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How Was the Solar System Formed? – The Nebular Hypothesis

Since time immemorial, humans have been searching for the answer of how the Universe came to be. However, it has only been within the past few centuries, with the Scientific Revolution, that the predominant theories have been empirical in nature. It was during this time, from the 16th to 18th centuries, that astronomers and physicists began to formulate evidence-based explanations of how our Sun, the planets, and the Universe began.

When it comes to the formation of our Solar System, the most widely accepted view is known as the Nebular Hypothesis . In essence, this theory states that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago. Originally proposed to explain the origin of the Solar System, this theory has gone on to become a widely accepted view of how all star systems came to be.

Nebular Hypothesis:

According to this theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.

From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc .

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury , Venus , Earth , and Mars . Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets ( Jupiter , Saturn , Uranus , and Neptune ) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line ). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid Belt , Kuiper Belt , and Oort Cloud .

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved. At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

History of the Nebular Hypothesis:

The idea that the Solar System originated from a nebula was first proposed in 1734 by Swedish scientist and theologian Emanual Swedenborg. Immanuel Kant, who was familiar with Swedenborg’s work, developed the theory further and published it in his Universal Natural History and Theory of the Heavens  (1755). In this treatise, he argued that gaseous clouds (nebulae) slowly rotate, gradually collapsing and flattening due to gravity and forming stars and planets.

A similar but smaller and more detailed model was proposed by Pierre-Simon Laplace in his treatise Exposition du system du monde (Exposition of the system of the world), which he released in 1796. Laplace theorized that the Sun originally had an extended hot atmosphere throughout the Solar System, and that this “protostar cloud” cooled and contracted. As the cloud spun more rapidly, it threw off material that eventually condensed to form the planets.

The Laplacian nebular model was widely accepted during the 19th century, but it had some rather pronounced difficulties. The main issue was angular momentum distribution between the Sun and planets, which the nebular model could not explain. In addition, Scottish scientist James Clerk Maxwell (1831 – 1879) asserted that different rotational velocities between the inner and outer parts of a ring could not allow for condensation of material.

It was also rejected by astronomer Sir David Brewster (1781 – 1868), who stated that:

“those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process… [Under such a view] the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere.”

By the early 20th century, the Laplacian model had fallen out of favor, prompting scientists to seek out new theories. However, it was not until the 1970s that the modern and most widely accepted variant of the nebular hypothesis – the solar nebular disk model (SNDM) – emerged. Credit for this goes to Soviet astronomer Victor Safronov and his book Evolution of the protoplanetary cloud and formation of the Earth and the planets (1972) . In this book, almost all major problems of the planetary formation process were formulated and many were solved.

For example, the SNDM model has been successful in explaining the appearance of accretion discs around young stellar objects. Various simulations have also demonstrated that the accretion of material in these discs leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.

While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe, and has been used to explain the formation of many of the exoplanets that have been discovered throughout our galaxy.

Although the nebular theory is widely accepted, there are still problems with it that astronomers have not been able to resolve. For example, there is the problem of tilted axes. According to the nebular theory, all planets around a star should be tilted the same way relative to the ecliptic. But as we have learned, the inner planets and outer planets have radically different axial tilts.

Whereas the inner planets range from almost 0 degree tilt, others (like Earth and Mars) are tilted significantly (23.4° and 25°, respectively), outer planets have tilts that range from Jupiter’s minor tilt of 3.13°, to Saturn and Neptune’s more pronounced tilts (26.73° and 28.32°), to Uranus’ extreme tilt of 97.77°, in which its poles are consistently facing towards the Sun.

Also, the study of extrasolar planets have allowed scientists to notice irregularities that cast doubt on the nebular hypothesis. Some of these irregularities have to do with the existence of “hot Jupiters” that orbit closely to their stars with periods of just a few days. Astronomers have adjusted the nebular hypothesis to account for some of these problems, but have yet to address all outlying questions.

Alas, it seems that it questions that have to do with origins that are the toughest to answer. Just when we think we have a satisfactory explanation, there remain those troublesome issues it just can’t account for. However, between our current models of star and planet formation, and the birth of our Universe, we have come a long way. As we learn more about neighboring star systems and explore more of the cosmos, our models are likely to mature further.

We have written many articles about the Solar System here at Universe Today. Here’s The Solar System , Did our Solar System Start with a Little Bang? , and What was Here Before the Solar System?

For more information, be sure to check out the origin of the Solar System and how the Sun and planets formed .

Astronomy Cast also has an episode on the subject – Episode 12: Where do Baby Stars Come From?

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5 Replies to “How Was the Solar System Formed? – The Nebular Hypothesis”

So… the transition from the geocentric view and eternal state the way things are evolved with appreciation of dinosaurs and plate tectonics too… and then refining the nebular idea… the Nice model… the Grand Tack model… alittle more? Now maybe the Grand Tack with the assumption of mantle breaking impacts in the early days – those first 10 millions years were heady times!

And the whole idea of “solar siblings” has been busy the last few years…

Nice overview, and I learned a lot. However, there are some salient points that I think I have picked up earlier:

“something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.”

The study of star forming molecular clouds shows that same early, large stars form that way. In the most elaborate model which makes Earth isotope measurements easiest to predict, by free coupling the processes, the 1st generation of super massive stars would go supernova in 1-10 million years.

That blows a 1st geeration of large bubbles with massive, compressed shells that are seeded with supernova elements, as we see Earth started out with. The shells would lead to a more frequent 2nd generation of massive stars with a lifetime of 10-100 million years or so. These stars have powerful solar winds.

That blows a 2nd generation of large bubbles with massive, compressed shells, The shells would lead to a 3d generation of ~ 500 – 1000 stars of Sun size or less. In the case of the Sun the resulting mass was not enough to lead to a closed star cluster as we can see circling the Milky Way, but an open star cluster where the stars would mix with other stars over the ~ 20 orbits we have done around the MW.

“The ices that formed these planets were more plentiful”.

The astronomy course I attended looked at the core collapse model of large planets. (ASs well as the direct collapse scenario.) The core grew large rapidly and triggered gas collapse onto the planet from the disk, a large factor being the stickiness of ices at the grain stage. The terrestrial planets grow by slower accretion, and the material may have started to be cleared from the disk. by star infall or radiation pressure flow outwards, before they are finished.

An interesting problem for terrestrial planets is the “meter size problem” (IIRC the name). It was considered hard to grow grains above a cm, and when they grow they rapidly brake and fall onto the star.

Now scientists have come up with grain collapse scenarios, where grains start to follow each other for reasons of gravity and viscous properties of the disk, I think. All sorts of bodies up to protoplanets can be grown quickly and, when over the problematic size, will start to clear the disk rather than being braked by it.

“But as we have learned, the inner planets and outer planets have radically different axial tilts.”

Jupiter can be considered a clue, too massive to tilt by outside forces. The general explanation tend to be the accretion process, where the tilt would be randomized. (Venus may be an exception, since some claim it is becoming tidally locked to the Sun – Mercury is instead locked in a 3:2 resonance – and it is in fact now retrograde with a putative near axis lock.) Possible Mercury bit at least Earth and Mars (and Moon) show late great impacts.

A recent paper show that terrestrial planets would suffer impacts on the great impact scale, between 1 to 8 as norm with an average of 3. These would not be able to clear out an Earth mass atmosphere or ocean, so if Earth suffered one such impact after having volatiles delivered by late accretion/early bombardment, the Moon could result.

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The Origin and Evolution of the Earth Class 11 Notes Geography

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11 Geography notes Chapter 2 The Origin and Evolution of the Earth

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This chapter deals with 1. Origin of the earth 2. Early theories 3. Modern Theories 4. Big Bang theory 5. The star formation 6. formation of planets 7. Our solar system 8. The moon 9. Evolution of the earth 10. Development of lithosphere 11. Evolution of Atmosphere and hydrosphere 12. Origin of life

ANGULAR MOMENTUM

It is unlikely that such a nebula would be created with no angular momentum, so it is probably initially spinning slowly. Because of conservation of angular momentum, the cloud spins faster as it contracts. The Spinning Nebula Flattens Because of the competing forces associated with gravity, gas pressure, and rotation, the contracting nebula begins to flatten into a spinning pancake shape

MODERN THEORIES

Origin of the universe The Big Bang Theory, also called as expand universe hypothesis. Edwin Hubble in 1920 provided the evidence that the universe is expanding. The galaxies move farther as the time passes.

Laboratory The Expanding Balloon

In The Expanding Universe, Charles Jenkins tells us that galaxies are moving away from each other. The universe appears to be growing larger. We can use a balloon to help us understand what is going on in the cosmos. In this activity, the balloon represents the universe. Bits of tape on the surface of the balloon represents some of the galaxies located throughout the universe.

Create your own expanding universe with these simple materials Tools & Materials

The Experiment

Observe how the distances between the galaxies changed as the balloon universe expanded. Do you see any pattern or trend?

Stages of Big Bang theory

(i) In the beginning, all matter was in the form of tiny ball(singular atom) with unimaginable small volume, infinite temperature and infinite density (ii) About 13.7 billion years ago the tiny ball exploded violently. The expansion continues even today. (iii) As a result, some energy was converted into matter (iv) Within fraction of second there was rapid expansion (v) The expansion slowdown after three minutes and first atom formed (vi) After 300000 years the temperature dropped down to 4,500 K and gave rise to atomic matter. (vii) The universe became transparent.

Steady State Theory

The star formation

Formation of planets -Stages in the development of planets

1. Our solar system consists of eight planets 2. The nebula of our solar system started collapsing around 5-5.6 b. y. a 3. The planets were formed about 4.6 b. y .a 4. Our solar system consists of 8 planets, 63 moons, millions of asteroids comets, huge quantity of gas and dust. 5. There are two types of planets inner planets and outer planets.

THE DIFFERENCE BETWEEN INNER PLANETS AND OUTER PLANETS

THE MOON: THE EARLIER EXPLANATION

A long time ago in a planet system close to home. A giant impact made the Moon The Moon is mantle material from the Earth and impactor. Earth today is mantle+core from early Earth + impactor

EVOLUTION OF THE EARTH

EVOLUTION OF LITHOSPHERE

1. There was volatile state during its primordial stage 2. Due to high-density temperature increased 3. The material started separating depending on their density Light material came outside and heavy material went inside the earth 4. It cooled and condensed into solid which is called lithosphere 5. At the time of formation of the moon, the earth again became hot 6. Due to differentiation different layers formed

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• Class 11 Geography Notes Chapter 2 The Origin and Evolution Of The Earth

As students of geography, it’s important to understand class 11 geography notes chapter 2 the origin and evolution of the Earth for CBSE Board . In this chapter, we will discuss the different theories about the origin of the Earth, the evolution of the Earth’s surface and interior, and the various geological processes that have shaped the Earth over time. (The origin and evolution of the earth notes)

Starry nights have always attracted us since childhood. You may also have thought of these stars and had numerous questions in your mind. Questions such as how many stars are there in the sky? How did they come into existence? Can one reach the end of the sky? Maybe many more such questions are still there in your mind. In this chapter, you will learn how these “twinkling little stars” were formed. With that, you will eventually also read the story of the origin and evolution of the earth.

The Origin and Evolution Of The Earth

There were many hypotheses put forth by philosophers and scientists regarding the origin of the earth.

Nebular Hypothesis (Early Hypothesis)

One of the earlier and more popular arguments was by German philosopher Immanuel Kant which was revised by mathematician Laplace in 1796 . It is known as Nebular Hypothesis. According to this hypothesis, the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.

Binary Theories

In 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped extension of material was separated from the solar surface. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets. Later on, the arguments considered a companion to the sun to have coexisted. These arguments are called binary theories.

Big Bang Theory (Modern Hypothesis)

The most popular argument regarding the origin of the universe is the Big Bang Theory. It is also called expanding universe hypothesis. The Big Bang Theory considers the following stages in the development of the universe.

• In the beginning, all matter forming the universe existed in one place in the form of a“tiny ball” (singular atom) with an unimaginably small volume, infinite temperature, and infinite density.
• At the Big Bang, the “tiny ball” exploded violently. This led to a huge expansion. It is now generally accepted that the event of the big bang took place 13.7 billion years before the present.
• Within 300,000 years from the Big Bang, the temperature dropped to 4,500 K (Kelvin) and gave rise to atomic matter. The universe became transparent

Formation Of Stars

The distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for the development of galaxies. A galaxy contains a large number of stars. Galaxies spread over vast distances that are measured in thousands of light-years. The diameters of individual galaxies range from 80,000-150,000 light years. A galaxy starts to form through the accumulation of hydrogen gas in the form of a very large cloud called a nebula.

Eventually, the growing nebula develops localized clumps of gas. These clumps continue to grow into even denser gaseous bodies, giving rise to the formation of stars. The formation of stars is believed to have taken place some 5-6 billion years ago.

What is a Light Year?

A light year is a measure of distance. The distance traveled by light in a span of 1 year is called 1 light year. 

1 Light year = 9.461×10 12 km

The mean distance between the sun and the earth is 149,598,000 km. In terms of light years, it is 8.311 minutes.

Formation Of Planets

• The stars are localized lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas core.
• In the next stage, the gas cloud starts getting condensed and the matter around the core develops into small rounded objects. These small-rounded objects by the process of cohesion develop into what are called planetesimals. Larger bodies start forming by collision, and gravitational attraction causes the material to stick together. Planetesimals are a large number of smaller bodies.
• In the final stage, these large numbers of small planetesimals accrete to form fewer large bodies in the form of planets.

Solar System

Our Solar system consists of eight planets. The nebula from which our Solar system is supposed to have been formed started its collapse and core formation sometime 5-5.6 billion years ago and the planets were formed about 4.6 billion years ago. Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets, and huge quantities of dust grains and gases.

class 11 geography notes ch 2 the origin and evolution of earth

The moon is the only natural satellite of the earth. It is now generally believed that the formation of the moon, as a satellite of the earth, is an outcome of a ‘giant impact’ or what is described as “the big splat”. A body the size of one to three times that of mars collided with the earth sometime shortly after the earth was formed. It blasted a large part of the earth into space. This portion of blasted material then continued to orbit the earth and eventually formed the present moon about 4.44 billion years ago.

Evolution Of Earth

Do you know that the planet earth initially was a barren, rocky, and hot object with a thin atmosphere of hydrogen and helium? This is far from the present-day picture of the earth. Hence, there must have been some events– processes, which may have caused this change from a rocky, barren, and hot earth to a beautiful planet with ample amounts of water and a conducive atmosphere favoring the existence of life. In the following section, you will find out how the period, between 4,600 million years and the present, led to the evolution of life on the surface of the planet.

There are three stages in the evolution of the present atmosphere. The first stage is marked by the loss of the primordial atmosphere. In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere. Finally, the composition of the atmosphere was modified by the living world through the process of photosynthesis. Sometime around 3,800 million years ago, life began to evolve. However, around 2,500-3,000 million years before the present, the process of photosynthesis got evolved. Life was confined to the oceans for a long time.

Origin Of Life

The record of life that existed on this planet in different periods is found in rocks in the form of fossils. The microscopic structures closely related to the present form of blue algae have been found in geological formations much older than 3,000 million years. It can be assumed that life began to evolve sometime 3,800 million years ago.

Important Terms

• Planetesimals: The gas cloud starts getting condensed and the matter around the core develops into small-rounded objects. These small-rounded objects by the process of cohesion develop into what are called planetesimals.
• Universe: All matter, energy, heavenly bodies, and all that is there in space is as a group called the universe.
• Galaxy: A galaxy is a cluster of millions of stars and solar systems.
• Outer Planets: Jupiter Saturn Uranus Neptune and Pluto are called Outer Planets.
• Inner Planets: Mercury, Venus, Earth, and Mars are called Inner Planets.
• Nebula: Slow circular moving gaseous clouds are called nebula.
• The big splat: The origin of the moon as a satellite of the earth is the result of a big collision which is called “the big splat”
• Differentiation: The process through which the earth-forming material got separated into different layers is called differentiation.
• Dwarf Planet: According to International Astronomical Union (IAU) on August 24, 2006, a planet is a celestial body that:- 1. Orbits around the sun. 2. Has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape. The non-satellites bodies fulfilling these two rules are called dwarf planets. Pluto is now considered a dwarf planet. Ceres, Eris, Makemake, and Haumea are some other dwarf planets.
• Jovian: Jovian means Jupiter-like.

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Q: What is the Nebular Hypothesis?

The Nebular Hypothesis is a theory that explains the formation of the Solar System, including the Earth, as a result of a cloud of gas and dust that collapsed under its own gravity.

Q: What is Plate Tectonics Theory?

Plate Tectonics Theory is the scientific theory that describes the movement of the Earth’s lithosphere, which is divided into a number of plates that move and interact with each other at their boundaries.

Q: How are mountains formed?

Mountains are formed by a variety of geological processes, including volcanic activity, tectonic plate movement, and erosion.

Q: What is the significance of the Paleozoic Era?

The Paleozoic Era is significant because it was a time of great biodiversity and the emergence of many of the major groups of animals that still exist today.

Q: How can we protect the Earth for future generations?

We can protect the Earth for future generations by reducing our carbon footprint, conserving natural resources, and promoting sustainable practices in agriculture, industry, and everyday life.

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The formation of the Earth is a long-standing mystery that has captivated scientists for centuries. Modern theories propose that our planet was born from a vast cloud of gas and dust, known as the solar nebula, which also gave rise to the Sun and other planets in our solar system. These theories provide insights into the processes that shaped the Earth’s early history and continue to influence our understanding of planetary formation.

Let us discuss the origin of Earth from modern theories’ perspective:

Modern Theories

Important modern theories related to the origin of the earth are as follows:

Origin of the Universe

The prevailing explanation for the origin of the universe is the Big Bang Theory, also known as the expanding universe hypothesis. This theory, supported by Edwin Hubble’s observations in 1920, suggests that the universe is expanding, with galaxies moving farther apart as time progresses. To visualize this concept, one can use a balloon analogy: marking points on the balloon to represent galaxies and inflating the balloon to simulate the expansion of the universe. However, while the distances between the points (galaxies) increase, the points themselves do not expand, contrary to what the balloon analogy suggests.

According to the Big Bang Theory:

• Initially, all matter in the universe existed in a singular atom, resembling a tiny ball with infinite temperature and density.
• The “tiny ball” exploded at the Big Bang, resulting in a massive expansion. This event is estimated to have occurred approximately 13.7 billion years ago, and the expansion continues to the present day. During the initial rapid expansion, some energy was converted into matter, leading to the formation of atoms within the first few minutes after the Big Bang.
• Within about 300,000 years after the Big Bang, the universe cooled down, allowing atomic matter to form, and the universe became transparent.

The expansion of the universe implies an increase in the space between galaxies. While an alternative concept proposed by Hoyle suggested a steady-state universe, accumulating evidence supports the idea of an expanding universe, which is currently favored by the scientific community.

The Star Formation

In the early universe, matter and energy were distributed unevenly, leading to variations in gravitational forces. These differences caused matter to be drawn together, laying the groundwork for the formation of galaxies. Galaxies, each containing numerous stars, span vast distances measured in thousands of light-years. Individual galaxies typically have diameters ranging from 80,000 to 150,000 light-years.

Galaxies begin to form when hydrogen gas accumulates in the form of large clouds called nebulae. Over time, these nebulae develop localized clumps of gas that gradually grow denser, eventually leading to the formation of stars. This process of star formation is estimated to have occurred around 5-6 billion years ago.

A light year is a unit of distance, not time, representing the distance light travels in one year at a speed of 300,000 kilometers per second. For example, the mean distance between the sun and the earth is approximately 149,598,000 kilometers, which translates to approximately 8.311 minutes in terms of light-years.

Formation of Planets

The development of planets is believed to occur in several stages:

(i) Initially, stars form as localized lumps of gas within a nebula. Gravitational forces within these lumps lead to the formation of a core and a rotating disc of gas and dust surrounding it.

(ii) As the gas cloud condenses, matter around the core coalesces into small-rounded objects known as planetesimals. These planetesimals gradually grow larger through collisions and gravitational attraction, eventually forming larger bodies.

(iii) In the final stage, numerous planetesimals merge to form fewer, larger bodies, ultimately resulting in the formation of planets.

Modern Theories Regarding the Origin of the Earth

The disk instability theory.

According to this theory, when our Sun was forming from a huge cloud of gas and dust called the solar nebula, there was a disk of material spinning around it. This disk was very dense and massive. Over time, some regions in the disk became denser than others due to the gravitational pull of the material itself.

These dense clumps or rings continued to attract more and more material from the surrounding disk, growing larger and larger. Eventually, these clumps became so massive that they collapsed under their own gravitational force, forming hot, dense cores. These cores acted as seeds for the formation of planets.

As the cores grew bigger by sweeping up the remaining gas and dust from the disk, they eventually became planetary embryos or protoplanets. These protoplanets collided and merged with each other over millions of years, finally forming the planets we see today, including the Earth. This theory explains how planets like Earth could have formed relatively quickly from the solar nebula.

Big Bang Theory regarding Earth’s Origin

According to the Big Bang Theory, the entire universe began as an incredibly hot, dense singularity about 13.8 billion years ago. This singularity exploded in a massive event called the Big Bang, releasing an enormous amount of energy and matter.

As the universe expanded and cooled, the matter and energy from the Big Bang condensed into clouds of gas and dust. These clouds, under the influence of gravity, began to collapse and form the first stars and galaxies, including the Milky Way galaxy where our solar system resides.

Within these galaxies, the clouds of gas and dust continued to collapse and form new stars, including our Sun. The leftover gas and dust surrounding the Sun eventually formed a rotating disk, known as the solar nebula. It was from this solar nebula that the planets, including the Earth, formed through the process described by the Nebular Hypothesis or the Disk Instability Theory.

Nebular Hypothesis of Earth’s Origin

According to the Nebular Hypothesis, our solar system, including the Earth, formed from a vast, rotating cloud of gas and dust called the solar nebula. This nebula was a leftover remnant from the formation of our Sun.

As the solar nebula rotated, it began to flatten into a disk-like shape due to the rotation. The central region of the disk became denser and hotter, eventually forming the Sun. The remaining gas and dust in the disk continued to swirl around the Sun.

Over time, the particles in the disk started to clump together due to gravity. These clumps, called planetesimals, collided and merged with each other, gradually forming larger and larger bodies. These larger bodies eventually became the planets, including the Earth. The Earth formed from the accretion of these planetesimals over millions of years.

Initially, the Earth was a molten ball of hot gases and liquids due to the immense heat generated by the collisions and radioactive decay. As the Earth cooled down, the heavier materials sank to the center, forming the core, while the lighter materials rose to the surface, forming the crust and atmosphere.

The Nebular Hypothesis provides a compelling explanation for how the Earth and other planets formed from the same rotating cloud of gas and dust that gave rise to the Sun, through a process of accretion and condensation of matter over a long period.

While the details may vary among different theories, the overarching principle is that the Earth and other planets formed from the same primordial cloud of gas and dust that gave rise to the Sun. These theories provide a compelling explanation for the Earth’s formation and continue to be refined as new evidence emerges from ongoing research and space exploration

Origin of the Earth: Modern Theories: FAQs

How long did it take for the earth to form.

The formation of the Earth is believed to have taken millions of years, as the process involved the gradual accumulation and merging of planetesimals and protoplanets over an extended period.

What was the Earth like in its early stages?

Initially, the Earth was a molten ball of hot gases and liquids, formed from the immense heat generated by the collisions and radioactive decay during its formation. As it cooled, the heavier materials sank to form the core, while the lighter materials rose to the surface, forming the crust and atmosphere.

How does the formation of the Earth relate to the formation of other planets?

According to the prevailing theories, the Earth and other planets in our solar system formed from the same solar nebula, following similar processes of accretion and condensation of matter over time.

What evidence supports these theories?

Evidence supporting these theories comes from various sources, including the study of meteorites (which are believed to be remnants of the early solar system), observations of planet-forming disks around other stars, and computer simulations of planetary formation processes.

Are there any alternative theories for the Earth’s formation?

While the Nebular Hypothesis and related theories are widely accepted, there are alternative hypotheses proposed by some scientists, such as the Rare Earth hypothesis or the idea that the Earth formed from the collision of two planetary embryos. However, these alternative theories are less widely supported by the available evidence.

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Earth - Origin & Evolution [Geography Notes For UPSC]

NCERT notes on important topics for the UPSC civil services exam . These notes will also be useful for other competitive exams like banking PO, SSC, state civil services exams and so on. In this article, you can read all about the origin and evolution of our Earth including the various theories of the origin of the Earth for the IAS exam .

Earth – Origin & Evolution (UPSC Notes):- Download PDF Here

Early Theories of Origin of Earth

The major early theories of the earth’s origin are discussed below.

• Nebular Hypothesis – This theory was developed by Immanuel Kant and modified in 1796 by Pierre Laplace. According to this hypothesis, the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.
• In 1900, Chamberlain and Moulton considered that a wandering star approached the sun which resulted in the formation of a cigar-shaped extension of material that got separated from the solar surface. This separated material continued to revolve around the sun and slowly got condensed into planets.
• The binary theories considered a companion to be coexisting with the sun.
• In 1950, the Nebular Hypothesis was revised by Otto Schmidt (in Russia) and Carl Weizascar (in Germany). According to them, the sun was surrounded by a solar nebula consisting mainly of hydrogen and helium along with dust. The friction and collision of particles led to the formation of a disk-shaped cloud and planets were formed through the process of accretion.

Origin of Earth Modern Theories

Big Bang Theory

• The Big Bang Theory explains the origin of the universe. It is also called the expanding universe hypothesis.
• In 1927, Abbe Georges Lemaitre, a Belgian astronomer was the first to provide a theory on the origin of the Universe. It was Edwin Hubble who provided evidence that the universe is expanding.
• According to this theory, all matter that formed the universe existed in one point (tiny ball) called singularity having an unimaginable small volume, infinite temperature and infinite density.
• The great event of the big bang happened some 13.7 billion years ago. The tiny ball exploded which led to a huge expansion and this expansion continues even today. There was rapid expansion within fractions of a second after the bang. Thereafter, the expansion slowed down. With the expansion some of the energy was converted into matter. Within the first three minutes from the big bang event, the first atom began to form.
• Within 300,000 years from the big bang, temperature dropped down to 4500 K and gave rise to atomic matter. The majority of atoms formed were hydrogen, along with helium and traces of lithium. Huge clouds of these elements fused through gravity to form stars and galaxies.
• The steady state theory considered the universe to be roughly  the same at any point of time.
• However, with greater evidence becoming available about the expanding universe, the Big Bang theory was confirmed which proposes that the universe originated from a single violent explosion of a very minute amount (tiny ball) of matter of high density and temperature.

Formation of Stars

Star formation is the process by which dense regions within molecular clouds in interstellar space called star forming regions or stellar nurseries collapse under their own gravitational attraction and form stars. The formation of stars is believed to have taken place some 5-6 billion years ago.

Stages in the Formation of Stars:

• Nebula – It is a cloud of gas (mainly hydrogen and helium) and dust in space. It is a star’s birth place.
• Protostar – It is an early stage of a star formation where nuclear fusion is yet to begin. It looks like a star but its core is not yet hot enough for nuclear fusion to take place.
• T Tauri Star – It represents an intermediate stage between a protostar and a low mass main sequence star like the sun. It is a young, low weight star, less than 10 million years old that is still undergoing gravitational contraction. 
• Main Sequence Star – At this stage, the core temperature is enough to start the fusion reactions i.e, fusing hydrogen atoms to form helium atoms. The sun is the main sequence star.
• Red Giant – A red giant is formed during the later stages of the evolution as the star runs out of hydrogen fuel at its centre. However, it still fuses hydrogen into helium in a shell surrounding a hot, dense degenerate helium core. This fusion of hydrogen into helium around the core releases much greater energy and pushes much harder against gravity and expands the volume of the star.
• Fusion of Heavier Elements – As the star expends, helium molecules fuse at the core which prevents the core from collapsing. When the fusion of helium ends, the core shrinks and begins fusing carbon. This process repeats until iron appears at the core. The iron fusion reaction absorbs energy, which causes the core to collapse. This implosion transforms massive stars into supernovae and smaller stars (sun) into white dwarfs. 
• Supernovae and Planetary Nebulae – Planetary nebula is an outer layer of gas and dust that is lost when the star changes from Red Giant to White Dwarf. This white dwarf becomes black dwarf when it stops emitting light.

Supernova is the explosive death of a bigger star and it obtains the brightness of 100 million suns for a short time. Neutron stars are produced after a supernova (protons and electrons combine to produce neutron stars).

Our Solar System

Our solar system consists of the sun (the star), eight planets, 63 moons, asteroids, comets and huge amounts of dust-grains and gases. The solar system is believed to have been formed about 5 – 5.6 billion years ago and the planets were formed about 4.6 billion years ago. The eight planets namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune revolve around the sun in fixed elliptical orbits.

• Mercury, Venus, Earth and Mars are called the inner planets and also terrestrial planets, which means earth-like as they are made up of rocks and metals. The other four planets – Jupiter, Saturn, Uranus and Neptune are called outer planets as well as jovian (Jupiter-like) or Gas Giant planets. They are mostly larger than terrestrial planets and have a thick atmosphere mainly of helium and hydrogen. 
• Dwarf planets are tiny planets in our solar system. Any celestial body orbiting around the sun, weighing for self gravity and nearly round in shape is called a dwarf planet.
• Mercury – zero
• Venus – zero
• Earth – one
• Mars – two
• Jupiter – about 53
• Saturn – about 53
• Uranus – about 27
• Neptune – 13
• The moon is the only natural satellite of the earth. The word satellite means “companion”. The satellites move around a planet from west to east. They do not have their own light but reflect the  light of the sun. The moon takes 27 days and 7 hours and 43 minutes for both its rotation and revolution around the earth. It is the fifth largest natural satellite of the solar system.
• It is believed that the formation of the moon is a result of a giant impact called ‘big splat’. A large body (somewhat one to three times the size of Mars) collided with the earth just after it was  formed. Due to this heavy impact, a large part of earth got separated. This portion of blasted material continued to revolve around the earth and eventually formed the present moon (4.44 billion years ago).

Read more Geography Notes for UPSC in the link.

Evolution of the Earth

The earth was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium.

• The different layers of the earth starting from the surface are crust, mantle, outer core and inner core. From crust to the core, density increases.
• Atmosphere – The solar wind was most intense nearer the sun; so it blew off lots of gas and dust from the terrestrial planets – Mercury, Venus, Earth and Mars. During the process of cooling of earth, gases and water vapour escaped from the interior of the earth thereby starting the evolution of the present atmosphere. The early atmosphere mainly had water vapour, carbon dioxide, nitrogen, methane, ammonia and small amounts of free oxygen. The process of release of gases from the interior of the earth is called “degassing”.
• Hydrosphere – The frequent volcanic eruptions provided the atmosphere with water vapour and gases. With the cooling of the earth, water vapours condensed and brought rain. The carbon dioxide in the atmosphere got dissolved in rainwater which further lowered the temperature leading to more condensation and more rain. The rain water got collected into depressions to give rise to oceans. Life was limited to oceans only for a long time. Oceans got saturated with oxygen through the process of photosynthesis and then some 2,000 million years ago oxygen began to flood the atmosphere. 

Geological Time Scale

The summary of evolution of life from unicellular bacteria to the modern man is given in the Geological Time Scale below –

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Extra Questions For Class 11 Geography The Origin and Evolution of the Earth 2

Q 1 – What is Nebula?

Ans. The nebula is the name given to the slowly rotating cloud of gas, which later condensed and solidified to give birth to different planets.

Q 2 – Name our galaxy.

Ans. Milky way.

Q 3 – Who first proposed the Nebular hypothesis?

Ans. The German philosopher, Immanuel Kant, proposed the Nebular hypothesis in 1755

Q 4 – How is it known that the moon was born along with the earth?

Ans. Radiometric dating of the rocks from the moon shows that it was born along with the earth.

Q 5 – Name all the nine planets in their order from the sun.

Ans. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

  Q 6 – Name the largest and the smallest planet of our solar system.

Ans. Jupiter is the largest and Mercury is the smallest planet of our solar system.

Q 7 – Name two scholars who had identical views on the origin of planets.

Ans. Immanuel Kant, a German scholar and Laplace, a French mathematician, expressed identical views on Nebular hypothesis.

Q 8 – Name two English scientists who put forward the Collision hypothesis of the origin of planets.

Ans. They were Sir James Jeans and Sir Harold Jeffreys.

Q 9 – Name the planet with a maximum number of satellites in our solar system

Ans. Saturn has the maximum number of satellites (20) in our solar system

Q 10 – Which planets in our solar system have no satellite at all?

Ans. Mercury and Venus have no satellites so far.

Q 11 – What are the various stages the sun has passed through during its formation

Ans. They are Nebula → Supernova → Protostar → Infant Sun → Sun.

Q 12 – Which other planet is known to have just one satellite-like our earth?

Ans. Pluto.

Q 13 – What is a meteorite?

Ans. A meteorite is a piece of rock or metal from outer space that hits the earth’s surface.

Q 14 – A piece of rock or metal from outer space falling on the earth?

Ans. Meteorite

Q 15 – The only planet with conditions favourable for the sustenance of life?

Ans. The earth.

Q 16 – The second-largest planet of our solar system?

Ans,. Saturn.

Q 17 – What is a protostar?

Ans. The infant son was the protostar.

Q 18 – Name two great ice areas of the present age.

Ans. Antarctica and Greenland.

Q 19 – When did drifting start in Pangea?

Ans. In the upper carboniferous age about 300 million years ago.

Q 20 – When was the earth formed?

Ans. The earth came into existence about 4600 million years ago.

Q 21 – How does earth’s surface appear from the space?

Ans. It appears blue from the space.

Q 22 – Why does earth appear as a round ball that shines bright and blue when viewed from space?

Ans. Because a large part of the earth’s surface is covered with water, which is blue in colour.

Q 23 – Why is the study of the origin of earth essential?

Ans. The study of the origin of the earth is essential because we are living on earth.

Q 24 – Why do planets differ in size, constituent matter and temperature?

Ans. They differ in size, matter and temperature because of their varying distances from the sun.

Q 25 – What are planetesimals?

Ans. Giant tongues of matter broke into small chunks or planetesimals, which went flying as cold bodies into orbits around the sun.

Q 26 – What is a protostar?

Ans. Protostar means original star. It is believed that the gaseous cloud exploded to form a supernova. The explosion caused shock waves that caused the denser portion of the cloud to collapse under their own ‘ gravity. The dense core grew larger and hotter and its gravity attracted more material. In this process, the hot core developed into a protostar. ‘ Finally, it became the infant sun.

Q 27 – Explain Heterogenous Accretion Hypothesis.

Ans. According to Heterogeneous Accretion Hypothesis, the composition of the material from which the earth accreted changed r with time giving rise to the layered structure of the earth. According to this model, the earth formed ‘ inside out’ with an, oxidised and volatile-rich nucleus and a more metal-rich and depolarised outer rings.

Q 28 – Differentiate between supernova and protostar.

Q 29 – What do you mean by plate tectonics?

Ans. Plate tectonics is a theory. According to it, the crust and mantle (lithosphere) are divided into plates and blocks. Different tectonic movements take place along their edges. These plates drift due to convection currents. Continents also drift along with these plates, known as tectonic plates.

Q 29 – The moon was born along with the earth. How?

Ans. Regarding the birth of the moon, there are many hypotheses. Radiometric dating of the rocks from the moon shows that it was born along with the earth. It either came out of the sun in a gaseous form but being too small was attracted by the earth, or it flew out of the earth due to a huge meteorite falling on the earth.

Q 30 – Why are inner planets heavier than the outer ones?

Ans. Initially, the nebula was revolving in the form of a flat disc. Due to high temperature, the heavy materials of the flat disc condensed in the inner part of the disc. These include iron and aluminium. Thus, inner planets Mercury, Venus, Earth and Mars were formed.

Q 31 – What is Collision hypothesis?

Ans. This hypothesis was proposed by Sir James Jeans and Sir Harold Jeffreys, a mathematician in England. According to this hypothesis, gaseous material was pulled away from the pre-existing sun by the gravitational force of a passing star.

Q 32 – Name the three layers of the earth’s interior.

• Ans. The Earth’s crust (lithosphere)
• The intermediate layer (mantle)
• The inner core (metallic core)

Q 33 – Why does the inner core of the earth behave like a solid?

Ans. The inner core of the earth should have been molten owing to high temperature in such a great depth, but it is not molten; it behaves like a solid because of the great pressure of the outer shells.

Q 34 – What do you understand by a light-year?

Ans. A light-year is a measure of distance and not of time. Light travels at a speed of 300,000 km/second. The distance that the light travels in one year is taken to be one light year. This is equal to 9.461 xlO12 km. The mean distance between the sun and the earth is Y‘ 149,598,000 km. In terms of light-years, it is 8,3 31 minutes.

Q 35 – How was the solid crust formed?

Ans. The solid crust was formed by the cooling and condensation of the gaseous material on the surface of the earth

Q 36 – What do you understand by the birth of the solar system?

Ans. The entire solar system forms a very small part of the galaxy that consists of many stars. Our galaxy is called the Milky Way. The gaseous cloud exploded to form a supernova. The explosion caused shock waves that caused the denser portion to collapse under its own gravity. In the process, the hot core developed into a protostar. Finally, the protostar became the infant sun.

Q 37 – What are the outer and inner planets?

Ans. The four planets Mercury, Venus, Earth and Mars are called inner planets. They lie between the sun and the belt of asteroids

The five planets Jupiter, Saturn, Uranus, Neptune and Pluto are called outer planets. These are also known as Jovian or Gas Giant planets.

Q 38 – Write a note on Heterogenous Accretion Hypothesis.

Ans. This hypothesis explains the layered structure of the earth. According to this theory, the earth is formed by the accretion of different ‘ types of materials. The materials changed with time. The earth formed from inside out due to the change of materials. It had an oxidised and hot, nucleus and more metal-rich rings.

Q 39 – Write short notes on : 1. Collision Hypothesis 2. Homogeneous Accretion Hypothesis 3. Origin of the Moon

Ans. 1. Collision Hypothesis – Sir James Jeans and Sir Harold Jeffreys gave this hypothesis. According to this hypothesis, gaseous material was pulled away from the pre-existing sun by the gravitational force of a passing star. By collision and gravitational attraction, the larger planetesimals swept up the smaller pieces and thus were formed the planets.

2. Homogenous Accretion Hypothesis – This hypothesis maintains that the earth accreted from an intermate mixture of silicate particles and metal particles. The material was assumed to have been formed in the solar nebula by a complex series of chemical and physical processes, which had occurred prior to the accretion of planets. According to a hypothesis, accretion of the earth occurred over a sufficiently long period (10—10s years) so that its gravitational potential energy was efficiently radiated away and it formed in an initially ‘cool’ and unmelted condition.

3. Origin of the moon – Radiometric dating of the rocks from the moon shows that it was born along with the earth. Apparently, there are two possibilities. It either came out of the sun in a gaseous form but being too small was attracted by the earth, or it flew out of the earth due to a huge meteorite falling on the earth. The area where the meteorite fell, a huge hollow was created, which is now filled up by an ocean and the landmass plunged to outer space to create the moon.

Q 40 – Discuss the various hypotheses for the evolution of the planets.

Ans. There are two hypotheses :

• Nebula hypothesis
• Collision hypothesis

1. Nebula hypothesis – In 1755, German philosopher Immanuel Kant, hypothesised that slowly rotating cloud of gas, called Nebula, in some unspecified manner, condensed into a number of discrete and globular bodies. The great French mathematician Laplace also proposed, more or less, the same theory in 1796.

According to Kant and Laplace, the original mass of gas-cooled and began to contract. The rotational speed increased as a consequence of the law of conservation of angular momentum. Then, successive rings of gaseous material were spun off from the central mass by centrifugal force. In the final stage, the rings condensed into planets.

  Q 41 – Collision hypothesis – Sir James Jeans and Sir Harold Jeffreys gave this hypothesis. According to this hypothesis, gaseous material was pulled away from the pre-existing sun by the gravitational force of a passing star. By collision and gravitational attraction, the larger planetesimals swept up the smaller pieces and thus were formed the planets.

Describe the evolution of the Solar system,

Ans. The entire solar system consists of the sun, the nine planets and their various satellites. The sun is a large brilliant star in the centre of the solar system. It is thought to be about 5 billion years old. The pressure and temperature at the centre of the Nebula (cloud of gas) that produced the solar system became so great that it triggered a nuclear reaction. Some of the hydrogen in the cloud fused into helium, releasing a great amount of energy.

The gaseous cloud exploded to form a supernova. The explosion caused shock waves that pushed the denser portion of the cloud to collapse under its own gravity. The dense core grew larger and hotter as its gravity attracted more material. In the process, the hot core developed into a protostar, that finally became the sun.

Describe the origin of life and die cause for its origin,

Ans. Modern scientists refer to the origin of life as a kind of chemical reaction, which first generated complex organic molecules and then assembled them. This assemblage was such that they could duplicate themselves converting inanimate matter into living substance. The record of life that existed on this planet in different periods is found in rocks in the form of fossils. The microscopic structures closely related to the present form of blue algae have been found in geological formations that are much older than some 3000 million years. It can be assumed that simpler forms precede these. Scientists consider that life began to evolve sometime 3800 million years ago.

The last phase in the evolution of the earth relates to the origin and evolution of life. The initial or even the early atmosphere of the earth was not conducive for the development of life.

Q 42 – Describe in short the evolution of atmosphere and hydrosphere (oceans).

Ans. There are three stages in the evolution of the present atmosphere. The first stage is the loss of the primordial atmosphere. In the second stage, the hot interior of the earth contributed to the evolution of the atmosphere. The third stage in the composition of the atmosphere was modified by the living world through the process of photosynthesis.

The early atmosphere, with hydrogen and helium, has been stripped off as a result of the solar winds. All the terrestrial planets are supposed to have lost their primordial atmosphere through the impact of solar winds. During the differentiation and cooling of the earth, gases and water vapour were released from the interior solid earth. This started the evolution of the present atmosphere. The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen.

The process through which the gases were outpoured from the interior is called degassing. Continuous volcanic eruptions contributed water vapour and gases to the atmosphere. As the earth cooled, the water vapour released started getting condensed. The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains, The rainwater falling on the surface got collected in the depressions to give rise to oceans. The earth’s oceans were formed within 500 million years from the formation of the earth. Oceans began to have the contribution of oxygen through the process of photosynthesis.

Q 43 – Why are the terrestrial planets rocky? Answer: Terrestrial planets are rocky because:

• Ans. The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles.
• The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the terrestrial planets.
• The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.

Q 44 – What is the basic difference in the arguments related to the origin of the earth given by (a) Kant and Laplace (b) Chamberlain and Moulton.

Ans. Kant and Laplace’s Principle: The hypothesis considered that the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating. According to this principle which emerged in 1796, the interior of the earth must be gaseous because the earth has originated from gas form.

2. Chamberlain and Moulton: In 1900, Chamberlain and Moulton considered that a wandering star approached the sun. As a result, a cigar-shaped extension of material was separated from the solar surface. As the passing star moved away, the material separated from the solar surface continued to revolve around the sun and it slowly condensed into planets. Later on, the arguments considered of a companion to the sun to have been coexisting. These arguments are called binary theories.

Q 45 – What is meant by the process of differentiation?

Ans. Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core. From the crust to the core, the density of the material increases. This process of the earth forming material got separated into different layers is called differentiation.

Q 46 – What was the nature of the earth surface initially?

Ans. The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. This is far from the present day picture of the earth. It is said that in initial stage the earth was in liquid form. Certainly, there must have been some events- processes, which may have caused this change from rocky, barren and hot earth to a beautiful planet with ample amount of water and conducive atmosphere favouring the existence of life.

Q 47 – What were the gases which initially formed the earth’s atmosphere?

Ans. Hydrogen’ and helium were the gases which initially formed the earth’s surface. The early atmosphere with hydrogen and helium is supposed to have been stripped off as a result of intense solar wind. This happened not only in the case of earth, but also in all the terrestrial planets which were supposed to have lost their primordial atmosphere through the impact of solar winds. During the cooling of the earth, gases and water vapour were released from the interior solid earth. Continuous volcanic eruptions contributed water vapour and gases at atmosphere. It was the first „ stage of atmosphere development.

Q 48 – Write an explanatory note on the ‘Big Bang Theory’.

Ans. The Big Bang Theory, also called as expanding universe hypothesis. Edwin Hubble in 1920 provided the evidence that the universe is expanding. The galaxies move farther as the time passes. It says that galaxies are moving away from each other. The universe appears to be growing larger.

The Big Bang Theory:-

In the beginning, ail matter forming the universe existed in one place in the form of a ‘tiny ball” with an unimaginably small volume, infinite temperature and infinite density.

2. At the Big Bang “tiny ball” exploded violently. This led to a huge expansion. It is now/ generally accepted that the event of big bang took place 13,7 billion years before the present. The expansion continues even to the present day. As it grew7, some energy was converted into matter. There was particularly rapid expansion within fractions of a second after the bang. Thereafter, the expansion has slowed down. Within first three minutes from the Big Bang event, the first atom began to form.

3. Within 300,000 years from the Big Bang, temperature dropped to 4,500k and gave rise to atomic matter. The universe became transparent.

4. The expansion of universe means increase in space between the galaxies. An alternative to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time. However, with  greater evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe.

Q 49 – List the stages in the evolution of the earth and explain each stage in brief.

Ans. The earth was mostly in a volatile state during its primordial stage. Due to gradual increase in density the temperature inside has increased. As a result the material inside started getting separated depending on their densities. This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface. With passage of time it cooled further and solidified and cpndensed into a smaller size. This later led to the development of the outer surface in the form of a crust. It is through the process of differentiation that the earth forming material got separated into different layers. Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core. From the crust to the core, the density of the material increases.

The origin of life as a kind of chemical reaction, which first generated complex organic molecules and assembled them, This assemblage was such that they could duplicate themselves concerting inanimate matter into living substance. The record of life that existed on this planet in different periods is found in rocks in the form of fossils. The microscopic structures closely related to the present form of the blue algae have been found in geological formations much older than some 3,000 million years. It can be assumed that life began to evolve sometime 3,800 million years ago,

Q 50 – What do you mean by the big splat?

Ans. The origin of the moon as a satellite of the earth is the result of big collision which is called “the big splat”.

Q 51 – Which hypothesis was given by Hoyle?

Ans. Hoyle gave concept of steady .state. It considered the universe to be roughly the same at any point of time. However, with greater evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe.

Q 52 – What do you mean by dwarf planets?

Ans. According to International Astronomical. Union (LAU) on August 24,2006, a planet is a celestial body that

• orbits around the sun
• has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape. The non-satellites bodies fulfilling these two rules are called dwarf planets. Pluto is now considered a dwarf planet. Ceres, Eris, Makemake, Haumea are some other dwarf planets.

Q 53 – What are different stages of development of atmosphere?

Ans . In the early stage the atmosphere with hydrogen and helium is supposed to have been stripped off as a result of intense solar winds. In the second stage during the cooling of the earth, gases and water vapour were released from the interior solid earth. Continuous volcanic eruptions contributed water vapour and gases to the atmosphere. As the earth cooled, the water vapour released started getting condensed. Third stage was the stage of photosynthesis.

Q 54 – What do you mean by degassing?

Ans. The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen. The process through which the gases were outpoured from the interior is called degassing.

Q 55 – Name different hypothesis associated with the formation of the earth.

Ans. Answer: Nebular Hypothesis: It was given by Laplace. Collision Hypothesis: It was given by Sir James and Harold Jeffrey. Accretion Hypothesis: It was given by Schmidt and Carl Weizascar. The Big Bang Theory: It was given by Edwin Hubble.

Q 56 – Who gave their opinions about the formation of the moon? What did they opine?

Ans. In 1838, Sir George Darwin suggested that initially the earth and the moon formed a single rapidly rotating body. The whole mass became a dumb-bell shaped body and eventually it broke. The material separated from the earth was formed as Moon and the place became the Pacific Ocean. It is not accepted now. The present theory is “the giant impact theory” or “big splat theory”. A large size body of Mars collided with the earth and that portion was separated from the earth. The same portion became as a moon which revolves around the earth.

Q 57 – What do you mean by light year?

Ans. A light year is equal to the number of kilometers travelled by light per second. It is a measure of distance and not of time. Light travels at a speed of 300,000 km/second. Therefore, the distances the light will travel in one year is taken to be one light year.

Q 58 – What do we get to know by the study of celestial bodies?

Ans. We get to know about the origin, evolution and function of each and every celestial bodies.

Q 59 – What do you mean by terrestrial planets?

Ans. The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Mercury, Venus, Earth and Mars are called terrestrial planets.

Q 60 – Which gases were found in excess in earlier atmosphere?

Ans. In the earlier atmosphere there was mainly hydrogen and helium.

Q 61 – How did atmosphere originate?

Ans. Gases were released from the earth’s interior such as water vapour and other gases. There were water vapour, nitrogen, carbon dioxide, methane, ammonia and little free oxygen. The process of outpouring the gases from the interior of the earth is called degassing. The process of differentiation created the present atmosphere.

Q 62 – How were stars formed?

Ans. The distribution of matter and energy was not even in the early universe. These initial density differences gave rise to differences in gravitational forces and it caused the matter to get drawn together. These formed the bases for development of galaxies which contains millions of stars.

Q 63 – What are jovian planets?

Ans. Jupiter, Saturn, Uranus and Neptune are called Jovian or Gas Giant planets. Jovian means jupiter-like. Most of them are much larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen.

Q 64 – Name the experts who modified nebular hypothesis.

Ans. In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany revised the ‘nebular hypothesis’, though differing in details. They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust. The friction and collision of particles led to formation of a disk-shaped cloud and the planets were formed through the process of accretion. However, scientists in later period took up the problems of origin of universe rather than that of just the earth or the planets.

Q 65 – What kind of evidences have been given by Edwin Hubble?

Ans. Edwin Hubble, in 1920, provided evidence that the universe is expanding. As time passes, galaxies move further and further apart.

Q 66 – What is the opinion of present day scientists about the origin of moon?

Ans. In 1838, Sir George Darwin suggested that initially the earth and the moon formed a single rapidly rotating body. The whole mass became a dumb-bell shaped body and eventually it broke. The material separated from the earth was formed as moon and the place became the Pacific Ocean. It is not accepted now.

The present theory is the “giant impact theory” or “big splat theory”. A large size body of Mars collided with the earth and that portion was separated from the earth. The same portion became as a moon which revolves around the earth. The moon was formed about 4.44 billion years ago.

Q 67 – What are the difference between terrestrial planet and jovian planets?

Ans. The main differences between the two are summarized below:

• The terrestrial planets were formed in the close vicinity of the planet star where it was too warm for gases to condense solid particles. Jovian planets were formed at a quite distant location.
• The solar wind was most intense nearer the sun, so it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.
• The terrestrial planets are smaller and their lower gravity could not hold the escaping gases. Jovian planets are bigger and have high gravity.

Q 68 – Inner planets are terrestrial while outer planets are Jovian. Why?

Ans. Inner planets are terrestrial while outer planets are Jovian because the terrestrial planets are smaller and their lower gravity could not hold the escaping gases. Jovian planets are bigger and have high gravity. Moreover, the solar wind was most intense nearer the sun, so it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.

Q 69 – Differentiate between inner planets and outer planets.

Ans. The difference between inner planets and outer planets inner planets outer planets

Q 70 – Explain how did life evolve on the earth?

Ans. The origin of life as a kind of chemical reaction, which first generated complex organic molecules and assembled them. This assemblage was such that they could duplicate themselves converting inanimate matter into living substance. The record of life that existed on this • planet in different periods is found in rocks in the form of fossils. The microscopic structures closely related to the present form of the blue algae have been found in geological formations m uch older than some 3,000 million years. It can be assumed that life began to evolve sometime 3,800 million years ago.

Q 71 – Explain different phases of evolution of planets.

Ans. Explain different phases of evolution of planets.

1,.Formation of Disc; The stars are localised lumps of gas within a nebula. The gravitational force within the lumps leads to the formation of a core to the gas cloud and a huge rotating disc of gas and dust develops around the gas core.

2. Formation of Planetesimals: In the next stage, the gas cloud starts and getting condensed and the matter around the core develops into small rounded objects. These small rounded objects by the process of collision develop into what is called planetesimals. Larger bodies start forming by collision and gravitational attraction causes the materials to stick together. Planetesimals are a large number of smaller bodies.

3. Formation of Planets: In the final stage, these large number of small planetesimals accrete to form fewer large bodies in the form of planets.

Q 72 – Explain the earliest theory associated with the origin of the earth.

Ans. A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth. One of the earlier and popular arguments was by German philosopher Immanuel Kant. Mathematician Laplace revised it in 1796. It is known as Nebular Hypothesis. According to this theory there was a hot and rotating gas cloud called Nebula in the space. From Nebula there was a gradual loss of heat due to its rotation which resulted in cooling of its outer surface. This gradual cooling caused contraction in size of Nebula, but its speed increased due to angular momentum. The outer layer was separated from the remaining part of Nebula.

The centre of Nebula became ‘Sun’ and the planets were formed of the smaller units. The lighter material gases and the heavier dust particle gave the fact of inner and outer planets. In 1950, Otto Schmidt in Russia and Carl Weizasear in Germany somewhat revised the ‘nebular hypothesis’, though differing in details. They considered that the sun was surrounded by solar nebula containing mostly the hydrogen and helium along with what may be termed as dust. The friction and collision of particles led to formation of a disk-shaped cloud and the planets were formed through the process of accretion.

Q 73 – Explain the modern theory associated with evolution of the earth. Ans.  In modern theory the evolution of the earth is associated with Big Bang Theory. It was put forth by Edwin Hubble in 1920. In the beginning, all matter forming the universe existed in one place in the form of a ‘tiny ball” \yith an unimaginably- small volume infinite temperature and infinite density. At the Big Bang “tiny ball” exploded violently. This led to a huge” expansion. it is now generally? accepted that the event of big. bang took place 13.7 billion years before the present. The expansion continues even to the present day. As it grew, some energy’ was converted into matter. There was particularly rapid expansion within fractions of a second’ after the bang. Thereafter, the expansion has slowed down. Within first Big Bang event, the first atom began to form.

Within 300,000 years from the Big Bang, temperature dropped to 4,500k and gave rise to atomic matter. The universe became transparent. The expansion of universe means increase in space between the galaxies. An alternative to this was Hoyle’s concept of steady state. It considered the universe to be roughly the same at any point of time. However, with greater evidence becoming available about the expanding universe, scientific community at present favours argument of expanding universe.

Q 74 – Explain the collision and accretion I hypothesis associated with the evolution of the earth

Ans. Collision hypothesis and accretion hypothesis are described below in short. 1.  Collision Hypothesis: It was given by Sir James and Harold Jeffrey.

• According to this theory, a large nebula ’wandering in the space came very close to smaller nebula (Sun) and its huge upsurge of matter on the surface of smaller nebula. The matter was detected from the smaller nebula and on cooling condensed into planets.

2. Accretion Hypothesis: It was given by? Schmidt and Carl Weizascar.

• According to them, solar system started out as a cloud of gas and dust drifting in a space called nebula. This gaseous cloud exploded violently to form supernova. The exploitation left the vast spinning cloud and gases and thus to collapse under its own gravity and develop as denser core.
• The denser core became larger and hotter and began to burge. Later it developed into protostar which finally evolved as ‘infant Sun.
•  Away from its central surface, particles of dust began to clump together and converted into first smaller fragments of rocks and then becoming larger bodies which were called planetesimals which collided with one another to form rocky inner planets like Mercury, Venus, Earth and Mars and the remaining were outer planets.

Q 75 – How’ was the layered structure of the earth developed?

Ans. The planet earth initially was a barren, rocky arid hot object with a thin atmosphere of hydrogen and helium. This is far from the present day picture of the earth. It is said that in initial stage the earth was in liquid form. Certainly, there must have been some events- processes, which may? have caused this change from rocky, barren and hot earth to a beautiful planet with ample amount of water and conducive atmosphere favouring the existence of life.

The earth was mostly in a volatile state during its primordial stage. Due to gradual increase in density the temperature inside has increased. As a result the material inside started getting separated depending on their densities. This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface. With passage of time it cooled further and solidified and condensed into a smaller size. This later led to the development of the outer surface in the form of a crust. It is through the process of differentiation that the earth forming material got separated into different layers

Q 76 – What role did carbon dioxide pipy in the formation of oceans?

Ans. The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains. The rainwater falling onto the surface got collected in the depressions to give rise to oceans. The earth’s oceans were formed within 500 million years from the formation of the earth.

Q 77 –How did earth develop its different layers?

Ans. During the formation of the moon, due to the giant impact, the earth was further heated up. It is through the process of differentiation that the earth forming material got separated into different layers. Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core. From the crust to the core, the density of the material increases.’

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10.02: Origin of the Solar System—The Nebular Hypothesis

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• Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
• Salt Lake Community College via OpenGeology

Our solar system formed at the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets [ 12 ]. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.

Planet Arrangement and Segregation

As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water [ 13 ]. This temperature differentiation resulted in the inner four planets of the solar system becoming rocky, and the outer four planets becoming gas giants.

Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming proto-planets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid core. Rocky planets built more rock on that core, while gas planets added gas and ice. Ice giants formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases.

The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and helium, which could not be collected by the weaker gravity of the smaller planets.

Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together [ 14 ]. These collisions created the asteroid belt, an unfinished planet, located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to the outer edge of the solar system [ 15 ].

Pluto and Planet Definition

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:

• Enough mass to have gravitational forces that force it to be rounded
• Not massive enough to create a fusion
• Large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets

12. Montmerle T, Augereau J-C, Chaussidon M, et al (2006) Solar System Formation and Early Evolution: the First 100 Million Years. In: From Suns to Life: A Chronological Approach to the History of Life on Earth. Springer New York, pp 39–95

13. Martin RG, Livio M (2012) On the evolution of the snow line in protoplanetary discs. Mon Not R Aston Soc Lett 425:L6–L9

14. Petit J-M, Morbidelli A, Chambers J (2001) The Primordial Excitation and Clearing of the Asteroid Belt. Icarus 153:338–347. https://doi.org/10.1006/icar.2001.6702

15. Walsh KJ, Morbidelli A, Raymond SN, et al (2011) A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475:206–209

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The Nebular Theory of Laplace Solar System

The nebular theory of Laplace is a theory that suggests that the Solar System was created by the gravitational collapse of a gaseous nebula. Read further to know more about it.

Table of Content

The first person to develop this theory was Pierre-Simon Laplace in 1796.

This theory was developed before we had a good understanding of gravity or nuclear forces, so it did not take into account these important forces. Laplace’s original idea has been modified over time to include these forces, and it is now known as the nebular hypothesis.

Nebular Hypothesis 

The nebular hypothesis is the most widely accepted model for the formation of our Solar System. It explains how all of the planets formed in their current orbits, and why they are made of different materials.

The basic idea behind the nebular hypothesis is that a large cloud of dust and gas (a nebula) collapsed under its own gravity. As the nebula collapsed, it spun faster and flattened out into a disk shape.

As the nebula continued to collapse, the material in the center became increasingly dense and hot. This central region eventually became our Sun, while the rest of the disk became our planets and moons.

The solar system we see today is thought to have formed in this way, over four billion years ago.

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Different Versions of Nebular Hypothesis

While there are many different versions of the nebular hypothesis, they all share a common set of ideas:

-A large cloud of dust and gas (a nebula) collapsed under its own gravity.

-As the nebula collapsed, it spun faster and flattened out into a disk shape.

-The material in the center became increasingly dense and hot, eventually forming our Sun.

-The rest of the disk became our planets and moons.

-This process took place over four billion years ago.

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Evidence Supporting Nebular Hypothesis

There are many pieces of evidence that support the nebular hypothesis, including:

-The chemical composition of the Sun and planets is similar to that of interstellar gas clouds.

– PLANETS ARE OBSERVED TO FORM IN OTHER STAR SYSTEMS (Eg. Protoplanetary disks have been observed around young stars).

– The Solar System has a well-defined plane of rotation (the ecliptic), which is thought to be a remnant of the original disk.

How the Solar System Was Formed?

The solar system was formed as a result of the gravitational collapse of a gaseous nebula. This led to the formation of the sun and planets. The solar system we see today is thought to have formed about four and a half billion years ago.

The nebula that gave birth to our solar system was a cloud of dust and gas that existed in interstellar space. It was probably similar to many other such nebulae that can be seen in our galaxy, the Milky Way. These nebulae are often called “stellar nurseries” because they are where new stars are born.

The gravity of a large star can cause nearby dust and gas particles to clump together. Over time, these clumps can grow into larger bodies known as protoplanets. If a protoplanet is massive enough, its gravity can cause it to collapse in on itself. This process can lead to the formation of a new star.

The nebula that formed our solar system was probably about 100 light-years across and contained between 50 and 100 stars. Our sun was just one of these stars. As the nebula collapsed, most of the stars moved away from each other. But our sun and a few other nearby stars remained together in what is known as a stellar cluster or birth cluster.

The sun and its close stellar neighbors would have continued to lose mass as they moved through the collapsing nebula. This mass loss eventually led to the formation of planets around the sun. The planets we see today are thought to have formed from the leftover dust and gas that surrounded the sun after it had finished forming.

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Conclusion 

The formation of our solar system was a slow and gradual process that took place over millions of years. Our understanding of how it happened is still evolving, and new discoveries are being made all the time. But we now know that the solar system we call home is just one small part of a vast and amazing universe.

Frequently Asked Questions

Get answers to the most common queries related to the UPSC Preparation.

What is the Nebular Theory of Laplace?

What are the main components of the theory, what is the evidence for the nebular theory of laplace, what are some criticisms of the nebular theory of laplace.

Ans: The Nebular Theory of Laplace is a theory that explains the formation of our solar system. It states that a large cloud of gas and dust (the nebula) collapsed under its own gravity to form our sun and planets.

Ans: There are four main components to the theory: (i) a rotating disk of gas and dust, (ii) gravitational collapse, (iii) accretion, and (iv) planetesimals. It also states that the solar system formed from the inside out, with the inner planets (Mercury, Venus, Earth, and Mars) forming first and the outer planets (Jupiter, Saturn, Uranus, Neptune) forming later.

Ans: There are several pieces of evidence that support the theory.

First, all of the planets in our solar system orbit our sun in roughly the same plane. 

Second, most of the planets rotate around their axes in the same direction as they orbit our sun. 

Third, all of the planets have nearly circular orbits. 

Fourth, all of the rocky worlds in our solar system (Mercury, Venus, Earth, and Mars) have iron cores surrounded by a silicate mantle. 

Fifth, the gas giants (Jupiter, Saturn, Uranus, and Neptune) have hydrogen and helium atmospheres. 

Sixth, comets orbit in a plane that is different from the planes of the planets. 

Finally, computer simulations have shown that a rotating disk of gas and dust can collapse to form a solar system.

Ans: One criticism is that it does not explain why our solar system has so much angular momentum. Another criticism is that it does not explain why all of the planets are orbiting in roughly the same plane. A third criticism is that it does not explain why most of the planets are rotating in the same direction as they orbit our sun.

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