[论文解读] Using observations of escaping H/He to constrain the atmospheric composition of sub-Neptunes

James G. Rogers, James E. Owen+2 more|arXiv (Cornell University)|Jan 20, 2026

Stellar, planetary, and galactic studies被引用 0

一句话总结

本文提出一种贝叶斯方法，通过观测逃逸的 H/He，将 sub-Neptune 的包裹层平均分子量 μ 限制在一定范围内，结合质量损失推导与 JWST/HST 遥感透射光谱，来限制大气组成与气溶胶压力。

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 utilises observations of escaping hydrogen or helium exospheres. This method is based on a simple timescale argument, which states that sub-Neptunes require a sufficiently large hydrogen or helium reservoir to explain on-going escape at their observed rates. This then naturally leads to an upper limit on atmospheric mean molecular weight. We apply this technique to archetypal sub-Neptunes, namely 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 JWST. 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 infra-red. 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 $\log f_ ext{env} = -1.67\pm0.78$, which is inconsistent with the hycean scenario at the $\sim 4σ$ level. This latter result requires further observational follow-up to confirm.

研究动机与目标

• Motivate the use of hydrogen/helium escape observations to bound envelope mean molecular weight in sub-Neptunes.
• Develop a simple analytical timescale-based constraint on envelope composition (upper μ) from observed mass loss.
• Apply the method to archetypal planets (TOI-776 b, TOI-776 c, GJ 436 b) and compare with JWST spectroscopy results.
• Integrate escape-derived constraints with atmospheric spectroscopy to refine atmospheric properties including aerosols.
• Reframe the analysis for the Hycean candidate K2-18 b and discuss implications for hydrogen-rich envelopes.

提出的方法

• Use a hydrogen mass loss timescale argument to derive an upper limit on envelope mean molecular weight μ.
• Adopt a Bayesian inference framework (UltraNest) to infer core mass, envelope fraction, μ, and related properties from mass, radius, and escape data.
• Model planetary structure with a simple envelope, deriving transit radius via a Rogers (2025) framework and H2–He–H2O opacities.
• Incorporate priors from literature for masses, radii, and mass-loss posteriors; account for uncertainties in stellar age and planet properties.
• Combine mass-loss constraints with JWST transmission spectroscopy constraints to infer joint μ limits and aerosol pressure levels.

Figure 1: The upper limit on envelope mean molecular weight is shown as a function of observed hydrogen mass loss rate for various envelope mass fractions. This applies to a $5$ Gyr old sub-Neptune with a core mass of $5$ M ⊕ and assuming a Solar hydrogen-to-helium mass fraction following Equations

实验结果

研究问题

• RQ1 Can observations of escaping H/He set an upper bound on the envelope mean molecular weight for sub-Neptunes?
• RQ2 How do mass-loss-derived μ limits compare with spectroscopic inferences of low/high μ atmospheres and high-altitude aerosols?
• RQ3 Can joint analysis of escaping hydrogen and JWST spectra break degeneracies between low-μ atmospheres with aerosols and high-μ atmospheres?
• RQ4 What implications do these constraints have for Hycean candidates like K2-18 b?

主要发现

• For a 5 M⊕ core and 1% envelope, the hydrogen mass loss implies X ≥ 0.53, translating to μ ≤ 3.09 g/mol under solar H2/He ratios.
• TOI-776 b, TOI-776 c, and GJ 436 b yield μ upper limits of 13.6, 12.8, and 10.2 g/mol respectively from mass-loss constraints alone.
• JWST spectroscopy for TOI-776 c shows a bimodal μ–aerosol pressure posterior; with aerosols above ~10^-2 bar, μ is constrained to be ≥ ~7 g/mol, and combined with mass loss gives μ ≤ 12.4 g/mol at 1σ.
• Joint analysis for TOI-776 c favors low-μ atmospheres with aerosols at low pressures when including escape data, potentially breaking degeneracies seen in transmission alone.
• Using HST Ly-α escape for K2-18 b, the inferred hydrogen envelope fraction is log f_env = -1.67 ± 0.78, which challenges the Hycean scenario at ≈4σ if the detection holds.

Figure 2: Marginalised posteriors are shown for the composition of TOI-776 b, TOI-776 c and GJ-436 b in the top, middle and bottom rows, respectively. The constraints come from observed masses, radii and escaping hydrogen mass loss rates from Schreyer et al. ( 2024 ); Loyd et al. ( 2025 ) . Here we