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# 3.5 Migration and planet interaction

Regardless of the mechanism involved, once planets have managed to form, they tend to interact with each other and with the remaining gas and planetesimals in the disc. Therefore, planets often end up in a different mass and orbital configuration than the one they had at formation. There are several ways this can happen.

## Interaction with remaining gas in the disc

The angular momentum exchange between a planet and the remaining gas in the disc causes the planet to migrate. Migration affects both Earth-sized rocky planets and giant planets, and is one possible mechanism for the formation of hot Jupiters like 51 Pegasi b, which almost certainly formed much further out and migrated inwards.

## Interaction with the remaining planetesimals

Giant planets can interact with the leftover planetesimals in the disc. The resulting exchange in angular momentum can cause the planetesimals to be ejected from the system.

## Planet–planet interaction

There is no guarantee that newly formed planets will be on stable orbits. Instabilities can cause the planets’ orbits to cross, and the net effect of this is usually the ejection of the smaller-mass body involved in the interaction, leaving the surviving planet on a highly eccentric orbit. This effect could explain the high eccentricities seen in many exoplanetary systems.

## Interaction with additional stellar companions

If a distant stellar companion is present and a planet is formed on an orbit that is misaligned with that of the binary companion star, the planet’s eccentricity will change due to the Kozai-Lidov effect. This is a dynamical phenomenon that affects systems where two bodies are orbiting each other in the presence of a third, more distant companion. The presence of the third body causes the position of the inner pair’s orbital periapsis to oscillate, leading to periodic exchanges between the planet’s orbital eccentricity and inclination on a timescale of many orbital periods. This is another possible explanation for the formation of hot Jupiters.

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