Small planet radius gap

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.

[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.