View source for Astronomy:Planetary-mass object

{{Short description|Size-based definition of celestial objects}}
[[File:25 solar system objects smaller than Earth.jpg|thumb|upright=1.6|The planetary-mass moons to scale, compared with Mercury, Venus, Earth, Mars, and Pluto (the other planetary-mass objects beyond Neptune have never been imaged up close). Borderline [[Astronomy:Proteus (moon)|Proteus]] and Nereid (about the same size as round Mimas) have been included. Unimaged [[Astronomy:Dysnomia (moon)|Dysnomia]] (intermediate in size between Tethys and Enceladus) is not shown; it is in any case probably not a solid body.<ref name="Brown2023">{{cite journal
  |first1 = Michael E. |last1 = Brown
  |first2 = Bryan |last2 = Butler
  |title      = Masses and densities of dwarf planet satellites measured with ALMA
  |journal    = The Planetary Science Journal
  |date       = July 2023
  |volume     = in press
  |issue      = 
  |id         = 
  |pages      = 11
  |doi-access = 
  |doi        = 
  |arxiv      = 2307.04848
  |bibcode    = 
  |s2cid      = }}</ref>]]

A '''planetary-mass object''' ('''PMO'''), '''planemo''',<ref name="Weintraub2014">{{cite book
| first1=David A. | last1=Weintraub | year=2014 | page=226
| title=Is Pluto a Planet?: A Historical Journey through the Solar System
| publisher=Princeton University Press | isbn=978-1400852970
| url=https://books.google.com/books?id=dW1_AwAAQBAJ&pg=PA226
}}</ref> or '''planetary body''' is, by [[Astronomy:Geophysical definition of planet|geophysical definition of celestial objects]], any celestial object massive enough to achieve [[Astronomy:Hydrostatic equilibrium|hydrostatic equilibrium]], but not enough to sustain core fusion like a [[Astronomy:Star|star]].<ref name=Basri_Brown_2006>{{cite journal
| first1=Gibor | last1=Basri
| first2=E. M. | last2=Brown
| title=Planetesimals to Brown Dwarfs: What is a Planet?
| journal=Annual Review of Earth and Planetary Sciences
| volume=34 | pages=193–216 | date=May 2006
| doi=10.1146/annurev.earth.34.031405.125058
| bibcode=2006AREPS..34..193B
|arxiv = astro-ph/0608417 | s2cid=119338327
}}</ref><ref name=Stern_Levison_2002>{{cite journal
| last1=Stern | first1=S. Alan
| last2=Levison | first2=Harold F.
| editor1-first=H. | editor1-last=Rickman
| title=Regarding the criteria for planethood and proposed planetary classification schemes
| journal=Highlights of Astronomy | volume=12
| page=208 | date=2002
| publisher=Astronomical Society of the Pacific
| location=San Francisco, CA
| bibcode=2002HiA....12..205S
| isbn=978-1-58381-086-6
| doi=10.1017/S1539299600013289
| doi-access=free
}}</ref>

The purpose of this term is to classify together a broader range of celestial objects than '[[Astronomy:Planet|planet]]', since many objects similar in geophysical terms do not conform to conventional expectations for a planet. Planetary-mass objects can be quite diverse in origin and location. They include planets, [[Astronomy:Dwarf planet|dwarf planet]]s, [[Astronomy:Planetary-mass moon|planetary-mass satellites]] and free-floating planets, which may have been ejected from a system ([[Astronomy:Rogue planet|rogue planet]]s) or formed through cloud-collapse rather than accretion ([[Astronomy:Sub-brown dwarf|sub-brown dwarf]]s).

== Usage in astronomy ==
While the term technically includes exoplanets and other objects, it is often used for objects with an uncertain nature or objects that do not fit in one specific class. Cases in which the term is often used:

* [[Astronomy:Rogue planet|isolated planetary-mass objects]] (iPMO; IPMO) are objects that are free-floating and have a low mass below deuterium burning and their nature as either an ejected free-floating planets or sub-brown dwarfs is not fully resolved (e.g. 2MASS J13243553+6358281,<ref>{{Cite journal |last1=Gagné |first1=Jonathan |last2=Allers |first2=Katelyn N. |last3=Theissen |first3=Christopher A. |last4=Faherty |first4=Jacqueline K. |last5=Bardalez Gagliuffi |first5=Daniella |last6=Artigau |first6=Étienne |date=2018-02-01 |title=2MASS J13243553+6358281 Is an Early T-type Planetary-mass Object in the AB Doradus Moving Group |journal=The Astrophysical Journal |volume=854 |issue=2 |pages=L27 |doi=10.3847/2041-8213/aaacfd |arxiv=1802.00493 |bibcode=2018ApJ...854L..27G |issn=0004-637X |doi-access=free }}</ref> PSO J060.3200+25.9644<ref>{{Cite journal |last1=Best |first1=William M. J. |last2=Liu |first2=Michael C. |last3=Magnier |first3=Eugene A. |last4=Bowler |first4=Brendan P. |last5=Aller |first5=Kimberly M. |last6=Zhang |first6=Zhoujian |last7=Kotson |first7=Michael C. |last8=Burgett |first8=W. S. |last9=Chambers |first9=K. C. |last10=Draper |first10=P. W. |last11=Flewelling |first11=H. |last12=Hodapp |first12=K. W. |last13=Kaiser |first13=N. |last14=Metcalfe |first14=N. |last15=Wainscoat |first15=R. J. |date=2017-03-01 |title=A Search for L/T Transition Dwarfs with Pan-STARRS1 and WISE. III. Young L Dwarf Discoveries and Proper Motion Catalogs in Taurus and Scorpius-Centaurus |journal=The Astrophysical Journal |volume=837 |issue=1 |pages=95 |doi=10.3847/1538-4357/aa5df0 |arxiv=1702.00789 |bibcode=2017ApJ...837...95B |issn=0004-637X |doi-access=free }}</ref> objects in [[Astronomy:NGC 1333|NGC 1333]]<ref>{{Cite journal |last1=Scholz |first1=Aleks |last2=Muzic |first2=Koraljka |last3=Jayawardhana |first3=Ray |last4=Almendros-Abad |first4=Victor |last5=Wilson |first5=Isaac |date=2023-05-01 |title=Disks around Young Planetary-mass Objects: Ultradeep Spitzer Imaging of NGC 1333 |journal=The Astronomical Journal |volume=165 |issue=5 |pages=196 |doi=10.3847/1538-3881/acc65d |arxiv=2303.12451 |bibcode=2023AJ....165..196S |issn=0004-6256 |doi-access=free }}</ref>)
* Objects with a mass range at the border of deuterium burning ([[Astronomy:VHS J1256–1257|VHS 1256-1257 b]],<ref>{{Cite journal |last1=Miles |first1=Brittany E. |last2=Biller |first2=Beth A. |last3=Patapis |first3=Polychronis |last4=Worthen |first4=Kadin |last5=Rickman |first5=Emily |last6=Hoch |first6=Kielan K. W. |last7=Skemer |first7=Andrew |last8=Perrin |first8=Marshall D. |last9=Whiteford |first9=Niall |last10=Chen |first10=Christine H. |last11=Sargent |first11=B. |last12=Mukherjee |first12=Sagnick |last13=Morley |first13=Caroline V. |last14=Moran |first14=Sarah E. |last15=Bonnefoy |first15=Mickael |date=2023-03-01 |title=The JWST Early-release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 μm Spectrum of the Planetary-mass Companion VHS 1256-1257 b |journal=The Astrophysical Journal |volume=946 |issue=1 |pages=L6 |doi=10.3847/2041-8213/acb04a |arxiv=2209.00620 |bibcode=2023ApJ...946L...6M |issn=0004-637X |doi-access=free }}</ref> [[Astronomy:BD+60 1417b|BD+60 1417b]]<ref>{{Cite journal |last1=Faherty |first1=Jacqueline K. |last2=Gagné |first2=Jonathan |last3=Popinchalk |first3=Mark |last4=Vos |first4=Johanna M. |last5=Burgasser |first5=Adam J. |last6=Schümann |first6=Jörg |last7=Schneider |first7=Adam C. |last8=Kirkpatrick |first8=J. Davy |last9=Meisner |first9=Aaron M. |last10=Kuchner |first10=Marc J. |last11=Bardalez Gagliuffi |first11=Daniella C. |last12=Marocco |first12=Federico |last13=Caselden |first13=Dan |last14=Gonzales |first14=Eileen C. |last15=Rothermich |first15=Austin |date=2021-12-01 |title=A Wide Planetary Mass Companion Discovered through the Citizen Science Project Backyard Worlds: Planet 9 |journal=The Astrophysical Journal |volume=923 |issue=1 |pages=48 |doi=10.3847/1538-4357/ac2499 |arxiv=2112.04678 |bibcode=2021ApJ...923...48F |issn=0004-637X |doi-access=free }}</ref>)
* Objects that orbit a star or brown dwarf, but its formation as exoplanets is challenging or impossible (VHS 1256-1257 b, CFHTWIR-Oph 98B<ref>{{Cite journal |last1=Fontanive |first1=Clémence |last2=Allers |first2=Katelyn N. |last3=Pantoja |first3=Blake |last4=Biller |first4=Beth |last5=Dubber |first5=Sophie |last6=Zhang |first6=Zhoujian |last7=Dupuy |first7=Trent |last8=Liu |first8=Michael C. |last9=Albert |first9=Loïc |date=2020-12-01 |title=A Wide Planetary-mass Companion to a Young Low-mass Brown Dwarf in Ophiuchus |journal=The Astrophysical Journal |volume=905 |issue=2 |pages=L14 |doi=10.3847/2041-8213/abcaf8 |arxiv=2011.08871 |bibcode=2020ApJ...905L..14F |issn=0004-637X |doi-access=free }}</ref>)

==Types==
=== Planetary-mass satellite ===
{{Main|Astronomy:Planetary-mass moon}}
[[File:Large Moons (4089199369).jpg|thumb|300px|Planetary-mass satellites larger than Pluto, the largest Solar dwarf planet.]]

The three largest satellites [[Astronomy:Ganymede (moon)|Ganymede]], [[Astronomy:Titan (moon)|Titan]], and [[Astronomy:Callisto (moon)|Callisto]] are of similar size or larger than the planet [[Astronomy:Mercury (planet)|Mercury]]; these and four more – [[Astronomy:Io (moon)|Io]], [[Astronomy:Moon|Moon]], [[Astronomy:Europa (moon)|Europa]], and [[Astronomy:Triton (moon)|Triton]] – are larger and more massive than the largest and most massive dwarf planets, [[Astronomy:Pluto|Pluto]] and [[Astronomy:Eris (dwarf planet)|Eris]]. Another dozen smaller satellites are large enough to have become round at some point in their history through their own gravity, tidal heating from their parent planets, or both. In particular, Titan has a thick atmosphere and stable bodies of liquid on its surface, like Earth (though for Titan the liquid is [[Chemistry:Methane|methane]] rather than water). Proponents of the geophysical definition of planets argue that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet. The term ''satellite planet'' is sometimes used for planet-sized satellites.<ref name="satelliteplanet">{{cite web |last=Villard |first=Ray |date=2010-05-14 |title=Should Large Moons Be Called 'Satellite Planets'? |url=http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |archive-url=https://web.archive.org/web/20100516201931/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |archive-date=2010-05-16 |access-date=2011-11-04 |publisher=Discovery News}}</ref>

=== Dwarf planets ===
[[File:Pluto_in_True_Color_-_High-Res.jpg|thumb|right|The dwarf planet Pluto]]{{Main|Dwarf planet}} A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. Planetary scientist and ''New Horizons'' principal investigator Alan Stern, who proposed the term 'dwarf planet', has argued that location should not matter and that only geophysical attributes should be taken into account, and that dwarf planets are thus a subtype of planet. The [[Organization:International Astronomical Union|International Astronomical Union]] (IAU) accepted the term (rather than the more neutral 'planetoid') but decided to classify dwarf planets as a separate category of object.<ref>{{cite web |url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |title=Resolution B5 Definition of a Planet in the Solar System |work=IAU 2006 General Assembly |publisher=International Astronomical Union |access-date=January 26, 2008}}</ref>
=== Planets and exoplanets===
{{Excerpt|Planet|paragraphs=1|only=paragraphs}}

=== Former stars ===
In close [[Astronomy:Binary star|binary star]] systems, one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar [[Astronomy:PSR J1719−1438|PSR J1719−1438]].<ref>{{cite journal |arxiv=1108.5201 |bibcode=2011Sci...333.1717B |doi=10.1126/science.1208890 |title=Transformation of a Star into a Planet in a Millisecond Pulsar Binary |date=2011 |last1=Bailes |first1=M. |display-authors=4 |last2=Bates |first2=S. D. |last3=Bhalerao |first3=V. |last4=Bhat |first4=N. D. R. |last5=Burgay |first5=M. |last6=Burke-Spolaor |first6=S. |last7=d'Amico |first7=N. |last8=Johnston |first8=S. |last9=Keith |first9=M. J. |journal=Science |volume=333 |issue=6050 |pages=1717–20 |pmid=21868629|s2cid=206535504 }}</ref> These shrunken white dwarfs may become a [[Astronomy:Helium planet|helium planet]] or [[Astronomy:Carbon planet|carbon planet]].

=== Sub-brown dwarfs ===
{{Main|Astronomy:Sub-brown dwarf}}

[[File:Artist's View of a Super-Jupiter around a Brown Dwarf (2M1207).jpg|thumb|Artist's impression of a super-Jupiter around the brown dwarf [[Astronomy:2M1207|2M1207]].<ref>{{cite web |date=19 February 2016 |title=Artist's View of a Super-Jupiter around a Brown Dwarf (2M1207) |url=http://www.spacetelescope.org/images/opo1605a/ |url-status=live |archive-url=https://web.archive.org/web/20210417134149/https://esahubble.org/images/opo1605a/ |archive-date=Apr 17, 2021 |access-date=22 February 2016 |website=ESA/Hubble}}</ref>]]

Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as [[Astronomy:Cha 110913−773444|Cha 110913−773444]]<ref name="Luhman">{{cite journal | journal=Astrophysical Journal |last1=Luhman |first1=K. L. |author2=Adame, Lucía |author3=D'Alessio, Paola |author4=Calvet, Nuria|author4-link=Nuria Calvet |title= Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk |volume=635 | issue=1 |pages=L93 |doi=10.1086/498868 |date= 2005 |bibcode=2005ApJ...635L..93L|arxiv = astro-ph/0511807 |s2cid=11685964}}
*{{cite press release |author=Whitney Clavin |date=2005-11-29 |title=A Planet With Planets? Spitzer Finds Cosmic Oddball |url=http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html |website=NASA |access-date=2022-07-29 |archive-date=2012-10-11 |archive-url=https://web.archive.org/web/20121011011111/http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html |url-status=dead }}</ref> and [[Astronomy:OTS 44|OTS 44]],<ref name=joergens2013_AA558>{{cite journal|last1=Joergens|first1=V.|display-authors=4|last2=Bonnefoy|first2=M.|last3=Liu|first3=Y.|last4=Bayo|first4=A.|last5=Wolf|first5=S.|last6=Chauvin|first6=G.|last7=Rojo|first7=P.|title=OTS 44: Disk and accretion at the planetary border|journal=Astronomy & Astrophysics|volume=558|number=7|date=2013|doi=10.1051/0004-6361/201322432|bibcode=2013A&A...558L...7J|arxiv = 1310.1936|pages=L7|s2cid=118456052}}</ref> or orbiting a larger object such as [[Astronomy:2MASS J04414489+2301513|2MASS J04414489+2301513]].

Binary systems of sub-brown dwarfs are theoretically possible; [[Astronomy:Oph 162225-240515|Oph 162225-240515]] was initially thought to be a binary system of a [[Astronomy:Brown dwarf|brown dwarf]] of 14 Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.<ref>{{cite journal | title=The Wide Brown Dwarf Binary Oph 1622–2405 and Discovery of A Wide, Low Mass Binary in Ophiuchus (Oph 1623–2402): A New Class of Young Evaporating Wide Binaries? |journal= Astrophysical Journal |author=Close, Laird M. |volume=660 | issue=2 |pages=1492–1506 |doi=10.1086/513417 |date=2007 |arxiv=astro-ph/0608574 |bibcode=2007ApJ...660.1492C | display-authors=4 | last2=Zuckerman | first2=B. | last3=Song | first3=Inseok | last4=Barman | first4=Travis | last5=Marois | first5=Christian | last6=Rice | first6=Emily L. | last7=Siegler | first7=Nick | last8=MacIntosh | first8=Bruce | last9=Becklin | first9=E. E. |s2cid= 15170262 }}</ref><ref>{{cite journal|last1=Luhman |first1=Kevin L. |last2=Allers |first2=Katelyn N. |last3=Jaffe |first3=Daniel T. |last4=Cushing |first4=Michael C. |last5=Williams |first5=Kurtis A. |last6=Slesnick |first6=Catherine L. |last7=Vacca |first7=William D. |date=April 2007 |journal=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |title=Ophiuchus 1622-2405: Not a Planetary-Mass Binary |volume=659 |issue=2 |pages=1629–1636 |url=http://www.iop.org/EJ/article/0004-637X/659/2/1629/70185.html |doi=10.1086/512539 |bibcode=2007ApJ...659.1629L |arxiv=astro-ph/0701242 |s2cid=11153196 }}</ref><ref>{{cite web |last=Britt |first=Robert Roy |date=2004-09-10 |title=Likely First Photo of Planet Beyond the Solar System |url=http://www.space.com/scienceastronomy/planet_photo_040910.html |url-status=live |archive-url=https://web.archive.org/web/20110127192049/https://www.space.com/326-photo-planet-solar-system.html |archive-date=Jan 27, 2011 |access-date=2008-08-23 |work=Space}}</ref>

=== Captured planets ===
[[Astronomy:Rogue planet|Rogue planet]]s in [[Astronomy:Open cluster|stellar clusters]] have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and <!--abstract seems to have a typo 10^6: rest of paper uses 10^5-->10<sup>5</sup> AU<!--see preceding comment-->. The capture efficiency decreases with increasing cluster volume, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system.<ref>[https://arxiv.org/abs/1202.2362 On the origin of planets at very wide orbits from the re-capture of free floating planets] {{Webarchive|url=https://web.archive.org/web/20220412011944/https://arxiv.org/abs/1202.2362 |date=2022-04-12 }}, Hagai B. Perets, M. B. N. Kouwenhoven, 2012</ref>

=== Rogue planets ===
{{Main|Astronomy:Rogue planet}}

Several [[Computer simulation|computer simulation]]s of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.<ref>{{cite journal | last=Lissauer | first= J. J. | title= Timescales for Planetary Accretion and the Structure of the Protoplanetary disk | journal= Icarus | volume= 69 | issue=2 | pages=249–265 | date=1987 | doi=10.1016/0019-1035(87)90104-7 | bibcode=1987Icar...69..249L| hdl= 2060/19870013947 | hdl-access=free }}</ref> 
Such objects are typically called ''rogue planets''.

==See also==
* [[Astronomy:Planetary mass|Planetary mass]]
* List of gravitationally rounded objects of the Solar System
* [[Astronomy:List of Solar System objects by size|List of Solar System objects by size]]

==References==
{{Reflist}}



[[Category:Planets]]

{{Sourceattribution|Planetary-mass object}}