Evolutionary pathways toward survival of a thick CO2- or SO2-rich atmosphere on the lava world TOI-561 b

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±0.4 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 (≲200 Earth oceans of hydrogen, S/H ≤10, and N/H ≤1), an oxidized mantle (fO₂ ≳ IW+4), a small iron core (≤ 0.40 for the core radius fraction), and low escape efficiency (ε≲ 10⁻³) in the hydrodynamic escape regime. At present, TOI-561 b is consistent with a global magma ocean beneath a thick (surface pressure ≈ 10³--10⁴ bar), high mean molecular weight atmosphere ($38−−60$ g mol⁻¹). Two archetypes emerge, differentiated by bulk sulfur content: a CO₂-dominated and an SO₂-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.

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BibTeX RIS

BibTeX

@misc{postolec2026evolutionary,
  title = {{Evolutionary pathways toward survival of a thick CO2- or SO2-rich atmosphere on the lava world TOI-561 b}},
  author = {Emma Postolec and Tim Lichtenberg and Johanna K. Teske and Harrison Nicholls and Mara Attia and Anjali Piette and Lisa Dang and Nicole L. Wallack and Mykhaylo Plotnykov and Alex McGinty and Samuel Boucher and Bo Peng and Diana Valencia},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.03144},
  eprint = {2609.03144},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.03144},
}

RIS

TY  - UNPB
TI  - Evolutionary pathways toward survival of a thick CO2- or SO2-rich atmosphere on the lava world TOI-561 b
AU  - Emma Postolec
AU  - Tim Lichtenberg
AU  - Johanna K. Teske
AU  - Harrison Nicholls
AU  - Mara Attia
AU  - Anjali Piette
AU  - Lisa Dang
AU  - Nicole L. Wallack
AU  - Mykhaylo Plotnykov
AU  - Alex McGinty
AU  - Samuel Boucher
AU  - Bo Peng
AU  - Diana Valencia
PY  - 2026
DA  - 2026/09/02
PB  - arXiv
DO  - 10.48550/arxiv.2609.03144
UR  - https://arxiv.org/abs/2609.03144
AB  - 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±0.4 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 (≲200 Earth oceans of hydrogen, S/H ≤10, and N/H ≤1), an oxidized mantle (fO_2 ≳ IW+4), a small iron core (≤ 0.40 for the core radius fraction), and low escape efficiency (ε≲ 10^−3) in the hydrodynamic escape regime. At present, TOI-561 b is consistent with a global magma ocean beneath a thick (surface pressure ≈ 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.
ER  -

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