Helium Depletion in Escaping Atmospheres of Sub-Neptunes: A Signature of Primary-to-Secondary Transition

TL;DR

This paper investigates how atmospheric escape driven by stellar XUV irradiation, coupled with magma-ocean degassing and volatile production, can drive a primary-to-secondary transition in short-period sub-Neptunes. By employing a 1D atmospheric–interior model that tracks hydrogen, helium, and water, the authors show that low-mass, close-in planets experience substantial helium depletion as hydrogen and water degas from the interior and escape, shrinking the envelope and altering planetary radii. A key finding is the strong correlation between planetary radius and the helium-escape rate, and the tendency for smaller planets (R_p ≲ 2.5 R_⊕) to harbor helium-poor, water-rich secondary atmospheres, potentially explaining non-detections of escaping helium. The study also highlights how interior composition (FeO content) and gas-accretion scenarios modulate the transition, with water production prolonging atmospheric lifetimes and influencing observables such as the 10830 Å helium line. Overall, the work provides a physical framework linking loss, degassing, and observable atmospheric signatures, aiding interpretation of helium-escape measurements and guiding future observations of sub-Neptunes.

Abstract

Short-period sub-Neptunes are common in extrasolar systems. These sub-Neptunes are generally thought to have primary atmospheres of protoplanetary-disk gas origin. However, atmospheric escape followed by degassing from their interiors can lead to the transition to secondary atmospheres depleted in gases less-soluble to magma, such as helium. These primary and secondary atmospheres can potentially be distinguished from observations of escaping hydrogen and helium. This study aims to elucidate the impact of the primary-secondary transition on atmospheric compositions of short-period sub-Neptunes. We simulate their evolution with atmospheric escape driven by stellar X-ray and extreme ultraviolet irradiation and degassing of hydrogen, helium, and water from their rocky interiors, with a one-dimensional structure model. We show that the transition takes place for low-mass, close-in planets which experience extensive atmospheric escape. These planets show the depletion of helium and enrichment of water in their atmospheres, because of their low and high abundances in the planetary interiors, respectively. A compilation of our parameter survey (the orbital period, planetary mass, envelope mass, and mantle FeO content) shows a correlation between the planet radius and the helium escape rate. We suggest that the transition from primary to secondary atmospheres may serve an explanation for helium non-detection for relatively-small ($\lesssim 2.5\ R_\oplus$) exoplanets.

Figures (12)

• Figure 1: Two models adapted for equilibrium conditions between magma ocean, atmosphere, and disk gas in the formation stage (Section \ref{['subsec:methods:workflow']}). (A) case with an exchange of hydrogen and helium between the planetary atmosphere and disk gas. (B) case without exchange of hydrogen and helium between the planetary atmosphere and disk gas, where the atmosphere unilaterally accretes onto the planet.
• Figure 2: Schematic illustration of the planetary evolution in this study.
• Figure 3: Evolution of a planet with the nominal parameter set (Table \ref{['table: parameters']}). Panels indicate (a) the mass fractions of hydrogen, helium, and water in the planetary atmosphere (denoted as $X$, $Y$, and $Z$), (b) the amounts of hydrogen, helium, and water in the magma ocean, (c) the planetary radius, (d) the envelope mass fraction (envelope mass/planetary mass), (e) the temperature at the magma-atmosphere boundary, and (f) the envelope mass.
• Figure 4: Dependence on planetary masses and orbital radii. The other parameters were assumed to be their nominal values (Table \ref{['table: parameters']}). (a)--(c) the mass fractions of hydrogen, helium, and water in the atmosphere, (d)--(f) the amounts of hydrogen, helium, and water in the magma ocean, (g) the planetary radius, and (h) and the envelope mass fraction. Vertical dashed lines correspond to the actual orbital distances ($d = 0.025$, $0.050$, and $0.075\ \mathrm{au}$) adopted in our calculations. For each $d$m the initial and final states are plotted with slight horizontal offsets to the left and right of the dashed lines, respectively, to distinguish them visually. Planets indicated with dashed circles did not retain their atmospheres over 5 Gyrs.
• Figure 5: Same as Figure \ref{['fig:survey_nominal']} but showing the dependence on the oxidation state of magma ocean. Three successive data points corresponds to $(W_{\ce{FeO}})_{\mathrm{ini}} = 0\, \mathrm{wt\%}$, $(W_{\ce{FeO}})_{\mathrm{ini}} = 8.24\, \mathrm{wt\%}$ (nominal), and $(W_{\ce{FeO}})_{\mathrm{ini}} = 49\, \mathrm{wt\%}$, from left to right. The other parameters were assumed to be their nominal values (Table \ref{['table: parameters']}) We note that missing data points indicate where the calculation failed to obtain a solution in the process of evolution.