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Peer-reviewedCC BY 4.0The Astrophysical Journal

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

  • Ji 吉 Wang 王1
  • Caroline Dorn2
  • Komal Bali2
  1. 1The Ohio State University
  2. 2Institute for Particle Physics and Astrophysics
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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 and 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 ∼16% (7 σ –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%–97.0% of the planets around FGK stars are rocky, whereas up to 61.6% (ranging from 4.1%–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) ≲0.01%, and more than 55.4% (82.7%) gaseous planets have a water mass fraction (WMF) ≲0.1% (≲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.

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Cite

BibTeXRIS
BibTeX
@article{王2026stellartype,
  title = {{A Stellar-type Dependence in the Rocky and Volatile Composition of Small Exoplanets}},
  author = {Ji 吉 Wang 王 and Caroline Dorn and Komal Bali},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aea407},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea407},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - A Stellar-type Dependence in the Rocky and Volatile Composition of Small Exoplanets
AU  - Ji 吉 Wang 王
AU  - Caroline Dorn
AU  - Komal Bali
PY  - 2026
DA  - 2026/10/08
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea407
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea407
AB  - 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 and 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 ∼16% (7 σ –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%–97.0% of the planets around FGK stars are rocky, whereas up to 61.6% (ranging from 4.1%–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) ≲0.01%, and more than 55.4% (82.7%) gaseous planets have a water mass fraction (WMF) ≲0.1% (≲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.
SN  - 0004-637X
SN  - 1538-4357
ER  -