2025/09/02 by Wanderley, Fábio, Cunha, Katia, Smith, Verne V. +11 · 1 citation
#Earth and Planetary Astrophysics (astro-ph.EP) #FOS: Physical sciences #Solar and Stellar Astrophysics (astro-ph.SR)
paper · doi:10.48550/arxiv.2509.01930
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∼1.6-2.0 R⊕, bordered by two peaks at R\rm 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=+0.01+0.03-0.04), which is different (2-3σ) 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 of S\rm p∼20-250 S⊕. The R\rm p-P\rm orb plane exhibits a sub-Neptune desert for P\rm orblt;2d, that appears at S\rm p>120 S⊕, being significantly smaller than S\rm pgt;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\rm p∼0.9ρ⊕, separating rocky exoplanets from sub-Neptunes. The density distribution within the sub-Neptune family itself reveals two peaks, at ρ\rm 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 lt;ρ\rm pgt;∼0.7ρ⊕ likely consists of volatile-rich water worlds.