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

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$_{\rm K_{\rm s}}$ calibration that is applied to the sample of M-dwarf planet hosts. The planetary radius distribution displays a gap over R$_{\rm p}$$\sim$1.6-2.0 R$_{\oplus}$, bordered by two peaks at R$_{\rm p}$$\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=$+0.01^{+0.03}_{-0.04}$), which is different (2-3$\sigma$) from m$\sim$$-$0.10 found previously for FGK dwarfs. This flat slope agrees with pebble accretion models, which include photoevaporation and inward orbital migration. The radius gap as a function of insolation is approximately constant over the range of S$_{\rm p}$$\sim$20-250 S$_{\oplus}$. The R$_{\rm p}$-P$_{\rm orb}$ plane exhibits a sub-Neptune desert for P$_{\rm orb}$$<$2d, that appears at S$_{\rm p}$$>$120 S$_{\oplus}$, being significantly smaller than S$_{\rm p}$$>$650 S$_{\oplus}$ found in the FGK planet-hosts, indicating that the appearance of the sub-Neptune desert is a function of host-star mass. Published masses for 51 exoplanets are combined with our radii to determine densities, which exhibit a gap at $\rho_{\rm p}$$\sim$0.9$\rho_{\oplus}$, separating rocky exoplanets from sub-Neptunes. The density distribution within the sub-Neptune family itself reveals two peaks, at $\rho_{\rm p}$$\sim$0.4$\rho_{\oplus}$ and $\sim$0.7$\rho_{\oplus}$. Comparisons to planetary models find that the low-density group are gas-rich sub-Neptunes, while the group at $<$$\rho_{\rm p}$$>$$\sim$0.7$\rho_{\oplus}$ likely consists of volatile-rich water worlds.