Mini-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).[1]

Artist's conception of a mini-Neptune or "gas dwarf"

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.[2]

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.[3] An estimated dividing line between a rocky planet and a gaseous planet is around 1.6–2.0 Earth radii.[4][5] 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.[6][1] 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.[7][8][9][10][11][12]

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 M🜨. 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.[13][14][15] A low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature.[16]

Neptune-like planets are considerably rarer than sub-Neptunes, despite being only slightly bigger.[17][18] This "radius cliff" separates sub-Neptunes (radius < 3 Earth radii) from Neptunes (radius > 3 Earth radii).[17] 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.[17]

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/cm3) that indicates either a substantial water content[19] or possibly a thick gas envelope.[20] However, more recent evidence suggests that it may be more dense than previously thought, and could be an ocean planet instead.[21]

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. 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),
  4. 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
  5. When Does an Exoplanet's Surface Become Earth-Like?, blogs.scientificamerican.com, 20 June 2012
  6. 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.
  7. Benjamin J. Fulton et al. "The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
  8. Courtney D. Dressing et al. "The Mass of Kepler-93b and The Composition of Terrestrial Planets"
  9. Leslie A. Rogers "Most 1.6 Earth-Radius Planets are not Rocky"
  10. Lauren M. Weiss, and Geoffrey W. Marcy. "The mass-radius relation for 65 exoplanets smaller than 4 Earth radii"
  11. 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"
  12. Geoffrey W. Marcy et al. "Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets"
  13. 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.
  14. Mass-radius relationships for exoplanets, Damian C. Swift, Jon Eggert, Damien G. Hicks, Sebastien Hamel, Kyle Caspersen, Eric Schwegler, and Gilbert W. Collins
  15. 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. American Astronomical Society. 875 (1): 29. doi:10.3847/1538-4357/ab0d93.
  16. Mass-Radius Relationships for Very Low Mass Gaseous Planets, Konstantin Batygin, David J. Stevenson, 18 Apr 2013
  17. "Why are there so many sub-Neptune exoplanets?". 17 December 2019.
  18. Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis, Edwin S. Kite, Bruce Fegley Jr., Laura Schaefer, Eric B. Ford, 5 Dec 2019
  19. 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.
  20. Earth-mass exoplanet is no Earth twin – Gaseous planet challenges assumption that Earth-mass planets should be rocky
  21. Timmer, John (15 December 2022). "Scientists may have found the first water worlds". Ars Technica. Retrieved 17 December 2022.

Further reading
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