Editing Planetary migration (section)

=== Gravitational scattering ===
{{main|Gravitational scattering}}

Another possible mechanism that may move planets over large orbital radii is <em>gravitational scattering</em> by larger planets or, in a protoplanetary disk, gravitational scattering by over-densities in the fluid of the disk.<ref name="cloutier">{{cite journal |author=R. Cloutier |author2=M-K. Lin |title=Orbital migration of giant planets induced by gravitationally unstable gaps: the effect of planet mass |journal=Monthly Notices of the Royal Astronomical Society |arxiv=1306.2514 |date=2013 |bibcode=2013MNRAS.434..621C |doi=10.1093/mnras/stt1047 |volume=434 |issue=1 |pages=621–632|doi-access=free |s2cid=118322844 }}</ref> In the case of the [[Solar System]], Uranus and Neptune may have been gravitationally scattered onto larger orbits by close encounters with Jupiter and/or Saturn.<ref name="thommes">{{cite journal |author=E. W. Thommes |author2=M. J. Duncan |author3=H. F. Levison |title=The Formation of Uranus and Neptune among Jupiter and Saturn |journal=Astronomical Journal |arxiv=astro-ph/0111290 |date=2002 |volume=123 |issue=5 |pages=2862 |doi=10.1086/339975 |bibcode=2002AJ....123.2862T|s2cid=17510705 }}</ref><ref name="Gomes" /> Systems of exoplanets can undergo similar dynamical instabilities following the dissipation of the gas disk that alter their orbits and in some cases result in planets being ejected or colliding with the star.

Planets scattered gravitationally can end on highly eccentric orbits with perihelia close to the star, enabling their orbits to be altered by the tides they raise on the star. The eccentricities and inclinations of these planets are also excited during these encounters, providing one possible explanation for the observed eccentricity distribution of the closely orbiting exoplanets.<ref name="Ford_Rasio_2008">{{cite journal |last1=Ford |first1=Eric B. |last2=Rasio |first2=Frederic A. |title=Origins of Eccentric Extrasolar Planets: Testing the Planet-Planet Scattering Model |journal=The Astrophysical Journal |date=2008 |volume=686 |issue=1 |pages=621–636|doi=10.1086/590926 |arxiv=astro-ph/0703163 |bibcode=2008ApJ...686..621F|s2cid=15533202 }}</ref> The resulting systems are often near the limits of stability.<ref name="Raymond_etal_2009">{{cite journal |last1=Raymond |first1=Sean N. |last2=Barnes |first2=Rory |last3=Veras |first3=Dimitri |last4=Armitage |first4=Phillip J. |last5=Gorelick |first5=Noel |last6=Greenberg |first6=Richard |title=Planet-Planet Scattering Leads to Tightly Packed Planetary Systems |journal=The Astrophysical Journal Letters |date=2009 |volume=696 |issue=1 |pages=L98–L101 |doi=10.1088/0004-637X/696/1/L98 |arxiv=0903.4700 |bibcode=2009ApJ...696L..98R|s2cid=17590159 }}</ref> As in the Nice model, systems of exoplanets with an outer disk of planetesimals can also undergo dynamical instabilities following resonance crossings during planetesimal-driven migration. The eccentricities and inclinations of the planets on distant orbits can be damped by [[dynamical friction]] with the planetesimals with the final values depending on the relative masses of the disk and the planets that had gravitational encounters.<ref name="Raymond_etal_2010">{{cite journal |last1=Raymond |first1=Sean N. |last2=Armitage |first2=Philip J. |last3=Gorelick |first3=Noel |title=Planet-Planet Scattering in Planetesimal Disks: II. Predictions for Outer Extrasolar Planetary Systems |journal=The Astrophysical Journal |date=2010 |volume=711 |issue=2 |pages=772–795 |doi=10.1088/0004-637X/711/2/772|arxiv=1001.3409 |bibcode=2010ApJ...711..772R|s2cid=118622630 }}</ref>