Small planet radius gap explained

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[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.[9] The bimodality in the distribution was confirmed with higher-precision data in the California-Kepler Survey in 2017,[1] which was shown to match the predictions of the photoevaporative mass-loss hypothesis later that year.

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. As a result, 'valley' is often used in place of 'gap'.[2] [3] 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, 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

Notes and References

  1. Web site: As Planet Discoveries Pile Up, a Gap Appears in the Pattern . Quanta Magazine . Rebecca . Boyle . 2019-05-16 . 2020-06-24.
  2. Van Eylen . V . Agentoft . Camilla . Lundkvist . M S . Kjeldsen . H . Owen . J E . Fulton . B J . Petigura . E . Snellen . I . 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 . 2018-07-06 . 0035-8711 . 10.1093/mnras/sty1783 . 4786–4795. free . 1710.05398 .
  3. Armstrong . David J. . Meru . Farzana . Bayliss . Daniel . Kennedy . Grant M. . Veras . Dimitri . A Gap in the Mass Distribution for Warm Neptune and Terrestrial Planets . The Astrophysical Journal . American Astronomical Society . 880 . 1 . 2019-07-17 . 2041-8213 . 10.3847/2041-8213/ab2ba2. 1906.11865 . L1. 2019ApJ...880L...1A . free .
  4. McDonald . George D. . Kreidberg . Laura . Lopez . Eric . The Sub-Neptune Desert and Its Dependence on Stellar Type: Controlled by Lifetime X-Ray Irradiation . The Astrophysical Journal . American Astronomical Society . 876 . 1 . 2019-04-29 . 1538-4357 . 10.3847/1538-4357/ab1095 . 2105.00142 . 22. 2019ApJ...876...22M . free .
  5. Owen . James E. . Wu . Yanqin. Yanqin Wu . KEPLER PLANETS: A TALE OF EVAPORATION . The Astrophysical Journal . IOP Publishing . 775 . 2 . 2013-09-12 . 0004-637X . 10.1088/0004-637x/775/2/105 . 105. 1303.3899 . 2013ApJ...775..105O .
  6. 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. . The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets . The Astronomical Journal . 154 . 3 . 2017-08-24 . 1538-3881 . 10.3847/1538-3881/aa80eb . 109 . . 1703.10375 . 2017AJ....154..109F . free .
  7. Owen . James E. . Wu . Yanqin. Yanqin Wu . The Evaporation Valley in the Kepler Planets . The Astrophysical Journal . American Astronomical Society . 847 . 1 . 2017-09-20 . 1538-4357 . 10.3847/1538-4357/aa890a . 29. 1705.10810 . 2017ApJ...847...29O . free .
  8. Youdin . Andrew N. . 2011-11-20 . THE EXOPLANET CENSUS: A GENERAL METHOD APPLIED TO KEPLER . The Astrophysical Journal . 742 . 1 . 38 . 10.1088/0004-637X/742/1/38 . 0004-637X. 1105.1782 . 2011ApJ...742...38Y . 118614975 .
  9. Lopez . Eric D. . Fortney . Jonathan J. . Miller . Neil . 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 . 2012-11-21 . 0004-637X . 10.1088/0004-637x/761/1/59 . 59. 1205.0010 . 2012ApJ...761...59L . free .
  10. Web site: BJ Fulton Wins 2018 Robert J. Trumpler Award for 'Landmark' Exoplanet Discovery Using Keck Observatory . W.M. Keck Observatory . 2018-09-10 . 2018-09-11.
  11. Web site: IfA graduate receives prestigious award for work on extrasolar planets . University of Hawaiʻi System News . 2018-08-15 . 2018-09-11.
  12. Venturini . Julia . Helled . Ravit . 17 October 2017 . The Formation of Mini-Neptunes . The Astrophysical Journal . 848 . 2 . 95 . 10.3847/1538-4357/aa8cd0 . 1709.04736 . 2017ApJ...848...95V . free .
  13. 2002.05243. Mousis. Olivier. Deleuil. Magali. Aguichine. Artyom. Marcq. Emmanuel. Naar. Joseph. Lorena Acuña Aguirre. Brugger. Bastien. Goncalves. Thomas. Irradiated Ocean Planets Bridge Super-Earth and Sub-Neptune Populations. The Astrophysical Journal . 2020. 896 . 2 . L22 . 10.3847/2041-8213/ab9530 . 2020ApJ...896L..22M . free .

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