Small planet radius gap

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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]

Contents

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

See also

Related Research Articles

Photoevaporation is the process where energetic radiation ionises gas and causes it to disperse away from the ionising source. The term is typically used in an astrophysical context where ultraviolet radiation from hot stars acts on clouds of material such as molecular clouds, protoplanetary disks, or planetary atmospheres.

<span class="mw-page-title-main">Hot Jupiter</span> Class of high mass planets orbiting close to a star

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<span class="mw-page-title-main">Super-Earth</span> Planet with a mass between Earth and Uranus

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<span class="mw-page-title-main">Mini-Neptune</span> Planet smaller than Neptune with a gas atmosphere

A Mini-Neptune 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.

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<span class="mw-page-title-main">GJ 3470 b</span> Hot Neptune orbiting GJ 3470

GJ 3470 b is an exoplanet orbiting the star GJ 3470, located in the constellation Cancer. With a mass of just under 14 Earth-masses and a radius approximately 4.3 times that of Earth's, it is likely something akin to Neptune despite the initially strong belief that the planet was not covered in clouds like the gas giants in the Solar System.

<span class="mw-page-title-main">Super-Neptune</span> Planet larger than Neptune but smaller than Saturn

A super-Neptune is a planet that is more massive than the planet Neptune. These planets are generally described as being around 5–7 times as large as Earth with estimated masses of 20–80 ME; beyond this they are generally referred to as gas giants. A planet falling within this mass range may also be referred to as a sub-Saturn.

<span class="mw-page-title-main">K2-18</span> Red dwarf star in the constellation Leo

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<span class="mw-page-title-main">V1298 Tauri</span> Star in the constellation Taurus

V1298 Tauri is a young weakly-lined T Tauri star that is part of the Taurus-Auriga association in the Taurus Molecular Cloud. Alternatively it is part of a proposed moving group, called Group 29 that is slightly older. The system has four transiting exoplanets, discovered with the Kepler space telescope in the K2 mission. One of the planets was discovered in August 2019 and the other three were discovered in November 2019 by the same team.

<span class="mw-page-title-main">Kepler-1638</span> G-type star in the constellation Cygnus

Kepler-1638 is a G-type main-sequence star located about 5,000 light years away in the constellation of Cygnus. One known exoplanet has been found orbiting the star: Kepler-1638b.As of January 2021, Kepler-1638 is the farthest star with a known exoplanet orbiting in the habitable zone.

<span class="mw-page-title-main">K2-18b</span> Mini-Neptune orbiting the red dwarf K2-18

K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 124 light-years (38 pc) away from Earth. The planet is a mini-neptune about 2.6 times the radius of Earth, with a 33-day orbit within the star's habitable zone. This means it receives about a similar amount of starlight as the Earth receives from the Sun. Initially discovered with the Kepler space telescope, it was later observed by the James Webb Space Telescope in order to study the planet's atmosphere.

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. 479 (4). Oxford University Press (OUP): 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. 880 (1). American Astronomical Society: 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. 876 (1). American Astronomical Society: 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. 775 (2). IOP Publishing: 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. 847 (1). American Astronomical Society: 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. 761 (1). IOP Publishing: 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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