Fulton gap

The Fulton gap (also called the photoevaporation valley[1][2] or Sub-Neptune Desert[3]) is an observed scarcity of planets with radii between 1.5 and 2 times Earth's radius, likely due to photoevaporation-driven mass loss.[4][5][6] A bimodality in the Kepler exoplanet population was first observed in 2013, and was noted as possibly confirming an emerging hypothesis that photoevaporation could drive atmospheric mass loss on close-in, low-mass planets.[4][7] 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.[4][7] The bimodality in the distribution was confirmed with higher-precision data in the California-Kepler Survey in 2017,[5][8] which was shown to match the predictions of the photoevaporative mass-loss hypothesis later that year.[6]

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[5]. As a result, 'valley' is often used in place of 'gap'.[1][2][6] 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,[9][10] although the existence of this radius gap had been noted along with its underlying mechanisms as early as 2012[7] and 2013.[4]