Rocky planets may not need water to host alien life

For decades, the search for life beyond Earth has centered on one key ingredient: water. If a planet didn’t have it, scientists tended to cross it off the list of potentially habitable worlds.

But that assumption may be far too narrow. A new study suggests that life might not need water at all.

Instead, an entirely different type of liquid – one that behaves very differently from water – could provide a safe haven for living chemistry on planets where water can barely exist.

Rethinking habitability without water

Researchers at the Massachusetts Institute of Technology (MIT) have been exploring the possibilities of ionic liquids – fluids made of salts that remain liquid below 100 degrees Celsius.

The team’s experiments show that these liquids could form naturally from chemicals already expected on some rocky planets and moons.

In the lab, the researchers found that mixing sulfuric acid with certain nitrogen-containing organic compounds produced an ionic liquid.

Sulfuric acid is a common byproduct of volcanic activity, while nitrogen-based organics have been detected on asteroids and planets in our solar system – suggesting they may also exist elsewhere in the galaxy.

Potential hosts for simple life forms

Unlike water, ionic liquids don’t evaporate easily. They can remain stable in hotter, lower-pressure conditions – the kinds of environments that would destroy liquid water.

Even more intriguing, some proteins can remain stable in these liquids, making them potential hosts for simple forms of life.

Rachana Agrawal led the study as a postdoc in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

“We consider water to be required for life because that is what’s needed for Earth life. But if we look at a more general definition, we see that what we need is a liquid in which metabolism for life can take place,” said Agrawal.

“Now if we include ionic liquid as a possibility, this can dramatically increase the habitability zone for all rocky worlds.”

Accidental discovery in the lab

The discovery came by accident. The team had been studying Venus – a planet shrouded in clouds of sulfuric acid – as part of ongoing efforts to search for life in its atmosphere.

If a spacecraft collects samples from these clouds, scientists would need to evaporate the acid to search for any leftover organic compounds.

Agrawal and fellow researcher Sara Seager were testing ways to do this when they stumbled on something unexpected. They mixed sulfuric acid with glycine, an organic compound, and tried to evaporate it under low pressure.

Most of the sulfuric acid disappeared – but a stubborn layer of liquid stayed behind. The team soon realized this was no ordinary residue.

Ionic liquids on exoplanets

The acid had reacted with glycine, swapping hydrogen atoms and producing a liquid mixture of salts, or an ionic liquid, that stayed stable over a wide range of temperatures and pressures.

“From there, we took the leap of imagination of what this could mean,” said Agrawal. “Sulfuric acid is found on Earth from volcanoes, and organic compounds have been found on asteroids and other planetary bodies.”

“So, this led us to wonder if ionic liquids could potentially form and exist naturally on exoplanets.”

On Earth, ionic liquids are mostly made in industrial settings. Naturally occurring examples are rare. One exception is when the venoms of two rival ant species mix. But could they form elsewhere in space?

Life in non-water liquids

The team tested over 30 nitrogen-containing organics with sulfuric acid under different temperatures and pressures. They even mixed the ingredients on basalt rock, a common type of planetary crust.

“In high school, you learn that an acid wants to donate a proton. And oddly enough, we knew from our past work with sulfuric acid (the main component of Venus’ clouds) and nitrogen-containing compounds, that a nitrogen wants to receive a hydrogen,” noted Seager. “It’s like one person’s trash is another person’s treasure.”

The results were surprising. Ionic liquids formed at temperatures up to 180 degrees Celsius and under pressures far lower than Earth’s atmosphere. Even when excess acid seeped into the rock, a droplet of ionic liquid remained.

“We were just astonished that the ionic liquid forms under so many different conditions,” said Seager.

“If you put the sulfuric acid and the organic on a rock, the excess sulfuric acid seeps into the rock pores, but you’re still left with a drop of ionic liquid on the rock. Whatever we tried, ionic liquid still formed.”

Tiny oases for alien life?

The findings open the door to a striking possibility: planets that are too hot and dry for water could still have patches of liquid – small “oases” where life might emerge.

“We’re envisioning a planet warmer than Earth, that doesn’t have water, and at some point in its past or currently, it has to have had sulfuric acid, formed from volcanic outgassing,” explained Seager.

“This sulfuric acid has to flow over a little pocket of organics. And organic deposits are extremely common in the solar system.”

Those pockets could last for years or even millennia, providing stable environments where ionic-liquid-based life might survive.

“We just opened up a Pandora’s box of new research,” Seager said. “It’s been a real journey.”

The full study was published in the journal Proceedings of the National Academy of Sciences.

Image Credit: NASA/JPL-Caltech.

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