A recipe for ionic liquids on exoplanets

Ionic liquids—viscous fluids made of salts—could be made naturally on other planets, according to research led by Rachana Agrawal and Sara Seager at the Massachusetts Institute of Technology and Janusz Pętkowski of the Wrocław University of Science and Technology (Proc. Natl. Acad. Sci. U.S.A. 2025, DOI: 10.1073/pnas.2425520122). Like water, pure ionic liquids can stably dissolve an array of proteins. The researchers say this feature potentially makes them an alternative solvent for life on a new class of planets very different from Earth—waterless worlds with almost no atmosphere.

In the search for life beyond our planet, there is an assumption that liquid water is the most likely—if not the only—solvent for life. That assumption means that the “habitable zone” around a star is defined as the distance where a planet could have liquid water on its surface, ruling out many discovered exoplanets deemed too warm. By contrast, ionic liquids can remain liquid at a wide range of temperatures and pressures, including conditions where water would freeze or evaporate. “If life can be based on solvents other than water, then it means that planetary habitability is not strictly tied to the presence of liquid water,” says Pętkowski. “We should broaden our search to include planetary environments that cannot host liquid water . . . [and] ionic liquids are unique candidates for planetary solvents,” he says.

On Earth, ionic liquids are mostly made synthetically, and there are millions of potential combinations that could still be discovered. To see if ionic liquids could occur on other worlds, the team tried to create one from common planetary materials.

The researchers began by mixing organic compounds found on meteorites and asteroids with concentrated sulfuric acid. After the organics had fully dissolved, the team removed excess sulfuric acid using a vacuum chamber, leaving behind a transparent, highly viscous liquid. The team also found that other combinations of temperatures and pressures form ionic liquids from a variety of organic molecules with a nitrogen atom, including a mixture of amino acids, aliphatic amines, and nucleic bases.

“In principle, any positively charged organic molecule could act as a cation to various anions,” Pętkowski says. “Ionic liquids can even form at room temperature and pressure, without the need of excess sulfuric acid,” he adds.

The team was also able to form ionic liquids under more realistic planetary-like conditions—on the surface of basaltic rocks at various settings (80 °C and 10^-5 bar, as well as room temperature and pressure, both in dry air and at ambient humidity). The liquids remained stable against further reactions. “No bodies are known to have ionic liquids in our own solar system because they are either too cold for the liquid phase or lack the necessary chemical components,” says Pętkowski. “But they can conceivably form on a rocky exoplanet,” he says.

The team says the results suggest that ionic liquids could form on a planet that has liquid sulfuric acid—possibly from sulfur dioxide and water vapor released by volcanic activity or from SO₂ reacting with basaltic or mafic rocks—and nitrogen-containing organics, which are common on many solid bodies.

Astronomer Amaury Triaud from the University of Birmingham is cautious about the amounts of ionic liquid that could be present at any given time. “But if small amounts of water were sufficient to help life start on Earth, then ionic liquids might be equally sufficient,” he says. But he cautions that “if future research tells us that large amounts of liquid are necessary, then ionic liquids might not provide a very likely route.”

Despite this, Triaud says that astronomical observers should stay alert. “Should we detect an unusual atmospheric signature on a planet that falls outside of the liquid-water habitable zone, we should keep an open mind and consider that this unusual signature might be produced by an organism being in an ionic liquid habitable zone,” he says.

Sarah Rugheimer, who studies atmospheric biosignatures in Earth-like planets at the University of Edinburgh, says this research is just the first of many steps before starting to look for ionic liquids in exoplanet atmospheres. “For these ionic liquids, the first places to look for life are in environments in our solar system or by alternative biochemistry lab experiments here on Earth,” she says. Rugheimer hopes such experiments could answer the question of whether ionic liquids are suitable for hosting life, and if so, what sorts of biosignatures should we then be looking for in the spectra of exoplanets.