Volatile-rich Evolution Of Molten Super-Earth L 98-59 d

Small low-density exoplanets are sculpted by strong stellar irradiation but their origin is unknown.

Two competing scenarios are that they formed either with rocky interiors and H2-He atmospheres (‘gas-dwarfs’), or instead with bulk compositions dominated by H₂O phases (‘water-worlds’). Observations of L 98-59 d have revealed its unusually low density and spectral characteristics, opening a window for disentangling its origin.

We constrain the possible range of evolutionary histories linking L 98-59 d’s birth conditions to these observations, using a coupled atmosphere-interior modelling framework. Observations are explained by a chemically-reducing mantle and early substantial (>1.8 mass%) sulfur and hydrogen content, inconsistent with both the gas-dwarf and water-world origin scenarios. Observed spectral features are explained by in-situ photochemical production of SO₂ in an H₂ background.

L 98-59 d’s interior comprises a permanent magma ocean, allowing long-term retention of volatiles within its interior over billions of years, consistent with surveyed Kepler Survey trends. Modelling of the `cosmic shoreline’ suggests that early volatile-metal budget is a determinant of atmospheric retention – here, we explicitly show that large initial budgets can explain JWST observations of some super-Earth planets (including DDT targets).

In breaking with the supposed gas-dwarf/water-world dichotomy, we reveal an evolutionary pathway defined by high-molar-mass atmospheres overlying magma oceans, and invite a more nuanced taxonomy of small exoplanets.

Harrison Nicholls, Tim Lichtenberg, Richard D. Chatterjee, Claire Marie Guimond, Emma Postolec, Raymond T. Pierrehumbert

Comments: In review. 4 figures and 4 pages in the main text
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2507.02656 [astro-ph.EP] (or arXiv:2507.02656v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.02656
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Submission history
From: Harrison Nicholls
[v1] Thu, 3 Jul 2025 14:19:28 UTC (1,457 KB)
https://arxiv.org/abs/2507.02656
Astrobiology