss, L. M. (2012). A Universe from Nothing: Why There Is Something Rather Than Nothing. Free Press.
• Lemaître, G. (1931). The Primeval Atom Hypothesis and the Problem of Clusters of Galaxies. Monthly Notices of the Royal Astronomical Society, 91(5), 483-490.
• Linde, A. (1990). Particle Physics and Inflationary Cosmology. Contemporary Concepts in Physics, 5, 295-339.
• Peebles, P. J. E. (1993). Principles of Physical Cosmology. Princeton University Press.
• Penrose, R. (2004). The Road to Reality: A Complete Guide to the Laws of the Universe. Vintage Books.
• Rees, M. J. (2000). Just Six Numbers: The Deep Forces That Shape the Universe. Basic Books.
• Weinberg, S. (1972). Gravitation and Cosmology: Principles and Applications of the General Theory of Relativity. John Wiley & Sons.

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Solar System Essay for Students and Children

500+ words essay on solar system.

Our solar system consists of eight planets that revolve around the Sun, which is central to our solar system . These planets have broadly been classified into two categories that are inner planets and outer planets. Mercury, Venus, Earth, and Mars are called inner planets. The inner planets are closer to the Sun and they are smaller in size as compared to the outer planets. These are also referred to as the Terrestrial planets. And the other four Jupiter, Saturn, Uranus, and Neptune are termed as the outer planets. These four are massive in size and are often referred to as Giant planets.

The smallest planet in our solar system is Mercury, which is also closest to the Sun. The geological features of Mercury consist of lobed ridges and impact craters. Being closest to the Sun the Mercury’s temperature sores extremely high during the day time. Mercury can go as high as 450 degree Celsius but surprisingly the nights here are freezing cold. Mercury has a diameter of 4,878 km and Mercury does not have any natural satellite like Earth.

Venus is also said to be the hottest planet of our solar system. It has a toxic atmosphere that always traps heat. Venus is also the brightest planet and it is visible to the naked eye. Venus has a thick silicate layer around an iron core which is also similar to that of Earth. Astronomers have seen traces of internal geological activity on Venus planet. Venus has a diameter of 12,104 km and it is just like Mars. Venus also does not have any natural satellite like Earth.

Earth is the largest inner planet. It is covered two-third with water. Earth is the only planet in our solar system where life is possible. Earth’s atmosphere which is rich in nitrogen and oxygen makes it fit for the survival of various species of flora and fauna. However human activities are negatively impacting its atmosphere. Earth has a diameter of 12,760 km and Earth has one natural satellite that is the moon.

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Mars is the fourth planet from the Sun and it is often referred to as the Red Planet. This planet has a reddish appeal because of the iron oxide present on this planet. Mars planet is a cold planet and it has geological features similar to that of Earth. This is the only reason why it has captured the interest of astronomers like no other planet. This planet has traces of frozen ice caps and it has been found on the planet. Mars has a diameter of 6,787 km and it has two natural satellites.

It is the largest planet in our solar system. Jupiter has a strong magnetic field . Jupiter largely consists of helium and hydrogen. It has a Great Red Spot and cloud bands. The giant storm is believed to have raged here for hundreds of years. Jupiter has a diameter of 139,822 km and it has as many as 79 natural satellites which are much more than of Earth and Mars.

Saturn is the sixth planet from the Sun. It is also known for its ring system and these rings are made of tiny particles of ice and rock. Saturn’s atmosphere is quite like that of Jupiter because it is also largely composed of hydrogen and helium. Saturn has a diameter of 120,500 km and It has 62 natural satellites that are mainly composed of ice. As compare with Jupiter it has less satellite.

Uranus is the seventh planet from the Sun. It is the lightest of all the giant and outer planets. Presence of Methane in the atmosphere this Uranus planet has a blue tint. Uranus core is colder than the other giant planets and the planet orbits on its side. Uranus has a diameter of 51,120 km and it has 27 natural satellites.

Neptune is the last planet in our solar system. It is also the coldest of all the planets. Neptune is around the same size as the Uranus. And it is much more massive and dense. Neptune’s atmosphere is composed of helium, hydrogen, methane, and ammonia and it experiences extremely strong winds. It is the only planet in our solar system which is found by mathematical prediction. Neptune has a diameter of 49,530 km and it has 14 natural satellites which are more than of Earth and Mars.

Scientists and astronomers have been studying our solar system for centuries and then after they will findings are quite interesting. Various planets that form a part of our solar system have their own unique geological features and all are different from each other in several ways.

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