Early theories of the Earth's origin

Detail

1. Where this section fits in the timeline

• Big Bang, about 13.7 billion years ago: the universe begins. → The Sun and the solar system form about 4.6 billion years ago. → The Earth forms. → Life begins.
• The solar system is the Sun plus everything held by its gravity: the planets, their moons, asteroids and comets.
• The early theories in this section tried to explain only one part of this chain: how the Earth and the planets were born.
• Modern dating figures:
• The Earth, meteorites and so the whole solar system are about 4.54 billion years old, with an error of less than 1% [2].
• Meteorites date to about 4.57 billion years [2].

• (NCERT: about 4.6 billion years.)

• Why scientists date meteorites, not Earth rocks:

• The Earth's oldest rocks have been recycled and destroyed by plate tectonics (the slow movement of the large plates of the Earth's crust) [2].
• So scientists use meteorites. These are pieces left over from the time the solar system formed.

2. Two families of theories

Feature	Nebular (monistic) theories	Encounter / "binary" (dualistic) theories
How many stars are involved	One: the Sun and its own cloud	Two: the Sun and a passing star
Where planet material comes from	A cloud of gas and dust around the young Sun	Material pulled out of the Sun by the passing star
Main thinkers	Kant; Laplace (1796); Otto Schmidt and Carl Weizsacker (1950)	Chamberlin and Moulton (early 1900s); Jeans and Jeffreys (1919)
Status today	Accepted, in a refined form (the solar-nebula model)	Rejected

3. Nebular hypothesis (Kant and Laplace)

• Nebular hypothesis: the idea that the planets formed from a cloud of material around a young Sun that rotated slowly. A nebula is a cloud of gas and dust in space.
• Who proposed it:
• The German philosopher Immanuel Kant first proposed it.

• The French mathematician Pierre-Simon Laplace revised it in 1796.

• Kant's version:

• A rotating cloud of gas and dust (a solar nebula) was pulled together by its own gravity [5].

• It collapsed and flattened into a spinning disk. The Sun and the planets formed from this disk [5].

• Laplace's version, step by step:

• Cause: the young Sun cooled and contracted (shrank) [5].
• Process: as it shrank, it threw off rings of material [5].

• Result: each ring gathered together to form a planet [5].

• What it explained well: the planets go around the Sun in the same plane and in the same direction [5].

• Its weak point: it could not explain how angular momentum is shared between the Sun and the planets [5]. Angular momentum is the amount of turning motion a body has, from spinning on its axis and from moving in its orbit.

4. The encounter ("passing star" or binary) theories

a) Chamberlin and Moulton's planetesimal hypothesis (early 1900s)

• Who proposed it: the American geologist Thomas C. Chamberlin and the astronomer Forest Ray Moulton, both from Chicago [6].
• How it worked, step by step:
• Cause: a wandering star passed close to the Sun.
• Process: its gravity pulled cigar-shaped strands of material out of the Sun. These strands cooled into small, cold, solid bodies called planetesimals.

• Result: the planetesimals stuck together (accretion) and built the planets.

• Key idea: the planets were put together from small, cold, solid particles. They did not condense out of hot gas [6].

• Planetesimal: the word comes from "infinitesimal planets", meaning tiny planets [6].
• In the early 20th century, this hypothesis took the place of Laplace's nebular hypothesis as the more popular view [6].

b) Jeans and Jeffreys' tidal hypothesis (1919)

• Who proposed it: the British astronomer Sir James Jeans, later supported by Harold Jeffreys.
• Jeans decided that Laplace's nebular hypothesis was wrong. In its place he put forward a "catastrophic" or tidal theory [7].
• How it worked, step by step:
• Cause: a star came very close to the Sun but did not hit it [7].
• Process: its pull drew a stream of material out of the Sun, much as the Moon pulls up a tide [7].
• Result: this material condensed to form the planets [7].

c) Why the encounter theories failed

• The angular momentum problem:
• Most of the solar system's turning motion is carried by the planets in their wide orbits. The Sun holds very little of it.

• A star passing by could not give the pulled-out material enough sideways motion to put it into such wide, steady orbits.

• Too rare: stars are very far apart, so close encounters almost never happen. If planets needed one, planetary systems would be extremely rare.

• Result: these theories lost support. The modern solar-nebula model brought back a refined version of the nebular idea.

5. Revised nebular hypothesis (1950)

• Who proposed it: Otto Schmidt (Russia) and Carl Weizsacker (Germany), in 1950.
• How it worked, step by step:
• Starting point: the Sun was surrounded by a solar nebula made mostly of hydrogen and helium, plus dust.
• Process: the particles rubbed against each other (friction) and collided. This formed a disk-shaped cloud.

• Result: the planets formed from this disk by accretion. Accretion means small bodies sticking together again and again until they become large bodies.

• The modern version (solar-nebula model):

• Trigger: a dense cloud of gas and dust collapsed, possibly because of the shockwave from a nearby supernova (an exploding star) [3].
• Disk: the collapsing cloud became a spinning, swirling disk [3].
• Sun: most of the material was pulled to the centre and formed the Sun. The Sun holds about 99.8% of the solar system's mass [4].

• Planets: clumps farther out in the disk smashed into one another and grew into planets and moons [3].

• Why the inner and outer planets are different:

• Near the hot Sun, only rocky material could survive. This gave the rocky terrestrial planets, Mercury to Mars [3].
• Ice and gas settled in the cooler outer regions. This gave the gas and ice giants [3].
• The asteroid belt is made of leftover pieces that never came together into a planet [3].

6. From the Earth to the universe

• Later, scientists stopped asking only how the Earth began. They started asking how the universe began.
• This led to the Big Bang theory and the study of how stars and galaxies form.

Prelims Hooks

• Nebular hypothesis: Immanuel Kant (German philosopher) first proposed it. Laplace (mathematician) revised it in 1796.
• Revised nebular hypothesis (1950): Otto Schmidt (Russia) and Carl Weizsacker (Germany). A common trap is to swap their countries.
• Planetesimal hypothesis: Chamberlin and Moulton, early 1900s. A wandering star pulled cigar-shaped strands out of the Sun.
• Tidal hypothesis: Jeans and Jeffreys, 1919. It also used a passing star.
• Encounter (binary) theories use two stars. Nebular theories use one star and its cloud.
• Main reason the passing-star theories failed: they could not explain the angular momentum of the planets.
• Laplace's model: the cooling, shrinking Sun shed rings of material that became planets [5].
• The solar nebula was mostly hydrogen and helium, plus dust. The planets grew by accretion.
• Age of the Earth and solar system: about 4.54 billion years [2]. (NCERT: about 4.6 billion years.)
• The Sun holds about 99.8% of the solar system's mass [4]. Rocky planets formed near the Sun and gas giants farther out [3].

Mains Points

• How scientific ideas develop:
• First came the nebular idea (Kant and Laplace).
• In the early 1900s the encounter theories (Chamberlin-Moulton, Jeans-Jeffreys) challenged it.
• Physics (angular momentum) and evidence then brought back an improved nebular model (Schmidt-Weizsacker, 1950, and the modern solar-nebula model).

• This history works well as the introduction to a GS-I answer on the origin of the Earth.

• What the modern nebular model explains:

• All planets go around the Sun in one plane and in the same direction.
• Rocky planets lie inside and gas giants outside, because temperature changed with distance from the Sun.

• The asteroid belt is leftover material that never formed a planet [3][5].

• Why dating the Earth needs meteorites: plate tectonics has destroyed the Earth's oldest rocks, so the 4.54-billion-year age comes from meteorites [2]. This links the Earth's origin to the study of the Earth's interior and plate tectonics.