The primary factor determining how fast a planet spins is the angular momentum it accumulates during its formation from the protoplanetary disk.
Here's a more detailed breakdown:
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Accretion of Gas and Dust: Planets form from a swirling disk of gas and dust surrounding a young star. As a planet grows, it gravitationally attracts material from this disk. This accretion process is crucial because the infalling material carries angular momentum.
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Angular Momentum Transfer: The angular momentum of the accreted gas and dust is transferred to the growing planet. The more material accreted and the greater the initial angular momentum of that material, the faster the planet will spin. This is analogous to a figure skater pulling their arms in to spin faster; the total angular momentum remains constant, but the rotational speed increases.
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Core Mass and Gas Accretion: A sufficiently massive core (several times the mass of Earth) is required to accrete significant amounts of hydrogen and helium gas from the protoplanetary disk. As the reference indicates, accretion of gas dramatically increases the planet's total angular momentum, resulting in rapid rotation.
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Collisions: Late-stage collisions with other protoplanets can also significantly affect a planet's rotation rate and axial tilt. A major impact, like the one theorized to have formed the Moon, can drastically alter a planet's spin.
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Tidal Locking: Over extremely long timescales, tidal forces exerted by a star or a large moon can slow down a planet's rotation. This process, known as tidal locking, eventually results in the planet's rotation period matching its orbital period (as seen with the Moon and Earth).
Factors Influencing Spin Rate:
| Factor | Description | Effect on Spin Rate |
|---|---|---|
| Gas and Dust Accretion | The process of a planet gravitationally attracting material from the protoplanetary disk. | Higher accretion rate → Faster spin |
| Core Mass | Influences the ability to accrete substantial hydrogen and helium gas. | Larger core (sufficient for gas accretion) → Faster spin |
| Major Collisions | Late-stage impacts with other large bodies. | Can significantly alter spin rate and axial tilt. |
| Tidal Forces | Gravitational forces exerted by a star or large moon. | Can slow down or tidally lock a planet's rotation. |
| Initial Angular Momentum of Protoplanetary Disk | How quickly the initial cloud of gas and dust was rotating from which the planet forms | Faster initial disk rotation can lead to faster spinning planet |
In summary, a planet's spin rate is primarily determined by the amount of angular momentum it gains during formation through accretion and collisions, tempered by subsequent tidal interactions. The ability to accrete large amounts of gas plays a critical role in establishing rapid rotation.
