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