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tidal migration

(interaction between tide and orbiting body that changes the orbit)

Tidal migration is a type of planetary migration or moon orbital migration in which tides on/within an astronomical object play a role in changing the orbit of an object orbiting it, e.g., changing the orbit of a planet (or binary-star companion) orbiting a star or the orbit of a moon orbiting a planet. If host rotation and the orbit are in the same direction (prograde) and an orbit takes longer than a host rotation (orbital period is longer than the host rotation period), then the host's rotation is slowed, taking longer and the orbit enlarges, also taking longer, but growing more slowly, and without other influences (e.g., causing an escape from the host), the periods are eventually the same (tidal locking). The mechanism doing this is the host's tidal deformation, not responding instantly, ends up rotated forward from underneath the orbiting body, and the gravitational force on the orbiting body has a small forward component, slowly increasing its orbital speed (tidal acceleration), in turn, slowly enlarging its orbit, and the "equal and opposite reaction" is that the host's rotation is being slowed down. Though the orbital speed has increased, the larger orbit affects the orbital period more, which increases. This is the situation with the Moon's orbit around the Earth, and moons around Saturn and Jupiter, which rotate faster than Earth.

On the other hand, if the host's rotation period is longer than the orbital period, then the host's rotation increases (decreasing its rotation period), and the orbiting body migrates inward, given enough time, colliding with the host unless tidal forces pull it apart first. The host's tidal deformation lags behind the orbiting body, so it is dragged forward, increasing the rotation, and the orbital speed is slowed, resulting in a smaller orbit, once again, the size of the orbit reducing the orbital period more than the slower orbit speed increases it.

If the host's rotation and the orbit are in opposite directions (retrograde), the situation is somewhat similar to the latter: the orbiting body migrates inward for similar reasons, and will collide with the host or be torn apart, but in this case, the host's rotation is slowing.

The host's tides, i.e., constant deformation or rearrangement of the host's material, generates heat through friction, heating the host (tidal heating), though not necessarily significantly. The disturbance caused by the tide can trigger waves such as in the sizeable atmospheres of gas planets, and resonances between the orbit and the waves can drastically change the effects of the tide, e.g., speed up the orbital migration. This is termed resonance locking. If such waves within a host star produce changes in its luminosity, they are termed tidally excited oscillations.

Since both bodies are orbiting around each other, these effects apply to both bodies, including tendency to migrate, changes in the rotation period, and tidal heating. The larger body has more effect on the smaller body, and if there is a large size difference (e.g., the Sun and a planet, or Jupiter and one of its moons), then the effect on the larger is small. The smaller object is typically the earlier to be tidally locked (always the same side facing its host), such as the Moon with the Earth, and some of Jupiter's moons. Pluto and its moon, Charon, which are much closer in mass, are each tidally locked to the other, each always with the same side facing the other.


(tidal,orbits,migration,planets,moons)
Further reading:
https://en.wikipedia.org/wiki/Planetary_migration#Tidal_migration
http://en.wikipedia.org/wiki/Tidal_acceleration

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