The small planet radius gap (also called the Fulton gap,[1] photoevaporation valley,[2][3] or Sub-Neptune Desert[4]) is an observed scarcity of planets with radii between 1.5 and 2 times Earth's radius, likely due to photoevaporation-driven mass loss.[5][6][7] A bimodality in the Kepler exoplanet population was first observed in 2011[8] and attributed to the absence of significant gas atmospheres on close-in, low-mass planets. This feature was noted as possibly confirming an emerging hypothesis that photoevaporation could drive atmospheric mass loss[5][9] This would lead to a population of bare, rocky cores with smaller radii at small separations from their parent stars, and planets with thick hydrogen- and helium-dominated envelopes with larger radii at larger separations.[5][9] The bimodality in the distribution was confirmed with higher-precision data in the California-Kepler Survey in 2017,[6][1] which was shown to match the predictions of the photoevaporative mass-loss hypothesis later that year.[7]

Despite the implication of the word 'gap', the Fulton gap does not actually represent a range of radii completely absent from the observed exoplanet population, but rather a range of radii that appear to be relatively uncommon.[6] As a result, 'valley' is often used in place of 'gap'.[2][3][7] The specific term "Fulton gap" is named for Benjamin J. Fulton, whose doctoral thesis included precision radius measurements that confirmed the scarcity of planets between 1.5 and 2 Earth radii, for which he won the Robert J. Trumpler Award,[10][11] although the existence of this radius gap had been noted along with its underlying mechanisms as early as 2011,[8] 2012[9] and 2013.[5]

Within the photoevaporation model of Owen and Wu, the radius gap arises as planets with H/He atmospheres that double the core's radius are the most stable to atmospheric mass-loss. Planets with atmospheres larger than this are vulnerable to erosion and their atmospheres evolve towards a size that doubles the core's radius. Planets with smaller atmospheres undergo runaway loss, leaving them with no H/He dominated atmosphere.[7]

Other possible explanations

  • Runaway gas accretion by larger planets.[12]
  • Observational bias favoring easier detection of hot ocean planets with extended steam atmospheres.[13]

See also

References

  1. 1 2 Boyle, Rebecca (2019-05-16). "As Planet Discoveries Pile Up, a Gap Appears in the Pattern". Quanta Magazine. Retrieved 2020-06-24.
  2. 1 2 Van Eylen, V; Agentoft, Camilla; Lundkvist, M S; Kjeldsen, H; Owen, J E; Fulton, B J; Petigura, E; Snellen, I (2018-07-06). "An asteroseismic view of the radius valley: stripped cores, not born rocky". Monthly Notices of the Royal Astronomical Society. Oxford University Press (OUP). 479 (4): 4786–4795. arXiv:1710.05398. doi:10.1093/mnras/sty1783. ISSN 0035-8711.
  3. 1 2 Armstrong, David J.; Meru, Farzana; Bayliss, Daniel; Kennedy, Grant M.; Veras, Dimitri (2019-07-17). "A Gap in the Mass Distribution for Warm Neptune and Terrestrial Planets". The Astrophysical Journal. American Astronomical Society. 880 (1): L1. arXiv:1906.11865. Bibcode:2019ApJ...880L...1A. doi:10.3847/2041-8213/ab2ba2. ISSN 2041-8213.
  4. ↑ McDonald, George D.; Kreidberg, Laura; Lopez, Eric (2019-04-29). "The Sub-Neptune Desert and Its Dependence on Stellar Type: Controlled by Lifetime X-Ray Irradiation". The Astrophysical Journal. American Astronomical Society. 876 (1): 22. arXiv:2105.00142. Bibcode:2019ApJ...876...22M. doi:10.3847/1538-4357/ab1095. ISSN 1538-4357.
  5. 1 2 3 4 Owen, James E.; Wu, Yanqin (2013-09-12). "KEPLER PLANETS: A TALE OF EVAPORATION". The Astrophysical Journal. IOP Publishing. 775 (2): 105. arXiv:1303.3899. Bibcode:2013ApJ...775..105O. doi:10.1088/0004-637x/775/2/105. ISSN 0004-637X.
  6. 1 2 3 Fulton, Benjamin J.; Petigura, Erik A.; Howard, Andrew W.; Isaacson, Howard; Marcy, Geoffrey W.; Cargile, Phillip A.; Hebb, Leslie; Weiss, Lauren M.; Johnson, John Asher; Morton, Timothy D.; Sinukoff, Evan; Crossfield, Ian J. M.; Hirsch, Lea A. (2017-08-24). "The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets". The Astronomical Journal. 154 (3): 109. arXiv:1703.10375. Bibcode:2017AJ....154..109F. doi:10.3847/1538-3881/aa80eb. ISSN 1538-3881.
  7. 1 2 3 4 Owen, James E.; Wu, Yanqin (2017-09-20). "The Evaporation Valley in the Kepler Planets". The Astrophysical Journal. American Astronomical Society. 847 (1): 29. arXiv:1705.10810. Bibcode:2017ApJ...847...29O. doi:10.3847/1538-4357/aa890a. ISSN 1538-4357.
  8. 1 2 Youdin, Andrew N. (2011-11-20). "THE EXOPLANET CENSUS: A GENERAL METHOD APPLIED TO KEPLER". The Astrophysical Journal. 742 (1): 38. arXiv:1105.1782. Bibcode:2011ApJ...742...38Y. doi:10.1088/0004-637X/742/1/38. ISSN 0004-637X. S2CID 118614975.
  9. 1 2 3 Lopez, Eric D.; Fortney, Jonathan J.; Miller, Neil (2012-11-21). "How Thermal Evolution and Mass-Loss Sculpt Populations of Super-Earths and Sub-Neptunes: Application to the Kepler-11 System and Beyond". The Astrophysical Journal. IOP Publishing. 761 (1): 59. arXiv:1205.0010. Bibcode:2012ApJ...761...59L. doi:10.1088/0004-637x/761/1/59. ISSN 0004-637X.
  10. ↑ "BJ Fulton Wins 2018 Robert J. Trumpler Award for 'Landmark' Exoplanet Discovery Using Keck Observatory". W.M. Keck Observatory. 2018-09-10. Retrieved 2018-09-11.
  11. ↑ "IfA graduate receives prestigious award for work on extrasolar planets". University of Hawaiʻi System News. 2018-08-15. Retrieved 2018-09-11.
  12. ↑ Venturini, Julia; Helled, Ravit (17 October 2017). "The Formation of Mini-Neptunes". The Astrophysical Journal. 848 (2): 95. arXiv:1709.04736. Bibcode:2017ApJ...848...95V. doi:10.3847/1538-4357/aa8cd0.
  13. ↑ Mousis, Olivier; Deleuil, Magali; Aguichine, Artyom; Marcq, Emmanuel; Naar, Joseph; Lorena Acuña Aguirre; Brugger, Bastien; Goncalves, Thomas (2020). "Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations". The Astrophysical Journal. 896 (2): L22. arXiv:2002.05243. Bibcode:2020ApJ...896L..22M. doi:10.3847/2041-8213/ab9530.
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