For decades, scientists searching for life beyond Earth have focused on one key requirement: liquid water.
Since water is essential for life here, it has long been assumed to be a necessity everywhere.
But a new study from MIT and collaborators suggests that other types of liquids—completely different from water—might also provide a home for life, greatly expanding the number of potentially habitable planets.
The research, published in Proceedings of the National Academy of Sciences, reveals that a type of fluid called an ionic liquid could form naturally on rocky planets and moons, even in places where water can’t exist. Ionic liquids are essentially salts that stay in liquid form at relatively low temperatures—below about 100°C.
Unlike water, they do not evaporate easily, and they can remain stable in environments that are hotter or under much lower pressure than those needed for liquid water.
The MIT-led team discovered that ionic liquids can form when sulfuric acid reacts with certain nitrogen-containing organic compounds.
Sulfuric acid can be produced by volcanic activity, while nitrogen-based organics have been found on asteroids, planets, and moons across our solar system, suggesting they may be common in other planetary systems too.
These unusual liquids may even be able to host certain biomolecules, such as proteins, that can remain stable in them. That means planets too hot or with too thin an atmosphere for water might still have “pockets” of liquid—possibly opening the door to entirely different kinds of life.
“We think of water as essential for life because that’s true for life on Earth,” said study lead author Rachana Agrawal, a former postdoctoral researcher in MIT’s Department of Earth, Atmospheric and Planetary Sciences.
“But if we take a broader view, all that’s really required is a liquid where metabolism can happen. Including ionic liquids in that definition greatly expands the habitable zone for rocky worlds.”
The discovery began with a project to explore whether life could exist in the clouds of Venus, which are filled with sulfuric acid.
Agrawal and MIT planetary scientist Sara Seager were testing ways to evaporate sulfuric acid from collected samples, using a low-pressure system. When they mixed sulfuric acid with the organic compound glycine, most of the acid evaporated—but a stubborn layer of liquid remained.
They soon realized that the acid had chemically reacted with the glycine, creating an ionic liquid that stayed stable across a wide range of temperatures and pressures. This accidental finding led them to wonder whether similar reactions could occur naturally on other planets.
In lab tests, the team mixed sulfuric acid with over 30 different nitrogen-containing organic compounds, at various temperatures and pressures, including conditions much hotter than Earth and far lower in pressure than our atmosphere.
They also tested the mixtures on basalt rock, a common planetary surface material. In nearly every case, ionic liquid formed and persisted, even after excess acid seeped into the rock.
The results showed that these liquids could form at up to 180°C and survive in extremely low-pressure environments—conditions far beyond water’s limits. On a hot, dry planet with past or present volcanic activity, sulfuric acid could flow over organic-rich patches of ground, creating tiny reservoirs of ionic liquid that might last for years or even millennia.
While it’s too soon to say whether life could actually exist in such liquids, the finding opens up new possibilities for habitability.
“We’ve just opened a Pandora’s box of research,” Seager said. “It’s exciting to think there might be entire classes of planets that could support life in a way we’ve never considered before.”
