Editing Exoplanet (section)

== Formation and evolution ==
{{See also|Accretion (astrophysics)|Nebular hypothesis|Planetary migration}}

Planets may form within a few to tens (or more) of millions of years of their star forming.<ref name=ddl2011>{{cite book|last=D'Angelo|first=G.|author2=Durisen, R. H. |author3=Lissauer, J. J.|chapter=Giant Planet Formation |bibcode=2010exop.book..319D| title=Exoplanets |publisher=University of Arizona Press, Tucson, AZ| editor=S. Seager. |pages=319–346|date=2011|chapter-url=http://www.uapress.arizona.edu/Books/bid2263.htm|arxiv=1006.5486 }}</ref><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>
The planets of the [[Solar System]] can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young [[protoplanetary disk]]s where planets are still forming<ref>{{cite journal|last1=Calvet|first1=Nuria|author1-link=Nuria Calvet|last2=D'Alessio|first2=Paola|last3=Hartmann|first3=Lee|last4=Wilner|first4=David|last5=Walsh|first5=Andrew|last6=Sitko|first6=Michael|title=Evidence for a developing gap in a 10 Myr old protoplanetary disk|journal=The Astrophysical Journal|date=2001|volume=568|issue=2|pages=1008–1016|doi=10.1086/339061|bibcode=2002ApJ...568.1008C|arxiv=astro-ph/0201425|s2cid=8706944}}</ref> to planetary systems of over 10 Gyr old.<ref>{{cite journal|last1=Fridlund|first1=Malcolm|last2=Gaidos|first2=Eric|last3=Barragán|first3=Oscar|last4=Persson|first4=Carina|last5=Gandolfi|first5=Davide|last6=Cabrera|first6=Juan|last7=Hirano|first7=Teruyuki|last8=Kuzuhara|first8=Masayuki|last9=Csizmadia|first9=Sz|last10=Nowak|first10=Grzegorz|last11=Endl|first11=Michael|last12=Grziwa|first12=Sascha|last13=Korth|first13=Judith|last14=Pfaff|first14=Jeremias|last15=Bitsch|first15=Bertram|last16=Johansen|first16=Anders|last17=Mustill|first17=Alexander|last18=Davies|first18=Melvyn|last19=Deeg|first19=Hans|last20=Palle|first20=Enric|last21=Cochran|first21=William|last22=Eigmüller|first22=Philipp|last23=Erikson|first23=Anders|last24=Guenther|first24=Eike|last25=Hatzes|first25=Artie|last26=Kiilerich|first26=Amanda|last27=Kudo|first27=Tomoyuki|last28=MacQueen|first28=Philipp|last29=Narita|first29=Norio|last30=Nespral|first30=David|last31=Pätzold|first31=Martin|last32=Prieto-Arranz|first32=Jorge|last33=Rauer|first33=Heike|last34=van Eylen|first34=Vincent|title=EPIC210894022b −A short period super-Earth transiting a metal poor, evolved old star|journal=Astronomy & Astrophysics|volume=604|pages=A16|date=28 April 2017|arxiv=1704.08284|doi=10.1051/0004-6361/201730822|s2cid=39412906}}</ref> When planets form in a gaseous protoplanetary disk,<ref name=dangelo_bodenheimer_2016>{{Cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |title=In Situ and Ex Situ Formation Models of Kepler 11 Planets|journal=The Astrophysical Journal|year=2016|volume=828|issue=1|pages=id. 33 (32 pp.)|doi=10.3847/0004-637X/828/1/33|arxiv = 1606.08088 |bibcode = 2016ApJ...828...33D |s2cid=119203398 |doi-access=free }}</ref> they accrete [[hydrogen]]/[[helium]] envelopes.<ref name=dangelo_bodenheimer_2013>{{cite journal|last=D'Angelo|first=G.|author2= Bodenheimer, P. |title=Three-Dimensional Radiation-Hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks|journal=[[The Astrophysical Journal]]|year=2013|volume=778|issue=1|pages=77 (29 pp.)|doi=10.1088/0004-637X/778/1/77|arxiv = 1310.2211 |bibcode = 2013ApJ...778...77D |s2cid=118522228}}</ref><ref name=dangelo2014>{{cite journal|last=D'Angelo|first=G.|author2=Weidenschilling, S. J. |author3=Lissauer, J. J. |author4=Bodenheimer, P. |title=Growth of Jupiter: Enhancement of core accretion by a voluminous low-mass envelope|journal=Icarus|date=2014|volume=241|pages=298–312|arxiv=1405.7305|doi=10.1016/j.icarus.2014.06.029|bibcode=2014Icar..241..298D|s2cid=118572605}}</ref> These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.<ref name=dangelo_bodenheimer_2016 /> This means that even terrestrial planets may start off with large radii if they form early enough.<ref>{{Cite journal | doi = 10.1093/mnras/stu085 |arxiv=1401.2765 |title=Origin and loss of nebula-captured hydrogen envelopes from 'sub'- to 'super-Earths' in the habitable zone of Sun-like stars |url=https://www.researchgate.net/publication/260647400 |journal=Monthly Notices of the Royal Astronomical Society |volume=439 |issue=4 |pages=3225–3238 |year=2014 |last1=Lammer |first1=H. |last2=Stokl |first2=A. |last3=Erkaev |first3=N. V. |last4=Dorfi |first4=E. A. |last5=Odert |first5=P. |last6=Gudel |first6=M. |last7=Kulikov |first7=Y. N. |last8=Kislyakova |first8=K. G. |last9=Leitzinger |first9=M. |doi-access=free |bibcode=2014MNRAS.439.3225L
|s2cid=118620603}}</ref><ref>{{cite journal |arxiv=1001.0917 |last1=Johnson |first1=R. E. |title=Thermally-Diven Atmospheric Escape |journal=The Astrophysical Journal |volume= 716 |issue= 2 |pages=1573–1578 |year=2010 |doi=10.1088/0004-637X/716/2/1573 |bibcode=2010ApJ...716.1573J |s2cid= 36285464}}</ref><ref>{{cite journal |arxiv=1006.0021|bibcode = 2010Icar..210..539Z |doi = 10.1016/j.icarus.2010.07.013 |volume=210 |issue=2 |title=Atmospheric mass loss by stellar wind from planets around main sequence M stars |journal=Icarus |pages=539–544 |year=2010 |last1=Zendejas |first1=J. |last2=Segura |first2=A. |last3=Raga |first3=A.C. |s2cid=119243879}}</ref> An example is [[Kepler-51b]] which has only about twice the mass of Earth but is almost the size of Saturn, which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.<ref name="kepler-51">{{Cite journal |doi=10.1088/0004-637X/783/1/53| title = Very Low Density Planets Around Kepler-51 Revealed with Transit Timing Variations and an Anomaly Similar to a Planet-Planet Eclipse Event |journal=The Astrophysical Journal |volume=783 |issue=1 |page=53 |year=2014 |last1=Masuda |first1=K. |bibcode=2014ApJ...783...53M |arxiv=1401.2885 |s2cid=119106865}}</ref>