At the dawn of the 20th century, the prevailing Nebular Hypothesis for the solar system’s origin faced a critical challenge: the problem of angular momentum. In 1905, two University of Chicago academics, geologist T.C. Chamberlin and astronomer Forest Ray Moulton, proposed a radical alternative: the Planetesimal Hypothesis. Their theory was dualistic, or biparental, meaning it required two stars instead of one. The theory suggested that a passing star ripped material from our early sun, which then condensed into small, solid bodies called “planetesimals” that eventually clumped together to form the planets. This dramatic stellar encounter was their proposed solution to the angular momentum puzzle and provided a comprehensive framework for explaining the origin of the Earth, its atmosphere, and its oceans.

A. Origin of the Earth

The theory begins with two initial heavenly bodies:

1. The Proto-Sun: A large star, but unlike in nebular theories, it was proposed to be relatively cool and composed of solid particles.
2. The Companion Star: A second, massive star that passed very close to the proto-sun.

As the companion star passed by, its immense gravitational pull created enormous tidal forces, tearing large quantities of material, or “jets,” away from the surface of the proto-sun. This ejected material soon cooled and condensed into countless small, solid bodies called planetesimals.

• The larger of these planetesimals became nuclei, whose gravity attracted and captured smaller planetesimals.
• Through this process of accretion, these nuclei gradually grew into the planets.
• The remaining, diminished proto-sun is our Sun today.

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B. The Three Stages of Earth’s Evolution

Chamberlin and Moulton outlined three overlapping stages for the Earth’s development.

1. First Stage: The Period of Planetesimal Accession

This is the period of the Earth’s initial formation and growth.

• Huge quantities of planetesimals were thrown out from the proto-sun. Some were pulled away by the intruding star, while others were captured by the proto-sun’s gravity and began to orbit it.
• Through the gradual aggregation of small planetesimals onto a larger nucleus, the Earth grew to its present size and shape.
• During this stage, the Earth was initially thought to be solid and without an atmosphere.

2. Second Stage: The Period of Dominant Volcanism

This stage covers the formation of the Earth’s interior, atmosphere, continents, and oceans.

• As the Earth grew, internal heat accumulated from various sources.
• This intense heat led to the selective melting of rock in the Earth’s outer parts.
• The result was widespread and violent volcanic activity, which fundamentally shaped the planet’s surface and atmosphere.

3. Third Stage: The Actual Geological Period

This is the ongoing stage where later geological features are formed.

• This period is characterised by the formation of fold mountains, faults, plateaus, and other features of the Earth’s crust due to internal and surface processes.

C. Evolution of Earth’s Components

Origin of Internal Heat

The heat inside the Earth increased as its size and mass grew. The primary sources were: i. Mutual Collision: The kinetic energy from the constant impacts of planetesimals was converted into heat. ii. Increasing Pressure: As the planet’s mass increased, the immense pressure in the interior generated significant heat. iii. Molecular Rearrangement: Heavier materials and compounds sank toward the centre, releasing gravitational potential energy as heat.

Evolution of the Atmosphere

The Earth’s atmosphere formed from two main sources:

• External Sources: As the young Earth’s gravity became stronger, it captured free atmospheric molecules (like hydrogen and helium) from space.
• Internal Sources: During the “Period of Dominant Volcanism,” outgassing released gases trapped within the planet’s interior. This process provided the early atmosphere with carbon dioxide (CO₂​), water vapour (H₂​O), and nitrogen (N_2​).

Evolution of Continents and Ocean Basins

• Widespread volcanism created a fragmented, uneven surface riddled with crevices and basins.
• The vast quantities of water vapour released during outgassing later condensed as the surface cooled.
• This water filled the volcanic craters and deep basins, first forming numerous lakes that eventually coalesced to create the primitive oceans. The higher-standing, thicker parts of the crust remained as continents.

D. Evaluation and Criticisms

The hypothesis was eventually discarded due to several major flaws pointed out by later scientists.

• Improbability of Encounter: The required close pass of two stars is an extraordinarily unlikely event, making it an unsatisfactory foundation for a theory.
• Encounter Dynamics: For planetesimals to be pulled out, the stars would have to pass at a distance not much greater than their diameters. Furthermore, material pulled from the Sun in such an encounter is far more likely to fall back into the Sun or be captured by the passing star than to form stable, nearly circular planetary orbits.
• Angular Momentum: While it was created to solve the angular momentum problem, later analysis showed it still couldn’t adequately explain the precise distribution of momentum we observe today.
• State of Matter: The theory proposes a “cold” accretion, resulting in an initially solid planet. However, significant geological evidence points to an initially molten Earth, which is necessary to explain the early differentiation of its core, mantle, and crust.
• Outer Planets: It provides no good explanation for the low-density, gaseous nature of the outer planets (Jupiter, Saturn, etc.). The accretion of solid planetesimals would form rocky bodies.
• Planetary Size: The theory cannot explain the observed pattern of planetary sizes: small rocky planets near the sun, then gas giants, with size decreasing again in the outer solar system.