Using observations of escaping H/He to constrain the atmospheric composition of sub-Neptunes
File(s)
Published version
Author(s)
Rogers, James G
Owen, James E
Schreyer, Ethan
Kirk, James
Type
Journal Article
Abstract
The internal composition of sub-Neptunes remains a prominent unresolved question in exoplanetary science. We present a technique to place constraints on envelope mean molecular weight that utilizes observations of escaping hydrogen or helium exospheres. This method is based on a simple time-scale argument, which states that sub-Neptunes require
a sufficiently large hydrogen or helium reservoir to explain ongoing escape at their observed rates. This then naturally leads to an upper limit on atmospheric mean molecular weight. We formalize this argument within a Bayesian inference model and apply it to the archetypal sub-Neptunes GJ-436 b, TOI-776 b and TOI-776 c, which have all been observed to be losing significant hydrogen content as well as relatively featureless transit spectra when observed with James Webb Space Telescope . Combining constraints from atmospheric escape and transit spectroscopy in the case of TOI-776 c allows us to tentatively rule out the high mean molecular weight scenario, pointing towards a low mean molecular weight atmosphere with high-altitude aerosols muting spectral features in the infrared. Finally, we reframe our analysis to the hycean candidate K2-18 b, which has also been shown to host a tentative escaping hydrogen exosphere. If such a detection
is robust, we infer a hydrogen-rich envelope mass fraction of log 10 fenv = −1 . 67 ± 0 . 78 , which is inconsistent with the hycean scenario at the ∼ 4 σ level. This latter result requires further observational follow-up to confirm.
a sufficiently large hydrogen or helium reservoir to explain ongoing escape at their observed rates. This then naturally leads to an upper limit on atmospheric mean molecular weight. We formalize this argument within a Bayesian inference model and apply it to the archetypal sub-Neptunes GJ-436 b, TOI-776 b and TOI-776 c, which have all been observed to be losing significant hydrogen content as well as relatively featureless transit spectra when observed with James Webb Space Telescope . Combining constraints from atmospheric escape and transit spectroscopy in the case of TOI-776 c allows us to tentatively rule out the high mean molecular weight scenario, pointing towards a low mean molecular weight atmosphere with high-altitude aerosols muting spectral features in the infrared. Finally, we reframe our analysis to the hycean candidate K2-18 b, which has also been shown to host a tentative escaping hydrogen exosphere. If such a detection
is robust, we infer a hydrogen-rich envelope mass fraction of log 10 fenv = −1 . 67 ± 0 . 78 , which is inconsistent with the hycean scenario at the ∼ 4 σ level. This latter result requires further observational follow-up to confirm.
Date Issued
2026-06-15
Date Acceptance
2026-05-27
Citation
Monthly Notices of the Royal Astronomical Society, 2026, 549 (4)
ISSN
0035-8711
Publisher
Oxford University Press (OUP)
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
549
Issue
4
Copyright Statement
© The Author(s) 2026. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
License URL
Publication Status
Published
Article Number
stag1043
Date Publish Online
2026-06-03