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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow/> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> 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 = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>+</mml:mo> <mml:mn>0.0</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.04</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.03</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ), which is different (2 σ –3 σ ) 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 p ∼ 20–250 S ⊕ . The R p – P orb plane exhibits a sub-Neptune desert for P orb &lt; 2 days, which appears at S p &gt; 120 S ⊕ , being significantly smaller than S p &gt; 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 ∼ 0.9 ρ ⊕ , separating rocky exoplanets from sub-Neptunes. The density distribution within the sub-Neptune family itself reveals two peaks, at ρ p ∼ 0.4 ρ ⊕ and ∼0.7 ρ ⊕ . Comparisons to planetary models find that the low-density group are gas-rich sub-Neptunes, while the group at &lt; ρ p &gt; ∼ 0.7 ρ ⊕ likely consists of volatile-rich water worlds.