Editing Planetary migration (section)

=== Gas disk ===
Observations suggest that gas in protoplanetary disks orbiting young stars have lifetimes of a few to several million years.<ref name=ep2017>{{cite journal |title=The dispersal of planet-forming discs: theory confronts observations|journal=Royal Society Open Science|date=2017|volume=4|issue=2|page=170114 |doi= 10.1098/rsos.170114 |arxiv = 1704.00214 |bibcode = 2017RSOS....470114E |last1=Ercolano| first1=B.| last2=Pascucci| first2=I.|pmid=28484640 |pmc=5414277 }}</ref>  If planets with masses of around an Earth mass or greater form while the gas is still present, the planets can exchange [[angular momentum]] with the surrounding gas in the [[protoplanetary disk]] so that their orbits change gradually. Although the sense of migration is typically inwards in locally isothermal disks, outward migration may occur in disks that possess entropy gradients.<ref name=dl2018>{{cite book|last=D'Angelo|first=G.|author2=Lissauer, J. J.|chapter=Formation of Giant Planets |bibcode=2018haex.bookE.140D| title=Handbook of Exoplanets |publisher=Springer International Publishing AG, part of Springer Nature| editor=Deeg H., Belmonte J. |pages= 2319–2343|date=2018|arxiv=1806.05649|doi=10.1007/978-3-319-55333-7_140|isbn=978-3-319-55332-0|s2cid=116913980}}</ref>