2025-08-11 マサチューセッツ工科大学 (MIT)
＜関連情報＞
- https://news.mit.edu/2025/planets-without-water-could-still-produce-certain-liquids-0811
- https://www.pnas.org/doi/10.1073/pnas.2425520122
温かく、水分の少ない岩石系系外惑星で表面にイオン液体を有するもの：惑星の居住可能性に関する新たな分類提案 Warm, water-depleted rocky exoplanets with surface ionic liquids: A proposed class for planetary habitability
Rachana Agrawal, Sara Seager, Iaroslav Iakubivskyi, +5 , and Janusz J. Petkowski
Proceedings of the National Academy of Sciences Published:August 11, 2025
DOI:https://doi.org/10.1073/pnas.2425520122
Significance
The search for habitable exoplanets has intensified with new telescopes and a growing number of exoplanets. Yet, many known exoplanets are too warm for surface liquid water and therefore considered inhospitable to life. Liquid is a fundamental requirement for life as we understand it, but whether that liquid has to be water is not known. We propose that such planets could still support life through ionic liquids, substances with negligible vapor pressure that remain liquid in warm, low-pressure conditions, even approaching a vacuum. Our experiments show that ionic liquids can form from planetary materials—sulfuric acid and nitrogen-containing organic compounds—offering a potential pathway for life on warm, thin-atmosphere, water-depleted worlds.
Abstract
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. 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.
