2025ApJ...993..233W

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_p ∼ 1.6-2.0 R_⊕, bordered by two peaks at R_p ∼ 1.2-1.6 R_⊕ (super-Earths) and 2.0-2.4 R_⊕ (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 (2σ-3σ) from m ∼ -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 ofS_p ∼ 20-250 S_⊕. TheR_p-P_orb plane exhibits a sub-Neptune desert for P_orb < 2 days, which appears at S_p > 120 S_⊕, being significantly smaller than S_p > 650 S_⊕ 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 ρ_p ∼ 0.9ρ_⊕, separating rocky exoplanets from sub-Neptunes. The density distribution within the sub-Neptune family itself reveals two peaks, at ρ_p ∼ 0.4ρ_⊕ _and ∼0.7ρ_⊕. Comparisons to planetary models find that the low-density group are gas-rich sub-Neptunes, while the group at <ρ_p> ∼ 0.7ρ_⊕ likely consists of volatile-rich water worlds.