The Tidal Hypothesis, proposed by Sir James Jeans in 1919 and later modified by Sir Harold Jeffreys, is an early 20th-century theory that attempted to explain the origin of our solar system.

While it was influential for a time because it seemed to explain the relative sizes and positions of the planets, the Tidal Hypothesis has since been discarded. It was replaced by the modern Solar Nebular Disk Model, largely because it failed to solve major problems related to the extreme unlikelihood of such a close stellar encounter and the distribution of angular momentum in the solar system.

Theory Given by Sir James Jeans in 1919

1. Stellar Encounter: The theory begins with a star much larger than our Sun passing very close to it.
2. Filament Ejection: The immense gravitational force of this intruding star pulled a long, cigar-shaped filament of hot gas from the Sun’s surface.
3. Planet Formation: As the intruding star moved away, this filament broke into fragments. These fragments then cooled and condensed to form the planets. The largest planets (Jupiter and Saturn) formed from the thick centre of the filament, while smaller planets formed from its tapered ends.
4. Satellite Formation: The theory suggested that moons were created in a similar way, from smaller filaments pulled from the newly formed, still-molten planets by the Sun’s gravity.

Merits

These were the points that initially made the theory attractive:

• Planet Size and Arrangement: The cigar shape of the filament offered a simple explanation for the arrangement of the planets. The largest planets (Jupiter and Saturn) formed from the thick central part, while the smaller planets (like Mercury and Mars) formed from the tapered ends.
• Satellite Systems: The theory applied the same logic to the moons of gas giants, explaining why some of their larger moons are found in the middle of their satellite systems.
• State of Outer Planets: It correctly reasoned that the larger planets, formed from more material, would take much longer to cool down and would thus remain in a gaseous state.
• Orbital Plane: The theory attempted to explain why the planets orbit on a plane slightly inclined to the Sun’s equator, attributing it to the gravitational pull of the passing star.

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Demerits

Despite its initial merits, the Tidal Hypothesis was discarded because it could not overcome several major scientific objections:

• Extreme Improbability: The chance of two stars passing close enough to cause such an event is incredibly low. This would make solar systems like ours exceptionally rare, which contradicts modern observations.
• Angular Momentum Problem: This was the most critical flaw. The planets, which hold less than 1% of the solar system’s mass, possess over 99% of its angular momentum (rotational energy). The theory could not explain how the slow-rotating material from the Sun could gain such immense angular momentum.
• Gaseous Dissipation: The hot gas filament pulled from the Sun would have been at a temperature of millions of degrees. Physics dictates that this gas would have expanded and dissipated into space long before it could cool enough to condense into solid planets.
• Compositional Differences: The theory doesn’t explain why the inner planets are dense and rocky while the Sun is composed almost entirely of hydrogen and helium.

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Modifications to the Original Theory by Harold Jeffreys, 1926

• Stellar Collision: According to Harold Jeffreys, there was an actual collision between the passing star and the Sun. This was a change from Jeans’ original idea of a near-miss.
• Origin of Rotation and Revolution: The glancing blow from the collision was proposed to explain the rotation (spin) of the planets on their axes and their revolution (orbit) around the Sun. The impact would have set the ejected material spinning.
• Need for Rapid Cooling: A major problem was that hot gas from the Sun would simply diffuse into space instead of clumping together. To solve this, it was suggested that smaller planets and satellites could not have formed by slow condensation from a purely gaseous state. Therefore, a rapid cooling process causing partial liquefaction or solidification was thought to be necessary for the material to hold together. This led to the idea that Earth, for example, cooled down to a completely liquid state in a very short time.

Shortcomings

The Tidal Hypothesis, including Jeffreys’ modifications, was ultimately discarded because it could not overcome several fundamental scientific problems.

• Low Probability of Collision: Stars are incredibly far apart from each other. The likelihood of a direct collision or even a very close encounter is extremely low. This makes it an improbable explanation for the formation of solar systems, which are now known to be common in the galaxy.
• Gaseous Dissipation: As a major criticism points out, the super-heated gaseous matter pulled from the Sun would have had immense internal pressure. It would have expanded and been lost in space almost immediately, rather than condensing to form planets.
• Compositional Mismatch: The theory implies that all planets should have the same chemical composition as the Sun. However, we know this is incorrect. The Sun is made mainly of light elements like hydrogen and helium, while the inner planets (like Earth) are dense and rocky, composed of much heavier elements. The hypothesis offered no explanation for this significant difference.

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