Habitable Zones

Warm, Water-depleted Rocky Exoplanets With Surface Ionic Liquids: A Proposed Class For Planetary Habitability

By Keith Cowing
Status Report
astro-ph.EP
August 13, 2025
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Warm, Water-depleted Rocky Exoplanets With Surface Ionic Liquids: A Proposed Class For Planetary Habitability
Sulfuric acid phase diagram. The pressure in atm (y-axis) vs. temperature in K (x-axis). The large purple shaded region represents the theoretical liquid-phase boundary for 98% w/w concentrated sulfuric acid (the rest water), adapted from Ballesteros et al. (31), based on their compiled experimental data from (47). Our light grey curve fit follows the data from (32), and diverges from the (31) below 475 K due to different thermodynamic sources. Above about 570 K, H₂SO₄ decomposes into SO₃ and H₂O rather than existing as a stable gas-phase species, shown by the hatched purple region. The solid red line marks an approximate thermal threshold above which organic compounds are expected to thermally break down, where ~470 K is conservative from the lower range of our measured decomposition temperatures (Table 1). The blue shade region indicates the liquid phase of water. The region to the left of the purple and red lines (and spanning about 100 K colder than the purple line (SI Table S9)) includes conditions under which planet surfaces may allow liquid H₂SO₄ to persist, dissolve organic material, and evaporate, leading to the formation of hydrogen sulfate ionic liquids. — astro-ph.EP

The discovery of thousands of exoplanets and the emergence of telescopes capable of exoplanet atmospheric characterization have intensified the search for habitable worlds.

Due to selection biases, many exoplanets under study are planets deemed inhospitable because their surfaces are too warm to support liquid water. We propose that such planets could still support life through ionic liquids: Liquid salts with negligible vapor pressure that can persist on warm planets with thin atmospheres, where liquid water cannot. Ionic liquids have not previously been considered as naturally occurring substances, and thus have not been discussed in planetary science.

Ionic liquids. (A) schematic of an ionic liquid composed of a hydrogen sulfate anion and a glycine cation. (B) Vials of selected ionic liquids composed of N-containing organics and hydrogen sulfate anions, compounds as labeled in the image. Ionic liquids are typically transparent, with a range of chemical characteristics including colors and viscosities. The ionic liquids were generated by evaporating excess sulfuric acid from mixtures of various organic compounds and concentrated sulfuric acid. -- PNAS

Ionic liquids. A) schematic of an ionic liquid composed of a hydrogen sulfate anion and a glycine cation. B) Vials of selected ionic liquids composed of N-containing organics and hydrogen sulfate anions, compounds as labeled in the image. Ionic liquids are typically transparent, with a range of chemical characteristics including colors and viscosities. The ionic liquids were generated by evaporating excess sulfuric acid from mixtures of various organic compounds and concentrated sulfuric acid. See Methods and SI for details. — astro-ph.EP

We demonstrate in laboratory experiments that ionic liquids can form from planetary materials: Sulfuric acid combined with nitrogen-containing organic molecules. Sulfuric acid can be volcanic in origin, and organic compounds are commonly found on planetary bodies.

The required planetary surface is water-depleted and must support sulfuric acid transiently in liquid phase to dissolve organics, followed by evaporation of excess liquid, conditions spanning approximately 300 K at 10^-7 atm to 350-470 K at 0.01 atm. Because ionic liquids have extremely low vapor pressures, they are not prone to evaporation, allowing small droplets or pools to persist without ocean-like reservoirs.

Ionic liquids’ minuscule vapor pressure at room temperature suggests possible stability on planets with negligible atmospheres, shielded by magnetic fields or rock crevices against harsh cosmic radiation. Ionic liquids can stably dissolve enzymes and other biomolecules, enabling biocatalysis and offering a plausible solvent for life, broadening the definition of habitable worlds.

Rachana Agrawal, Sara Seager, Iaroslav Iakubivskyi, Weston P. Buchanan, Ana Glidden, Maxwell D. Seager, William Bains, Jingcheng Huang, Janusz J. Petkowski

Comments: Published in PNAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Chemical Physics (physics.chem-ph)
Cite as: arXiv:2508.08437 [astro-ph.EP] (or arXiv:2508.08437v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.08437
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Related DOI:
https://doi.org/10.1073/pnas.2425520122
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Submission history
From: Janusz Petkowski
[v1] Mon, 11 Aug 2025 19:53:54 UTC (3,934 KB)
https://arxiv.org/abs/2508.08437
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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