Origin of Earth

Quick Review

• A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth.
• German philosopher Immanuel Kant gave the Nebular Hypothesis theory.
• Later in 1900, Chamberlain and Moulton gave arguments which were known as Binary theories.
• He considered that there were two heavenly bodies (stars) in the universe – Proto-Sun and its Companion Star or Intruding star.
• In 1950, Otto Schmidt in Russia and Carl Weizascar in Germany revised Nebular hypothesis theory.
• The most popular argument and modern theory regarding the origin of the universe is the Big Bang Theory.
• Edwin Hubble, in 1920, provided evidence that the universe is expanding. That’s why it is also called expanding universe hypothesis.

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Notes for you

Earlier Theories on Origin of Earth

A large number of hypotheses were put forth by different philosophers and scientists regarding the origin of the earth. They are:

Nebular Hypothesis

• According to Nebular Hypothesis, planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.
• The theory was given by German philosopher Immanuel Kant and Laplace (French Mathematician) revised it in 1796.
• Nebular Theory of Laplace (1796) tried to explain the formation of the solar system. But it had many drawbacks as the theory was based on assumptions.
• But one assumption it got right was that the solar system was born from a giant gas of dust called as nebula.
• A giant interstellar cloud known as the solar nebula (a vast, swirling cloud of gas and dust) gave birth to our solar system and everything in it.
• The nebula started its collapse and core formation some 5-5.6 billion years ago and the Sun and the planets were formed about 4.6 billion years ago.
• Drawbacks: Source of the origin of the nebula and why did only 9 rings come out from irregular ring detached from the nebula was not explained.

Binary theories on origin of earth:

• Binary theories tried to explain that the sun had a companion.
• Planetesmial Hypothesis: According to this theory a wandering star approached the sun. He assumed there were two heavenly bodies (stars) in the universe – Proto-Sun and its Companion Star or Intruding star.
• Hoyle’s supernova hypothesis: According to Hoyle initially there were two stars in the universe – Primitive Sun and Companion star. Over time, all of the hydrogen nuclei of companion star collapsed and violently exploded which resulted in the spread of enormous mass of dust which started revolving around the primitive sun in the form of a circular disc. The matter of this disc became building material for the formation of the future of planets.

Revised Nebular Hypothesis

• According to this theory, the initial universe comprised of stars & dust filling up the space in between.
• This dust started to revolve around the primitive rotating sun and gradually the dust stars accreting & condensing & thus forming the solar system.
• Drawbacks: He did not explain the mode of origin of these dusts. These dark matters were called ‘inter-stellar dusts’ by Schimidt.

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Modern Theories:

One of the most common modern theories regarding the origin of the universe is “Big Bang Theory”. Lets study about it.

Big Bang theory

• It is also called the expanding universe hypothesis.
• According to this theory, Everything in the universe has emerged from a point known as the singularity, 13.7 billion years ago.
• In the beginning, all matter or substance forming this universe existed at one place as a tiny ball. This tiny ball had an extremely small volume, infinite density and temperature.
• The universe expanded from such high density & high-temperature state.
• At the Big Bang (cosmic explosion), this ball blasted fiercely and forcefully from which all the matter of the universe was thrown out which eventually accreted to form stars, solar system and celestial bodies.

Evidence which supports Big Bang Theory:

1.  Red Shift: As an object (light) moves away from us, its wavelength increases & shift to the Red end of the spectrum (Called Red Shift).
2. CBMR: Electromagnetic Radiation which was present shortly after the big bang, we now observe it as background microwave radiation.

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Our Solar System

Our solar system formed 5 billion years ago. It consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grains and gases.

Types of Planet

Inner planet or terrestrial or Earth-like planet	Outer planet or Jovian or giant planets
Mercury, Venus, Earth, Mars	Jupiter, Saturn, Uranus,  Neptune, and Pluto.

What are Jovian planets?

Jovian planet is larger in size as compared to the terrestrial planet and has a thick atmosphere mostly made of helium and hydrogen. Jupiter, Saturn, Uranus and Neptune are jovian planets.

Difference between Terrestrial and Jovian planets:

Terrestrial Planet	Jovian planet
Terrestrial planets are formed by solid particles.	Jovian is formed by gaseous particles.
Solar winds were most intense nearer the sun so it blew off lots of gases and dust from terrestrial planets.	Similar removal does not happen in Jovian planets.
A terrestrial planet is smaller hence lower gravity could not hold the escaping gases.	A jovian planet is larger hence higher gravity could hold the escaping gases.

Formation of Planets

The formation of planets occur in different stages:

• The stars are localised gas lumps inside a nebula.
• Because of gravitational force within the lumps, a core of the gas cloud along with spinning discs of dust and gas are created.
• After this step, condensation of gaseous clouds and the matter present over the core changes into tiny rounded objects.
• Formation of planetesimals ( developed small round objects ) occur by cohesion process.
• By colliding with each other and sticking together by gravitational force they form larger bodies.
• Further, these large numbers of small planetesimals aggregate to develop into a smaller number of large bodies called planets.

Formation of the Sun

• The nebula began to collapse in on itself after becoming gravitationally unstable.
• This was possibly because of a nearby supernova sending shock waves rippling through space.
• Gravity then drew dust and gas to the centre of the nebular cloud.
• As more matter comes in, the centre got denser and hotter, increasing the gravity and pulling even more dust inwards causing a snowball effect.
• About 99.9% of the material fell into the centre and became the protosun (no sunlight yet).
• Once the centre of the cloud became hot enough it triggered nuclear fusion, and the Sun was born.

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