Beyond gas dwarfs and water worlds: hybrid envelopes among the M-dwarf sub-Neptunes characterised with JWST
Abstract
Sub-Neptunes orbiting M dwarfs are commonly interpreted either as rocky cores beneath primordial H/He envelopes (gas dwarfs) or as volatile-rich bodies formed beyond the water-ice line (water worlds). Because the two are degenerate in mass and radius once envelope evolution is treated self-consistently, the atmospheric mean molecular weight μ is the more direct discriminant. We compile the eight M-dwarf sub-Neptunes with published JWST transmission spectra and a constraint on μ, grade each constraint by the quality of the underlying evidence, and combine it with the restricted Jeans parameter and with energy-limited escape integrated over the system lifetime, using XUV histories from M-dwarf activity-age relations. Planets are classified by what their present composition implies about their origin. At the μ= 3.8 amu threshold adopted in the gas-dwarf literature, four planets fit neither end-member: their envelopes (3.8 < μ≤ 18 amu) are too heavy for primordial H/He, yet neither their composition nor their escape history requires a volatile-rich origin. Two of these assignments rest on molecular detections (LP 791-18 c and TOI-270 d) and two on bounded or conditional metallicities. One planet, LHS 1140 b, requires a volatile-rich origin; one, TOI-776 c, is tentatively a gas dwarf; two remain ambiguous. We propose the term hybrid worlds for the intermediate class, whose envelopes can be produced by magma-ocean equilibration, supercritical miscibility, preferential hydrogen loss or accretion interior to the ice line. The count depends on the adopted threshold (three planets at 5 amu, one at 10 amu), but no threshold leaves both end-member classes physically consistent. The sample is set by JWST target selection, so these fractions are not occurrence rates; a homogeneous survey testing whether μ is bimodal would test the proposed class directly.
Related
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- Coupled Orbital and Interior Evolution of Sub-Neptunes
BibTeX
@misc{navarrete2026beyond,
title = {{Beyond gas dwarfs and water worlds: hybrid envelopes among the M-dwarf sub-Neptunes characterised with JWST}},
author = {Juan F. Pagés Navarrete},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.34635},
eprint = {2609.34635},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.34635},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Beyond gas dwarfs and water worlds: hybrid envelopes among the M-dwarf sub-Neptunes characterised with JWST
AU - Juan F. Pagés Navarrete
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2609.34635
UR - https://arxiv.org/abs/2609.34635
AB - Sub-Neptunes orbiting M dwarfs are commonly interpreted either as rocky cores beneath primordial H/He envelopes (gas dwarfs) or as volatile-rich bodies formed beyond the water-ice line (water worlds). Because the two are degenerate in mass and radius once envelope evolution is treated self-consistently, the atmospheric mean molecular weight μ is the more direct discriminant. We compile the eight M-dwarf sub-Neptunes with published JWST transmission spectra and a constraint on μ, grade each constraint by the quality of the underlying evidence, and combine it with the restricted Jeans parameter and with energy-limited escape integrated over the system lifetime, using XUV histories from M-dwarf activity-age relations. Planets are classified by what their present composition implies about their origin. At the μ= 3.8 amu threshold adopted in the gas-dwarf literature, four planets fit neither end-member: their envelopes (3.8 < μ≤ 18 amu) are too heavy for primordial H/He, yet neither their composition nor their escape history requires a volatile-rich origin. Two of these assignments rest on molecular detections (LP 791-18 c and TOI-270 d) and two on bounded or conditional metallicities. One planet, LHS 1140 b, requires a volatile-rich origin; one, TOI-776 c, is tentatively a gas dwarf; two remain ambiguous. We propose the term hybrid worlds for the intermediate class, whose envelopes can be produced by magma-ocean equilibration, supercritical miscibility, preferential hydrogen loss or accretion interior to the ice line. The count depends on the adopted threshold (three planets at 5 amu, one at 10 amu), but no threshold leaves both end-member classes physically consistent. The sample is set by JWST target selection, so these fractions are not occurrence rates; a homogeneous survey testing whether μ is bimodal would test the proposed class directly.
ER -