Editing Planetary migration (section)

=== Tidal migration ===
Tides between the star and planet modify the semi-major axis and orbital eccentricity of the planet. If the planet is orbiting very near to its star, the tide of the planet raises a bulge on the star. If the star's rotational period is longer than the planet's orbital period the location of the bulge lags behind a line between the planet and the center of the star creating a torque between the planet and the star. As a result, the planet loses angular momentum and its semi-major axis decreases with time.

If the planet is in an eccentric orbit the strength of the tide is stronger when it is near perihelion. The planet is slowed the most when near perihelion, causing its aphelion to decrease faster than its perihelion, reducing its eccentricity. Unlike disk migration – which lasts a few million years until the gas dissipates – tidal migration continues for billions of years. Tidal evolution of close-in planets produces semi-major axes typically half as large as they were at the time that the gas nebula cleared.<ref>{{cite arXiv |title=Tidal evolution of close-in extra-solar planets |first1=Brian |last1=Jackson |first2=Richard |last2=Greenberg |first3=Rory |last3=Barnes |quote=Submitted [for publication] on 4 Jan 2008 |date=4 Jan 2008|eprint = 0801.0716|class = astro-ph}}</ref>