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{{Not to be confused|Sub-Neptune}}]
A '''mini-Neptune''' (sometimes known as a '''gas dwarf''' or '''transitional planet''') is a planet less massive than ]. but resembles Neptune in that it has a thick ]] atmosphere, probably with deep layers of ice, rock or liquid oceans (made of ], ], a mixture of both, or heavier volatiles).<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|doi=10.3847/0004-637X/828/1/33|arxiv = 1606.08088 |bibcode = 2016ApJ...828...33D }}</ref>
{{short description|Planet smaller than Neptune with a gas atmosphere}}
A '''Mini-Neptune''' (sometimes known as a '''gas dwarf''' or '''transitional planet''') is a planet less massive than ] but resembling Neptune in that it has a thick ]-] atmosphere, probably with deep layers of ice, rock or liquid oceans (made of ], ], a mixture of both, or heavier volatiles).<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|doi=10.3847/0004-637X/828/1/33|arxiv = 1606.08088 |bibcode = 2016ApJ...828...33D |s2cid=119203398 |doi-access=free }}</ref>


A '''gas dwarf''' is a ] with a ] that has accumulated a thick envelope of hydrogen, helium, and other ], having as result a total radius between 1.7 and 3.9 ] ({{Earth radius|1.7–3.9}}). The term is used in a three-tier, ]-based classification regime for short-period ]s, which also includes the rocky, ] with less than {{Earth radius|1.7}} and planets greater than {{Earth radius|3.9}}, namely ]s and ]s.<ref>, Buchhave et al.</ref> A gas dwarf is a ] with a ] that has accumulated a thick envelope of hydrogen, helium, and other ], having, as a result, a total radius between 1.7 and 3.9 ] ({{Earth radius|1.7–3.9}}). The term is used in a three-tier, ]-based classification regime for short-period ]s, which also includes the rocky, ] with less than {{Earth radius|1.7}} and planets greater than {{Earth radius|3.9}}, namely ]s and ]s.<ref>, Buchhave et al.</ref>


==Properties== ==Properties==
Theoretical studies of such planets are loosely based on knowledge about Uranus and Neptune. Without a thick atmosphere, it would be classified as an ] instead.<ref>, E.J.W. de Mooij (1), M. Brogi (1), R.J. de Kok (2), J. Koppenhoefer (3,4), S.V. Nefs (1), I.A.G. Snellen (1), J. Greiner (4), J. Hanse (1), R.C. Heinsbroek (1), C.H. Lee (3), P.P. van der Werf (1),</ref> An estimated dividing line between a rocky planet and a gaseous planet is around 1.6-2.0 Earth radii.<ref>, Daniel C. Fabrycky, Jack J. Lissauer, Darin Ragozzine, Jason F. Rowe, Eric Agol, Thomas Barclay, Natalie Batalha, William Borucki, David R. Ciardi, Eric B. Ford, John C. Geary, Matthew J. Holman, Jon M. Jenkins, Jie Li, Robert C. Morehead, Avi Shporer, Jeffrey C. Smith, Jason H. Steffen, Martin Still</ref><ref>, blogs.scientificamerican.com, 20 June 2012</ref> Planets with larger radii and measured masses are mostly low-density and require an extended atmosphere to simultaneously explain their masses and radii, and observations are showing that planets larger than approximately 1.6 Earth-radius (and more massive than approximately 6 Earth-masses) contain significant amounts of volatiles or H–He gas, likely acquired during formation.<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=]|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 }}</ref><ref name="dangelo_bodenheimer_2016"/> Such planets appear to have a diversity of compositions that is not well-explained by a single mass–radius relation as that found for denser, rocky planets.<ref>Benjamin J. Fulton et al. "</ref><ref>Courtney D. Dressing et al. ""</ref><ref>Leslie A. Rogers ""</ref> Similar results are confirmed by other studies.<ref>Lauren M. Weiss, and Geoffrey W. Marcy. ""</ref><ref>Geoffrey W. Marcy, Lauren M. Weiss, Erik A. Petigura, Howard Isaacson, Andrew W. Howard and Lars A. Buchhave. ""</ref><ref>Geoffrey W. Marcy et al. ""</ref> As for mass, the lower limit can vary widely for different planets depending on their compositions; the dividing mass can vary from as low as one to as high as 20 {{Earth mass}}. Theoretical studies of such planets are loosely based on knowledge about Uranus and Neptune. Without a thick atmosphere, they would be classified as an ] instead.<ref>{{cite journal | arxiv=1111.2628 | doi=10.1051/0004-6361/201117205 | title=Optical to near-infrared transit observations of super-Earth GJ 1214b: Water-world or mini-Neptune? | date=2012 | last1=De Mooij | first1=E. J. W. | last2=Brogi | first2=M. | last3=De Kok | first3=R. J. | last4=Koppenhoefer | first4=J. | last5=Nefs | first5=S. V. | last6=Snellen | first6=I. A. G. | last7=Greiner | first7=J. | last8=Hanse | first8=J. | last9=Heinsbroek | first9=R. C. | last10=Lee | first10=C. H. | last11=Van Der Werf | first11=P. P. | journal=Astronomy & Astrophysics | volume=538 | pages=A46 | bibcode=2012A&A...538A..46D }}</ref> An estimated dividing line between a rocky planet and a gaseous planet is around 1.6–2.0 Earth radii.<ref>{{cite journal | arxiv=1202.6328 | doi=10.1088/0004-637X/790/2/146 | title=ARCHITECTURE OF ''KEPLER'' 'S MULTI-TRANSITING SYSTEMS. II. NEW INVESTIGATIONS WITH TWICE AS MANY CANDIDATES | date=2014 | last1=Fabrycky | first1=Daniel C. | last2=Lissauer | first2=Jack J. | last3=Ragozzine | first3=Darin | last4=Rowe | first4=Jason F. | last5=Steffen | first5=Jason H. | last6=Agol | first6=Eric | last7=Barclay | first7=Thomas | last8=Batalha | first8=Natalie | last9=Borucki | first9=William | last10=Ciardi | first10=David R. | last11=Ford | first11=Eric B. | last12=Gautier | first12=Thomas N. | last13=Geary | first13=John C. | last14=Holman | first14=Matthew J. | last15=Jenkins | first15=Jon M. | last16=Li | first16=Jie | last17=Morehead | first17=Robert C. | last18=Morris | first18=Robert L. | last19=Shporer | first19=Avi | last20=Smith | first20=Jeffrey C. | last21=Still | first21=Martin | last22=Van Cleve | first22=Jeffrey | journal=The Astrophysical Journal | volume=790 | issue=2 | page=146 | bibcode=2014ApJ...790..146F }}</ref><ref>, blogs.scientificamerican.com, 20 June 2012</ref> Planets with larger radii and measured masses are mostly low-density and require an extended atmosphere to simultaneously explain their masses and radii, and observations show that planets larger than approximately 1.6 Earth-radius (and more massive than approximately 6 Earth-masses) contain significant amounts of volatiles or H–He gas, likely acquired during formation.<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=]|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="dangelo_bodenheimer_2016"/> Such planets appear to have a diversity of compositions that is not well-explained by a single mass–radius relation as that found for denser, rocky planets.<ref>{{cite journal | doi=10.3847/1538-3881/aa80eb | doi-access=free | title=The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets | date=2017 | last1=Fulton | first1=Benjamin J. | last2=Petigura | first2=Erik A. | last3=Howard | first3=Andrew W. | last4=Isaacson | first4=Howard | last5=Marcy | first5=Geoffrey W. | last6=Cargile | first6=Phillip A. | last7=Hebb | first7=Leslie | last8=Weiss | first8=Lauren M. | last9=Johnson | first9=John Asher | last10=Morton | first10=Timothy D. | last11=Sinukoff | first11=Evan | last12=Crossfield | first12=Ian J. M. | last13=Hirsch | first13=Lea A. | journal=The Astronomical Journal | volume=154 | issue=3 | page=109 | arxiv=1703.10375 | bibcode=2017AJ....154..109F }}</ref><ref>{{cite journal | arxiv=1412.8687 | doi=10.1088/0004-637X/800/2/135 | title=THE MASS OF Kepler-93b AND THE COMPOSITION OF TERRESTRIAL PLANETS | date=2015 | last1=Dressing | first1=Courtney D. | last2=Charbonneau | first2=David | last3=Dumusque | first3=Xavier | last4=Gettel | first4=Sara | last5=Pepe | first5=Francesco | last6=Collier Cameron | first6=Andrew | last7=Latham | first7=David W. | last8=Molinari | first8=Emilio | last9=Udry | first9=Stéphane | last10=Affer | first10=Laura | last11=Bonomo | first11=Aldo S. | last12=Buchhave | first12=Lars A. | last13=Cosentino | first13=Rosario | last14=Figueira | first14=Pedro | last15=Fiorenzano | first15=Aldo F. M. | last16=Harutyunyan | first16=Avet | last17=Haywood | first17=Raphaëlle D. | last18=Johnson | first18=John Asher | last19=Lopez-Morales | first19=Mercedes | last20=Lovis | first20=Christophe | last21=Malavolta | first21=Luca | last22=Mayor | first22=Michel | last23=Micela | first23=Giusi | last24=Motalebi | first24=Fatemeh | last25=Nascimbeni | first25=Valerio | last26=Phillips | first26=David F. | last27=Piotto | first27=Giampaolo | last28=Pollacco | first28=Don | last29=Queloz | first29=Didier | last30=Rice | first30=Ken | journal=The Astrophysical Journal | volume=800 | issue=2 | page=135 | bibcode=2015ApJ...800..135D | display-authors=1 }}</ref><ref>{{cite journal | arxiv=1407.4457 | doi=10.1088/0004-637X/801/1/41 | title=''MOST'' 1.6 EARTH-RADIUS PLANETS ARE NOT ROCKY | date=2015 | last1=Rogers | first1=Leslie A. | journal=The Astrophysical Journal | volume=801 | issue=1 | page=41 | bibcode=2015ApJ...801...41R }}</ref><ref>{{cite journal | arxiv=1312.0936 | doi=10.1088/2041-8205/783/1/L6 | title=The Mass-Radius Relation for 65 Exoplanets Smaller Than 4 Earth Radii | date=2014 | last1=Weiss | first1=Lauren M. | last2=Marcy | first2=Geoffrey W. | journal=The Astrophysical Journal | volume=783 | issue=1 | pages=L6 | bibcode=2014ApJ...783L...6W }}</ref><ref>{{cite journal | arxiv=1404.2960 | doi=10.1073/pnas.1304197111 | title=Occurrence and core-envelope structure of 1–4× Earth-size planets around Sun-like stars | date=2014 | last1=Marcy | first1=Geoffrey W. | last2=Weiss | first2=Lauren M. | last3=Petigura | first3=Erik A. | last4=Isaacson | first4=Howard | last5=Howard | first5=Andrew W. | last6=Buchhave | first6=Lars A. | journal=Proceedings of the National Academy of Sciences | volume=111 | issue=35 | pages=12655–12660 | doi-access=free | pmid=24912169 | bibcode=2014PNAS..11112655M }}</ref><ref>{{cite journal | arxiv=1401.4195 | doi=10.1088/0067-0049/210/2/20 | title=MASSES, RADII, AND ORBITS OF SMALL ''KEPLER'' PLANETS: THE TRANSITION FROM GASEOUS TO ROCKY PLANETS | date=2014 | last1=Marcy | first1=Geoffrey W. | last2=Isaacson | first2=Howard | last3=Howard | first3=Andrew W. | last4=Rowe | first4=Jason F. | last5=Jenkins | first5=Jon M. | last6=Bryson | first6=Stephen T. | last7=Latham | first7=David W. | last8=Howell | first8=Steve B. | last9=Gautier | first9=Thomas N. | last10=Batalha | first10=Natalie M. | last11=Rogers | first11=Leslie | last12=Ciardi | first12=David | last13=Fischer | first13=Debra A. | last14=Gilliland | first14=Ronald L. | last15=Kjeldsen | first15=Hans | last16=Christensen-Dalsgaard | first16=Jørgen | last17=Huber | first17=Daniel | last18=Chaplin | first18=William J. | last19=Basu | first19=Sarbani | last20=Buchhave | first20=Lars A. | last21=Quinn | first21=Samuel N. | last22=Borucki | first22=William J. | last23=Koch | first23=David G. | last24=Hunter | first24=Roger | last25=Caldwell | first25=Douglas A. | last26=Van Cleve | first26=Jeffrey | last27=Kolbl | first27=Rea | last28=Weiss | first28=Lauren M. | last29=Petigura | first29=Erik | last30=Seager | first30=Sara | journal=The Astrophysical Journal Supplement Series | volume=210 | issue=2 | page=20 | bibcode=2014ApJS..210...20M | display-authors=1 }}</ref>


Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via ] than larger planets and planets farther out.<ref>{{cite journal | citeseerx = 10.1.1.122.9085 | title = Transonic hydrodynamic escape of hydrogen from extrasolar planetary atmospheres | author1 = Feng Tian | first2 = Owen B. | last2 = Toon | first3 = Alexander A. | last3 = Pavlov | first4 = H. | last4 = De Sterck | journal = The Astrophysical Journal | volume = 621 | issue = 2 | pages = 1049–1060 |date=March 10, 2005 |bibcode = 2005ApJ...621.1049T |doi = 10.1086/427204 }}</ref><ref>, Damian C. Swift, Jon Eggert, Damien G. Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler, and Gilbert W. Collins</ref> The lower limit for mass can vary widely for different planets depending on their compositions; the dividing mass can vary from as low as one to as high as 20 {{Earth mass}}. Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via ] than larger planets and planets farther out.<ref>{{cite journal | citeseerx = 10.1.1.122.9085 | title = Transonic hydrodynamic escape of hydrogen from extrasolar planetary atmospheres | author1 = Feng Tian | first2 = Owen B. | last2 = Toon | first3 = Alexander A. | last3 = Pavlov | first4 = H. | last4 = De Sterck | journal = The Astrophysical Journal | volume = 621 | issue = 2 | pages = 1049–1060 |date=March 10, 2005 |bibcode = 2005ApJ...621.1049T |doi = 10.1086/427204 | s2cid = 6475341 }}</ref><ref>{{cite journal | arxiv=1001.4851 | doi=10.1088/0004-637X/744/1/59 | title=Mass-Radius Relationships for Exoplanets | date=2012 | last1=Swift | first1=D. C. | last2=Eggert | first2=J. H. | last3=Hicks | first3=D. G. | last4=Hamel | first4=S. | last5=Caspersen | first5=K. | last6=Schwegler | first6=E. | last7=Collins | first7=G. W. | last8=Nettelmann | first8=N. | last9=Ackland | first9=G. J. | journal=The Astrophysical Journal | volume=744 | issue=1 | page=59 | bibcode=2012ApJ...744...59S }}</ref><ref name="Martinez2019">{{cite journal|last1=Martinez|first1=Cintia F.|last2=Cunha|first2=Katia|last3=Ghezzi|first3=Luan|last4=Smith|first4=Verne V.|title=A Spectroscopic Analysis of the California-Kepler Survey Sample. I. Stellar Parameters, Planetary Radii, and a Slope in the Radius Gap|journal=The Astrophysical Journal|publisher=American Astronomical Society|volume=875|issue=1|date=2019-04-10|page=29 |doi=10.3847/1538-4357/ab0d93|doi-access=free|arxiv=1903.00174 |bibcode=2019ApJ...875...29M |hdl=10150/633733|hdl-access=free}}</ref> A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.<ref>{{cite journal | arxiv=1304.5157 | doi=10.1088/2041-8205/769/1/L9 | title=Mass-Radius Relationships for Very Low Mass Gaseous Planets | date=2013 | last1=Batygin | first1=Konstantin | last2=Stevenson | first2=David J. | journal=The Astrophysical Journal | volume=769 | issue=1 | pages=L9 | bibcode=2013ApJ...769L...9B }}</ref>


Neptune-like planets are considerably rarer than sub-Neptunes, despite being only slightly bigger.<ref name="cliff">{{Cite web|url=https://astrobites.org/2019/12/17/why-are-there-so-many-sub-neptune-exoplanets/|title = Why are there so many sub-Neptune exoplanets?|date = 17 December 2019}}</ref><ref>{{cite journal | arxiv=1912.02701 | doi=10.3847/2041-8213/ab59d9 | doi-access=free | title=Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis | date=2019 | last1=Kite | first1=Edwin S. | last2=Bruce Fegley Jr. | last3=Schaefer | first3=Laura | last4=Ford | first4=Eric B. | journal=The Astrophysical Journal Letters | volume=887 | issue=2 | pages=L33 | bibcode=2019ApJ...887L..33K }}</ref> This "radius cliff" separates ]s (radius < 3 Earth radii) from Neptunes (radius > 3 Earth radii).<ref name="cliff"/> This is thought to arise because, during formation when gas is accreting, the atmospheres of planets of that size reach the pressures required to force the hydrogen into the magma ocean, stalling radius growth. Then, once the magma ocean saturates, radius growth can continue. However, planets that have enough gas to reach saturation are much rarer, because they require much more gas.<ref name="cliff"/>
A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.<ref>*, ], David J. Stevenson, 18 Apr 2013</ref>

Neptune like planets are considerably rarer than sub-Neptunes, despite being only slightly bigger.<ref name="cliff">https://astrobites.org/2019/12/17/why-are-there-so-many-sub-neptune-exoplanets/</ref><ref>, Edwin S. Kite, Bruce Fegley Jr., Laura Schaefer, Eric B. Ford, 5 Dec 2019</ref> This “radius cliff” separates sub-Neptunes (radii < 3 Earth radii) from Neptunes (radii > 3 Earth radii).<ref name="cliff"/> This radius-cliff is thought to arise because during formation when gas is accreting, the atmospheres of planets that size reach the pressures required to force the hydrogen into the magma ocean stalling radius growth. Then, once the magma ocean saturates, radius growth can continue. However, planets that have enough gas to reach saturation are much rarer, because they require much more gas.<ref name="cliff"/>


==Examples== ==Examples==
The smallest known extrasolar planet that might be a gas dwarf is ], which is less ] but has a 60% larger volume and therefore has a density (2.1(+2.2/-1.2) grams per cubic centimetre) that indicates either a substantial water content<ref name = "Jontof-Hutter">{{cite journal |last=Jontof-Hutter |first=D |title=Mass of the Mars-sized Exoplanet Kepler-138b from Transit Timing |journal=Nature |volume=522 |issue=7556 |pages=321–323 |date=18 June 2015 |doi=10.1038/nature14494 |arxiv=1506.07067 |bibcode=2015Natur.522..321J |displayauthors=etal |last2=Rowe |first2=J |pmid=26085271}}</ref> or possibly a thick gas envelope.<ref></ref> The smallest known extrasolar planet that might be a gas dwarf is ], which is less ] but has a 60% larger volume and therefore has a density {{val|2.1|2.2|1.2|u=g/cm3}} that indicates either a substantial water content<ref name = "Jontof-Hutter">{{cite journal |last1=Jontof-Hutter |first1=D |title=Mass of the Mars-sized Exoplanet Kepler-138b from Transit Timing |journal=Nature |volume=522 |issue=7556 |pages=321–323 |date=18 June 2015 |doi=10.1038/nature14494 |arxiv=1506.07067 |bibcode=2015Natur.522..321J |display-authors=etal |last2=Rowe |first2=J |pmid=26085271|s2cid=205243944 }}</ref> or possibly a thick gas envelope.<ref></ref> However, more recent evidence suggests that it may be more dense than previously thought, and could be an ] instead.<ref>{{cite web |url=https://arstechnica.com/science/2022/12/scientists-revisit-kepler-findings-learn-two-planets-are-water-worlds/ |title=Scientists may have found the first water worlds |last=Timmer |first=John |date=15 December 2022 |website=] |access-date=17 December 2022}}</ref>


== See also == == See also ==
* ] * ]
* ]
* ] * ]
* ] * ]
* ] * ]
* ] * ]
* ] * ]
* ] * ]
* ] * ]
* ]



==References== ==References==
{{reflist}}
<references/>


==Further reading== ==Further reading==
*, Victoria Jaggard, National Geographic News, Published February 2, 2011 *, Victoria Jaggard, National Geographic News, Published February 2, 2011
*{{cite journal|last=Barnes|first=Rory|date=January 13, 2009|title=The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?|journal=The Astrophysical Journal|arxiv=0901.1698|bibcode = 2009ApJ...695.1006B |doi = 10.1088/0004-637X/695/2/1006|last2=Jackson|first2=Brian|last3=Raymond|first3=Sean N.|last4=West|first4=Andrew A.|last5=Greenberg|first5=Richard|volume=695|issue=2|pages=1006 }} *{{cite journal|last1=Barnes|first1=Rory|date=January 13, 2009|title=The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?|journal=The Astrophysical Journal|arxiv=0901.1698|bibcode = 2009ApJ...695.1006B |doi = 10.1088/0004-637X/695/2/1006|last2=Jackson|first2=Brian|last3=Raymond|first3=Sean N.|last4=West|first4=Andrew A.|last5=Greenberg|first5=Richard|volume=695|issue=2|pages=1006–1011 |s2cid=18849636}}


==External links== ==External links==
* *


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Latest revision as of 17:47, 28 August 2024

Not to be confused with Sub-Neptune.
Artist's conception of a mini-Neptune or "gas dwarf"
Planet smaller than Neptune with a gas atmosphere

A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen-helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made of water, ammonia, a mixture of both, or heavier volatiles).

A gas dwarf is a gas planet with a rocky core that has accumulated a thick envelope of hydrogen, helium, and other volatiles, having, as a result, a total radius between 1.7 and 3.9 Earth radii (1.7–3.9 R🜨). The term is used in a three-tier, metallicity-based classification regime for short-period exoplanets, which also includes the rocky, terrestrial-like planets with less than 1.7 R🜨 and planets greater than 3.9 R🜨, namely ice giants and gas giants.

Properties

Theoretical studies of such planets are loosely based on knowledge about Uranus and Neptune. Without a thick atmosphere, they would be classified as an ocean planet instead. An estimated dividing line between a rocky planet and a gaseous planet is around 1.6–2.0 Earth radii. Planets with larger radii and measured masses are mostly low-density and require an extended atmosphere to simultaneously explain their masses and radii, and observations show that planets larger than approximately 1.6 Earth-radius (and more massive than approximately 6 Earth-masses) contain significant amounts of volatiles or H–He gas, likely acquired during formation. Such planets appear to have a diversity of compositions that is not well-explained by a single mass–radius relation as that found for denser, rocky planets.

The lower limit for mass can vary widely for different planets depending on their compositions; the dividing mass can vary from as low as one to as high as 20 ME. Smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out. A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.

Neptune-like planets are considerably rarer than sub-Neptunes, despite being only slightly bigger. This "radius cliff" separates sub-Neptunes (radius < 3 Earth radii) from Neptunes (radius > 3 Earth radii). This is thought to arise because, during formation when gas is accreting, the atmospheres of planets of that size reach the pressures required to force the hydrogen into the magma ocean, stalling radius growth. Then, once the magma ocean saturates, radius growth can continue. However, planets that have enough gas to reach saturation are much rarer, because they require much more gas.

Examples

The smallest known extrasolar planet that might be a gas dwarf is Kepler-138d, which is less massive than Earth but has a 60% larger volume and therefore has a density 2.1+2.2
−1.2 g/cm that indicates either a substantial water content or possibly a thick gas envelope. However, more recent evidence suggests that it may be more dense than previously thought, and could be an ocean planet instead.

See also

References

  1. ^ D'Angelo, G.; Bodenheimer, P. (2016). "In Situ and Ex Situ Formation Models of Kepler 11 Planets". The Astrophysical Journal. 828 (1): id. 33. arXiv:1606.08088. Bibcode:2016ApJ...828...33D. doi:10.3847/0004-637X/828/1/33. S2CID 119203398.
  2. Three regimes of extrasolar planets inferred from host star metallicities, Buchhave et al.
  3. De Mooij, E. J. W.; Brogi, M.; De Kok, R. J.; Koppenhoefer, J.; Nefs, S. V.; Snellen, I. A. G.; Greiner, J.; Hanse, J.; Heinsbroek, R. C.; Lee, C. H.; Van Der Werf, P. P. (2012). "Optical to near-infrared transit observations of super-Earth GJ 1214b: Water-world or mini-Neptune?". Astronomy & Astrophysics. 538: A46. arXiv:1111.2628. Bibcode:2012A&A...538A..46D. doi:10.1051/0004-6361/201117205.
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