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| ==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> |
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>https://arxiv.org/abs/1111.2628</ref> An estimated dividing line between a rocky planet and a gaseous planet is around 1.6–2.0 Earth radii.<ref>https://arxiv.org/abs/1202.6328</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>https://doi.org/10.3847/1538-3881/aa80eb</ref><ref>https://arxiv.org/abs/1412.8687</ref><ref>https://arxiv.org/abs/1407.4457</ref><ref>https://arxiv.org/abs/1312.0936</ref><ref>https://arxiv.org/abs/1404.2960</ref><ref>https://arxiv.org/abs/1401.4195</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> |
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>https://arxiv.org/abs/1001.4851</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|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>https://arxiv.org/abs/1304.5157</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> |
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>https://arxiv.org/abs/1912.02701</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"/> | ||
| ==Examples== | ==Examples== | ||
Revision as of 07:40, 29 July 2024
Not to be confused with Sub-Neptune.
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, 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 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
- Chthonian planet
- Hot Jupiter
- Hot Neptune
- Hycean planet
- Mega-Earth
- Sub-Neptune
- Super-Earth
- Super-Jupiter
- Planet Nine
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.
- Three regimes of extrasolar planets inferred from host star metallicities, Buchhave et al.
- https://arxiv.org/abs/1111.2628
- https://arxiv.org/abs/1202.6328
- 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.
- https://doi.org/10.3847/1538-3881/aa80eb
- https://arxiv.org/abs/1412.8687
- https://arxiv.org/abs/1407.4457
- https://arxiv.org/abs/1312.0936
- https://arxiv.org/abs/1404.2960
- https://arxiv.org/abs/1401.4195
- 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. S2CID 6475341.
- https://arxiv.org/abs/1001.4851
- Martinez, Cintia F.; Cunha, Katia; Ghezzi, Luan; Smith, Verne V. (2019-04-10). "A Spectroscopic Analysis of the California-Kepler Survey Sample. I. Stellar Parameters, Planetary Radii, and a Slope in the Radius Gap". The Astrophysical Journal. 875 (1). American Astronomical Society: 29. doi:10.3847/1538-4357/ab0d93. hdl:10150/633733.
- https://arxiv.org/abs/1304.5157
- ^ "Why are there so many sub-Neptune exoplanets?". 17 December 2019.
- https://arxiv.org/abs/1912.02701
- 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
- Timmer, John (15 December 2022). "Scientists may have found the first water worlds". Ars Technica. Retrieved 17 December 2022.
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.
