A Stellar-Type Dependence in the Rocky and Volatile Composition of Small Exoplanets

Abstract: We investigate the rocky and volatile composition of small exoplanets by modeling the population-level distribution of densities using a mixture framework that links interior structure models to observable quantities. We analyze three complementary samples spanning different stellar environments: the Luque & Pallé M-dwarf sample, the DACE M-dwarf sample, and the DACE FGK sample. We consider a log-normal parameterization, which captures a characteristic core mass fraction (CMF) and the intrinsic dispersion to describe a single rocky population. The single rocky population inference suggests a higher CMF for small planets around FGK stars than those around M stars by $\sim$16\% (7-11 $σ$ depending on sample selection). We also consider a power-law parameterization, which probes clustering near compositional boundaries at CMF=0.32. The power-law parameterization provides an alternative interpretation: 89.9\% to 97.0\% of the planets around FGK stars are rocky whereas up to 61.6\% (ranging from 4.1\% to 61.6\%) of planets around M stars are rocky. In addition, we find that volatile mass fractions are highly concentrated. For example, to describe the DACE M-dwarf sample using a mixture of rocky, water-rich, and gas-rich planets, we find that more than 99.5\% gaseous planets have an atmospheric mass fraction (AMF) $\lesssim 0.01\%$, and more than 55.4\% (82.7\%) gaseous planets have a water mass fraction (WMF) $\lesssim 0.1\%$ ($\lesssim 1\%$). These results suggest that while volatile-bearing planets are common, their composition and prevalence depend strongly on stellar environment, and their volatile inventories are tightly constrained by formation and evolutionary processes.