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The geophysical planet definition is a proposed definition of what is and is not a planet that was formalized in response to criticism of the definition adopted by the International Astronomical Union.[1][2] The geophysical planet definition states: a planet is a sub-stellar mass object that has never undergone nuclear fusion that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid [is rounded due to self-gravity], regardless of its orbital parameters.[1][2][3] This is equivalent to the second clause of the IAU definition, excluding the first clause (that a planet be in orbit around the sun) and the third clause (that a planet has cleared the neighborhood around its orbit. As such dwarf planets and round moons are counted as planets according to this definition. Five bodies are currently recognised as dwarf planets by the IAU, Ceres, Pluto (the dwarf planet with the largest known radius[4]), Haumea, Makemake and Eris (the dwarf planet with the largest known mass[5]).[6] It has been suggested that for icy bodies the limit at which objects are likely to be in hydrostatic equilibrium is around 450 km, which would mean there are many more dwarf planets in the Kuiper belt and would make dwarf planets the most common type of planet in the Solar System under the geophysical planet definition.[7]

The proposed geophysical planet definition allows objects in the solar system to be grouped together on the basis of common physical properties regardless of location. An examination of spacecraft imagery of objects in the solar system reveals that the threshold at which an object is large enough to be round by self-gravity is also likely to be where geological activity becomes prevalent.[8] However, some "round" satellites such as Mimas and Callisto are geologically inactive. Others, such as Io, Europa and Enceladus are geologically active due to tidal heating, rather than the gravitational energy more closely associated with "roundness."

Early proponents of the geophysical planet definition include the lead organisers of a petition shortly after the IAU vote protesting the definition adopted by the IAU that attracted over 300 signatures.[9][10][3] Proponents of the geophysical planet definition have published papers reviewing the planetary science literature, which shows this definition has been occasionally used by some planetary scientists for decades, including after the IAU definition was established, and that asteroids have routinely regarded as "minor" planets although the usage has been somewhat inconsistent and has varied considerably.[11][12]

Since the geophysical planet definition does not specify that planets orbit the Sun, it also applies to exoplanets. It excludes objects that have ever undergone nuclear fusion so it may exclude some of the higher mass objects included in exoplanet catalogs and brown dwarfs. The Extrasolar Planets Encyclopaedia, Exoplanet Data Explorer and NASA Exoplanet Archive all include objects significantly more massive than the theoretical 13 Jupiter mass threshold[13] at which deuterium fusion begins for reasons including: uncertainties in how this limit would apply to a body with a rocky core, uncertainties in the masses of exoplanets, and discussions about whether deuterium burning or formation mechanism is the most appropriate criterion to use to define an upper cut-off.[14][15][16] Nonetheless, both the geophysical planet definition and the IAU definition require a planet to be "round" (in or close to hydrostatic equilibrium.) Determining the roundness of a body requires measurements across multiple chords however exoplanet detection techniques only provide the planet mass, ratio of the cross-sectional area to that of the host star, or the relative brightness. However, as of 2019, the smallest detected exoplanet (Kepler-1520b) has a mass of 0.02 times that of the Earth. By analogy to objects within our own solar system, that is most likely large enough to be rounded by self-gravity.


  1. ^ a b Runyon, K. D.; Stern, S. A.; Lauer, T. R.; Grundy, W.; Summers, M. E.; Singer, K. N. (March 2017). "A geophysical planet definition" (PDF). Lunar and Planetary Science Conference Abstracts. Retrieved 12 October 2019.
  2. ^ a b Runyon, Kirby D.; Stern, S. Alan (17 May 2018). "An organically grown planet definition — Should we really define a word by voting?". Astronomy. Retrieved 12 October 2019.
  3. ^ a b Flatow, Ira; Sykes, Mark (28 March 2008). "What Defines a Planet? (transcript)". NPR. Retrieved 12 October 2019.
  4. ^ Stern, S. A.; Bagenal, F. (16 October 2015). "The Pluto system: Initial results from its exploration by New Horizons". ScienceMag. Retrieved 14 October 2019.
  5. ^ Brown, Michael E.; Schaller, Emily L. (June 15, 2007). "The Mass of Dwarf Planet Eris". Science. 316 (5831): 1585. Bibcode:2007Sci...316.1585B. doi:10.1126/science.1139415. PMID 17569855.
  6. ^ "Naming of Astronomical Objects". International Astronomical Union. Retrieved 12 October 2019.
  7. ^ Tancredi, Gonzalo; Favre, Sofía (June 2008). "Which are the dwarfs in the Solar System?". Icarus. 195 (2): 851–862. doi:10.1016/j.icarus.2007.12.020. ISSN 0019-1035.
  8. ^ Sykes, Mark V. (March 2008). "The Planet Debate Continues". Science. 319 (5871): 1765. doi:10.1126/science.1155743. ISSN 0036-8075.
  9. ^ Chang, Kenneth (1 September 2006). "Debate Lingers Over Definition for a Planet". The New York Times. Retrieved 12 October 2019.
  10. ^ A Planet Definition Debate Alan Stern & Ron Ekers
  11. ^ Runyon, K. D.; Metzger, P. T.; Stern, S. A.; Bell, J. (July 2019). "Dwarf planets are planets, too: planetary pedagogy after New Horizons" (PDF). Pluto System After New Horizons Workshop Abstracts. 2133: 7016. Bibcode:2019LPICo2133.7016R. Retrieved 12 October 2019.
  12. ^ Metzger, Philip T.; Sykes, Mark V.; Stern, Alan; Runyon, Kirby (February 2019). "The reclassification of asteroids from planets to non-planets" (PDF). Icarus. 319: 21–32. arXiv:1805.04115v2. doi:10.1016/j.icarus.2018.08.026. ISSN 0019-1035. Retrieved 12 October 2019.
  13. ^ Saumon, D.; Hubbard, W. B.; Burrows, A.; Guillot, T.; Lunine, J. I.; Chabrier, G. (April 1996). "A Theory of Extrasolar Giant Planets" (PDF). The Astrophysical Journal. 460: 993. arXiv:astro-ph/9510046. Bibcode:1996ApJ...460..993S. doi:10.1086/177027. ISSN 0004-637X. Retrieved 12 October 2019.
  14. ^ Schneider, J.; Dedieu, C.; Le Sidaner, P.; Savalle, R.; Zolotukhin, I. (August 2011). "Defining and cataloging exoplanets: the database". Astronomy & Astrophysics. 532. A79. doi:10.1051/0004-6361/201116713. ISSN 0004-6361.
  15. ^ Wright, J. T.; Fakhouri, O.; Marcy, G. W.; Han, E.; Feng, Y.; Johnson, John Asher; Howard, A. W.; Fischer, D. A.; Valenti, J. A.; Anderson, J.; Piskunov, N. (April 2011). "The Exoplanet Orbit Database". Publications of the Astronomical Society of the Pacific. 123 (902): 412–422. doi:10.1086/659427. ISSN 1538-3873. Retrieved 12 October 2019.
  16. ^ "Exoplanet Criteria for Inclusion in the Archive". NASA Exoplanet Archive. 26 March 2019. Retrieved 12 October 2019.