Evolutionary pathways toward survival of a thick CO2- or SO2-rich atmosphere on the lava world TOI-561 b
Year: 2026
Type: preprint
Abstract: Rocky planets evolve through the exchange of volatiles between their interiors and atmospheres, an interplay still poorly constrained by observations. Remarkably, highly irradiated ultrashort-period (USP) exoplanets may offer a window into this exchange -- some retain low bulk densities compatible with volatile-rich envelopes surrounding rocky interiors, indicating possible secondary atmospheres. TOI-561 b is a prime example, with a bulk density of $4.3\pm0.4$ g cm$^{-3}$ and recent JWST observations favoring a thick volatile atmosphere overlying a dayside magma ocean. Here, we investigate the evolutionary pathways allowing TOI-561 b to retain a substantial atmosphere over gigayears using the PROTEUS coupled interior--atmosphere framework. We explore different core radius fractions, Bond albedos, atmospheric escape efficiencies, mantle redox states, and initial C--H--O--N--S volatile inventories, under in situ evolution and late inward migration. Over half of our simulations leave a bare interior too dense to match observations. Successful cases favor a volatile-rich origin ($\lesssim200$ Earth oceans of hydrogen, S/H $\le10$, and N/H $\le1$), an oxidized mantle ($f$O$_2 \gtrsim \mathrm{IW}+4$), a small iron core ($\le 0.40$ for the core radius fraction), and low escape efficiency ($ε\lesssim 10^{-3}$) in the hydrodynamic escape regime. At present, TOI-561 b is consistent with a global magma ocean beneath a thick (surface pressure $\approx 10^{3}$--$10^{4}$ bar), high mean molecular weight atmosphere ($38$--$60$ g mol$^{-1}$). Two archetypes emerge, differentiated by bulk sulfur content: a CO$_2$-dominated and an SO$_2$-dominated atmosphere. Migration is viable but not required to reproduce the observations. Our study illustrates how interior--atmosphere coupling governs atmospheric retention on irradiated rocky planets.
Source: arXiv (Cornell University)
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
SDG: Climate action
Open Access status: green
Funders: National Science Foundation, National Aeronautics and Space Administration, Alfred P. Sloan Foundation, Space Telescope Science Institute, UK Research and Innovation, European Commission, Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung, Rijksuniversiteit Groningen, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Canadian Space Agency, Natural Sciences and Engineering Research Council of Canada, Science and Technology Facilities Council
Awards: G103270039, G2191281307, G2262804166, G256731684, G3680819011, PRELIFE - Pathways, Reactions, and Environments leading to LIFE: An interdisciplinary approach., G6686480456, G6956711515, Charting the Evolution of Alien Worlds: Integrating Interior, Atmospheric, and Orbital Processes, Thermo-chemical Evolution and Formation of Super-Earths and Mini-Neptunes, Distinguishing oceans of water from magma on sub-Neptunes, G8673939453, G8827130585, Non-variant antigens of trypanosomes as potential target for vaccination
OpenAlex ID: W7208755667