Editing Planetary migration (section)

===Kozai cycles and tidal friction===
{{See also|Kozai mechanism}}
A planetary orbit that is inclined relative to the plane of a binary star can shrink due to a combination of Kozai cycles and tidal friction. Interactions with the more distant star cause the planet's orbit to undergo an exchange of eccentricity and inclination due to the Kozai mechanism. This process can increase the planet's eccentricity and lower its perihelion enough to create strong tides between the planet on the star increases. When it is near the star the planet loses angular momentum causing its orbit to shrink. 

The planet's eccentricity and inclination cycle repeatedly, slowing the evolution of the planets semi-major axis.<ref name="Fabrycky_Tremaine_2007">{{cite journal |last1=Fabrycky |first1=Daniel |last2=Tremaine |first2=Scott |title=Shrinking Binary and Planetary Orbits by Kozai Cycles with Tidal Friction |journal=The Astrophysical Journal |date=2007 |volume=669 |issue=2 |pages=1298–1315 |doi=10.1086/521702 |arxiv=0705.4285 |bibcode=2007ApJ...669.1298F|s2cid=12159532 }}</ref> If the planet's orbit shrinks enough to remove it from the influence of the distant star the Kozai cycles end. Its orbit will then shrink more rapidly as it is tidally circularized. The orbit of the planet can also become retrograde due to this process. Kozai cycles can also occur in a system with two planets that have differing inclinations due to gravitational scattering between planets and can result in planets with retrograde orbits.<ref name="Noaz_etal_2011">{{cite journal |last1=Naoz |first1=Smadar |last2=Farr |first2=Will M. |last3=Lithwick |first3=Yoram |last4=Rasio |first4=Frederic A. |last5=Teyssandier |first5=Jean|title=Hot Jupiters from secular planet-planet interactions |journal=Nature |date=2011 |volume=473 |issue=7346 |pages=187–189 |doi=10.1038/nature10076 |arxiv=1011.2501 |bibcode=2011Natur.473..187N |pmid=21562558|s2cid=4424942 }}</ref><ref name="Nagasawa_etal_2008">{{cite journal |last1=Nagasawa |first1=M. |last2=Ida |first2=S. |last3=Bessho |first3=T. |title=Formation of Hot Planets by a Combination of Planet Scattering, Tidal Circularization, and the Kozai Mechanism |journal=The Astrophysical Journal |date=2008 |volume=678 |issue=1 |pages=498–508 |doi=10.1086/529369 |arxiv=0801.1368 |bibcode=2008ApJ...678..498N|s2cid=14210085 }}</ref>