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| {{short description|Type of planet}} | {{short description|Type of planet}} | ||
| 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> | 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 |s2cid=119203398}}</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> | 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, 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 |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>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}}. | ||
| 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> | 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> | ||
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| 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> | 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 (radius < 3 Earth radii) from Neptunes (radius > 3 Earth radii).<ref name="cliff"/> This radius-cliff 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"/> | 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>, Edwin S. Kite, Bruce Fegley Jr., Laura Schaefer, Eric B. Ford, 5 Dec 2019</ref> This “radius cliff” separates sub-Neptunes (radius < 3 Earth radii) from Neptunes (radius > 3 Earth radii).<ref name="cliff"/> This radius-cliff 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"/> | ||
| ==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 | |
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 |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> | ||
| == See also == | == See also == | ||
| Line 34: | Line 34: | ||
| ==Further reading== | ==Further reading== | ||
| *, Victoria Jaggard, National Geographic News, Published February 2, 2011 | *, Victoria Jaggard, National Geographic News, Published February 2, 2011 | ||
| *{{cite journal| |
*{{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== | ||
Revision as of 00:50, 21 November 2021
A mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembles 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, it 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 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. 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. Similar results are confirmed by other studies. 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 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 radius-cliff 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) grams per cubic centimetre) that indicates either a substantial water content or possibly a thick gas envelope.
See also
- Chthonian planet
- Gas giant
- Hot Jupiter
- Hot Neptune
- Ice giant
- Mega-Earth
- Planet Nine
- Super-Earth
- Super-Jupiter
- Sub-Neptune
References
- ^ 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.
{{cite journal}}: CS1 maint: unflagged free DOI (link) - Three regimes of extrasolar planets inferred from host star metallicities, Buchhave et al.
- Optical to near-infrared transit observations of super-Earth GJ1214b: water-world or mini-Neptune?, 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),
- Architecture of Kepler's Multi-transiting Systems: II. New investigations with twice as many candidates, 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
- When Does an Exoplanet's Surface Become Earth-Like?, blogs.scientificamerican.com, 20 June 2012
- D'Angelo, G.; Bodenheimer, P. (2013). "Three-Dimensional Radiation-Hydrodynamics Calculations of the Envelopes of Young Planets Embedded in Protoplanetary Disks". The Astrophysical Journal. 778 (1): 77 (29 pp.). arXiv:1310.2211. Bibcode:2013ApJ...778...77D. doi:10.1088/0004-637X/778/1/77. S2CID 118522228.
- Benjamin J. Fulton et al. "The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
- Courtney D. Dressing et al. "The Mass of Kepler-93b and The Composition of Terrestrial Planets"
- Leslie A. Rogers "Most 1.6 Earth-Radius Planets are not Rocky"
- Lauren M. Weiss, and Geoffrey W. Marcy. "The mass-radius relation for 65 exoplanets smaller than 4 Earth radii"
- Geoffrey W. Marcy, Lauren M. Weiss, Erik A. Petigura, Howard Isaacson, Andrew W. Howard and Lars A. Buchhave. "Occurrence and core-envelope structure of 1-4x Earth-size planets around Sun-like stars"
- Geoffrey W. Marcy et al. "Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets"
- Feng Tian; Toon, Owen B.; Pavlov, Alexander A.; De Sterck, H. (March 10, 2005). "Transonic hydrodynamic escape of hydrogen from extrasolar planetary atmospheres". The Astrophysical Journal. 621 (2): 1049–1060. Bibcode:2005ApJ...621.1049T. CiteSeerX 10.1.1.122.9085. doi:10.1086/427204.
- Mass-radius relationships for exoplanets, Damian C. Swift, Jon Eggert, Damien G. Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler, and Gilbert W. Collins
- *Mass-Radius Relationships for Very Low Mass Gaseous Planets, Konstantin Batygin, David J. Stevenson, 18 Apr 2013
- ^ "Why are there so many sub-Neptune exoplanets?". 17 December 2019.
- Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis, Edwin S. Kite, Bruce Fegley Jr., Laura Schaefer, Eric B. Ford, 5 Dec 2019
- Jontof-Hutter, D; Rowe, J; et al. (18 June 2015). "Mass of the Mars-sized Exoplanet Kepler-138b from Transit Timing". Nature. 522 (7556): 321–323. arXiv:1506.07067. Bibcode:2015Natur.522..321J. doi:10.1038/nature14494. PMID 26085271. S2CID 205243944.
- Earth-mass exoplanet is no Earth twin – Gaseous planet challenges assumption that Earth-mass planets should be rocky
Further reading
- Six New Planets: Mini-Neptunes Found Around Sunlike Star, Victoria Jaggard, National Geographic News, Published February 2, 2011
- Barnes, Rory; Jackson, Brian; Raymond, Sean N.; West, Andrew A.; Greenberg, Richard (January 13, 2009). "The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?". The Astrophysical Journal. 695 (2): 1006–1011. arXiv:0901.1698. Bibcode:2009ApJ...695.1006B. doi:10.1088/0004-637X/695/2/1006. S2CID 18849636.
