paper

An Analysis of the Radius Gap in a Sample of Kepler, K2 and TESS exoplanets orbiting M Dwarf Stars

arXiv:2509.01930

Abstract

Planetary radii are derived for 218 exoplanets orbiting 161 M dwarf stars. Stellar radii are based on an analysis of APOGEE high-resolution near-IR spectra for a subsample of the M-dwarfs; these results are used to define a stellar radius-M $_{K_{s}}$ calibration that is applied to the sample of M-dwarf planet hosts. The planetary radius distribution displays a gap over R $_{p}$ $\sim$ 1.6-2.0 R $_{\oplus}$ , bordered by two peaks at R $_{p}$ $\sim$ 1.2-1.6 R $_{\oplus}$ (super-Earths) and 2.0-2.4 R $_{\oplus}$ (sub-Neptunes). The radius gap is nearly constant with exoplanetary orbital period (a power-law slope of m= $+ 0.0 1_{- 0.04}^{+ 0.03}$ ), which is different (2-3 $I ¨ $ ) from m $\sim$ $-$ 0.10 found previously for FGK dwarfs. This flat slope agrees with pebble accretion models, which include photoevaporation and inward orbital migration. The radius gap as a function of insolation is approximately constant over the range of S $_{p}$ $\sim$ 20-250 S $_{\oplus}$ . The R $_{p}$ -P $_{orb}$ plane exhibits a sub-Neptune desert for P $_{orb}$ $<$ 2d, that appears at S $_{p}$ $>$ 120 S $_{\oplus}$ , being significantly smaller than S $_{p}$ $>$ 650 S $_{\oplus}$ found in the FGK planet-hosts, indicating that the appearance of the sub-Neptune desert is a function of host-star mass. Published masses for 51 exoplanets are combined with our radii to determine densities, which exhibit a gap at $I ¨ _{p}$ $\sim$ 0.9 $I ¨ _{\oplus}$ , separating rocky exoplanets from sub-Neptunes. The density distribution within the sub-Neptune family itself reveals two peaks, at $I ¨ _{p}$ $\sim$ 0.4 $I ¨ _{\oplus}$ and $\sim$ 0.7 $I ¨ _{\oplus}$ . Comparisons to planetary models find that the low-density group are gas-rich sub-Neptunes, while the group at $<$ $I ¨ _{p}$ $>$ $\sim$ 0.7 $I ¨ _{\oplus}$ likely consists of volatile-rich water worlds.

Submitted to ApJ