small planet radius gap

{{Short description|Apparent uncommonness of planets between 1.5 times and twice the radius of Earth}}

File:SmallPlanetsComeInTwoSizes-20170619.png

The small planet radius gap (also called the Fulton gap,{{cite web |title=As Planet Discoveries Pile Up, a Gap Appears in the Pattern |work=Quanta Magazine |first=Rebecca |last=Boyle |date=2019-05-16 |url=https://www.quantamagazine.org/as-planet-discoveries-pile-up-a-gap-appears-in-the-pattern-20190516/ |access-date=2020-06-24}} photoevaporation valley,{{cite journal | last1=Van Eylen | first1=V | last2=Agentoft | first2=Camilla | last3=Lundkvist | first3=M S | last4=Kjeldsen | first4=H | last5=Owen | first5=J E | last6=Fulton | first6=B J | last7=Petigura | first7=E | last8=Snellen | first8=I | title=An asteroseismic view of the radius valley: stripped cores, not born rocky | journal=Monthly Notices of the Royal Astronomical Society | publisher=Oxford University Press (OUP) | volume=479 | issue=4 | date=2018-07-06 | issn=0035-8711 | doi=10.1093/mnras/sty1783 | pages=4786–4795| doi-access=free | arxiv=1710.05398 }}{{cite journal | last1=Armstrong | first1=David J. | last2=Meru | first2=Farzana | last3=Bayliss | first3=Daniel | last4=Kennedy | first4=Grant M. | last5=Veras | first5=Dimitri | title=A Gap in the Mass Distribution for Warm Neptune and Terrestrial Planets | journal=The Astrophysical Journal | publisher=American Astronomical Society | volume=880 | issue=1 | date=2019-07-17 | issn=2041-8213 | doi=10.3847/2041-8213/ab2ba2|arxiv=1906.11865 | page=L1| bibcode=2019ApJ...880L...1A | doi-access=free }} or Sub-Neptune Desert{{cite journal | last1=McDonald | first1=George D. | last2=Kreidberg | first2=Laura | last3=Lopez | first3=Eric | title=The Sub-Neptune Desert and Its Dependence on Stellar Type: Controlled by Lifetime X-Ray Irradiation | journal=The Astrophysical Journal | publisher=American Astronomical Society | volume=876 | issue=1 | date=2019-04-29 | issn=1538-4357 | doi=10.3847/1538-4357/ab1095 |arxiv=2105.00142 | page=22| bibcode=2019ApJ...876...22M | doi-access=free }}) is an observed scarcity of planets with radii between 1.5 and 2 times Earth's radius, likely due to photoevaporation-driven mass loss.{{cite journal | last1=Owen | first1=James E. | last2=Wu | first2=Yanqin|author2-link= Yanqin Wu | title=KEPLER PLANETS: A TALE OF EVAPORATION | journal=The Astrophysical Journal | publisher=IOP Publishing | volume=775 | issue=2 | date=2013-09-12 | issn=0004-637X | doi=10.1088/0004-637x/775/2/105 | page=105| arxiv=1303.3899 | bibcode=2013ApJ...775..105O }}{{cite journal |last1=Fulton |first1=Benjamin J. |last2=Petigura |first2=Erik A. |last3=Howard |first3=Andrew W. |last4=Isaacson |first4=Howard |last5=Marcy |first5=Geoffrey W. |last6=Cargile |first6=Phillip A. |last7=Hebb |first7=Leslie |last8=Weiss |first8=Lauren M. |last9=Johnson |first9=John Asher |last10=Morton |first10=Timothy D. |last11=Sinukoff |first11=Evan |last12=Crossfield |first12=Ian J. M. |last13=Hirsch |first13=Lea A. |title=The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets |journal=The Astronomical Journal |volume=154 |issue=3 |date=2017-08-24 |issn=1538-3881 |doi=10.3847/1538-3881/aa80eb |page=109 |ref={{sfnref |Fulton et al. |2017}}|arxiv=1703.10375 |bibcode=2017AJ....154..109F |doi-access=free }}{{cite journal | last1=Owen | first1=James E. | last2=Wu | first2=Yanqin|author2-link= Yanqin Wu | title=The Evaporation Valley in the Kepler Planets | journal=The Astrophysical Journal | publisher=American Astronomical Society | volume=847 | issue=1 | date=2017-09-20 | issn=1538-4357 | doi=10.3847/1538-4357/aa890a | page=29| arxiv=1705.10810 | bibcode=2017ApJ...847...29O | doi-access=free }} A bimodality in the Kepler exoplanet population was first observed in 2011{{Cite journal |last=Youdin |first=Andrew N. |date=2011-11-20 |title=THE EXOPLANET CENSUS: A GENERAL METHOD APPLIED TO KEPLER |url=https://iopscience.iop.org/article/10.1088/0004-637X/742/1/38 |journal=The Astrophysical Journal |volume=742 |issue=1 |pages=38 |doi=10.1088/0004-637X/742/1/38 |issn=0004-637X|arxiv=1105.1782 |bibcode=2011ApJ...742...38Y |s2cid=118614975 }} 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{{cite journal | last1=Lopez | first1=Eric D. | last2=Fortney | first2=Jonathan J. | last3=Miller | first3=Neil | title=How Thermal Evolution and Mass-Loss Sculpt Populations of Super-Earths and Sub-Neptunes: Application to the Kepler-11 System and Beyond | journal=The Astrophysical Journal | publisher=IOP Publishing | volume=761 | issue=1 | date=2012-11-21 | issn=0004-637X | doi=10.1088/0004-637x/761/1/59 | page=59| arxiv=1205.0010 | bibcode=2012ApJ...761...59L | doi-access=free }} 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. The bimodality in the distribution was confirmed with higher-precision data in the California-Kepler Survey in 2017, 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'. 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,{{cite web |title=BJ Fulton Wins 2018 Robert J. Trumpler Award for 'Landmark' Exoplanet Discovery Using Keck Observatory |website=W.M. Keck Observatory |date=2018-09-10 |url=http://www.keckobservatory.org/fulton_gap/ |access-date=2018-09-11}}{{cite web |title=IfA graduate receives prestigious award for work on extrasolar planets |website=University of Hawaiʻi System News |date=2018-08-15 |url=https://www.hawaii.edu/news/2018/08/15/benjamin-fulton-recognition/ |access-date=2018-09-11}} although the existence of this radius gap had been noted along with its underlying mechanisms as early as 2011, 2012 and 2013.

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.