Planets with hydrogen skies could harbor life

Microbes can live and grow in bottles of pure hydrogen, scientists now report. Their new finding could widen the range of worlds on which astronomers might seek evidence of alien life.

“We’re trying to expand people’s view of what should be considered a habitable planet,” says Sara Seager. “It seems to increase our chances that we may find life elsewhere.”

Seager is an astronomer at the Massachusetts Institute of Technology, in Cambridge. She studies exoplanets. They orbit stars other than our sun. Seager has been curious about whether these far-flung planets may host life. Some that astronomers have observed are rocky, like Earth. But their skies have been filled with different gases. Earth’s is made of mostly nitrogen gas with some oxygen. However, the air on some rocky exoplanets’ might be mostly hydrogen. So Seager and her colleagues set up an experiment in the lab to see if microbes could live in such an environment.

They used yeast and E. coli, a type of bacteria. Both are considered stand-ins for other single-celled life. The researchers housed these microbes in small bottles with some nutrient broth. Then, the team pulled all of the air out of in six bottles and replaced it with pure hydrogen gas, pure helium gas or a mixture of 80 percent nitrogen and 20 percent carbon dioxide. A final set of bottles was left with Earth’s air.

Every few hours, the researchers removed some of the microbes. Then they’d count how many cells were alive. As expected, they thrived in Earthly air. But the cell counts grew in all of the other atmosphere’s, too — just not as well.

The team shared its findings May 4 in Nature Astronomy.

Byproducts of life

The microbes didn’t just survive. They also gave off distinctive gases known as biosignatures. The one emitted by E. coli, for example, was ammonia.

If seen in the atmosphere of other planets, those gases might signal life below.

“E. coli is such a simple organism, yet it produces an incredible array” of gases, says Giada Arney. She works at NASA’s Goddard Space Flight Center in Greenbelt, Md. She was not involved in these new experiments. But as an astrobiologist, she studies the possibility of life on other worlds. “Knowing which gases can be produced by life,” she says, “is a necessary first step towards [confirming] them as possible detectable biosignatures on an exoplanet.”

But just seeking a hydrogen-rich atmosphere isn’t enough, says John Baross. He is an astrobiologist at the University of Washington in Seattle. A planet would need other ingredients for life. It would need to have the equivalent of the nutrient broth that Seager’s team fed their microbes. However, he explains, that broth could be a liquid water ocean — one that exchanges chemicals with some rocky surface.

Astrobiologists plan to search for signs of alien life soon. They will do this by looking at starlight that filters through the atmospheres of exoplanets. Future observatories, such as the James Webb Space Telescope, could search for those biosignatures.

But there’s a caveat (KAV-ee-ott) to the recent lab experiments. Seager’s group used pure hydrogen. It’s not clear whether rocky planets with an atmosphere of pure hydrogen even exist. Based on what’s known about how planets form, pure hydrogen atmospheres should be rare, says Daniel Koll. He is a planetary scientist at MIT, but not one involved in the new work.

While hydrogen atmospheres might be rare, they might be relatively easy to spot with telescopes. Hydrogen is very light. An atmosphere of all or mostly hydrogen should be puffy. It would extend up to 14 times farther from the planet’s surface than Earth’s nitrogen-dominated atmosphere does. That means more starlight would filter through the atmosphere on its way to Earth’s telescopes. And this could make it easier to probe those atmospheres for signs of life.

Planets with hydrogen skies might not be the only place to look for life, however. Experiments by Seager and her team showed that microbes could also live where there was air of mostly helium or nitrogen. However, those atmospheres would be thinner. And that would make them harder to detect around small, distant planets.

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Life’s oddballs

Seager says her team’s simple experiments might not surprise many biologists. After all, there are microbes living in hydrogen-rich ecosystems on Earth. Those microbes live in places like mines. That deep, the decay of calcium can create air pockets with 33 to 88 percent hydrogen by volume. That’s way more than is typical in open air on the surface. Our planet’s hydrogen is mostly locked up in water. Overall, Earth’s atmosphere contains much less than 1 percent hydrogen gas.

Earth’s atmosphere also contains oxygen. That’s the gas we need to stay alive. But E. coli can live without oxygen in the guts of many animals. And yeast goes without it too. When used to brew beer, for instance, it thrives without oxygen. That yeast breaks down glucose, a simple sugar, to make beer’s alcohol and carbon dioxide. But neither microbe is adapted to live comfortably in pure hydrogen. So Seager thought it was worth testing to see whether they could survive it.

And they did.

“This is a very in-your-face demonstration that if Earth life can exist under hydrogen-atmosphere conditions, then certainly alien life should be able to,” says Koll. When thinking about other planets, he warns, “We shouldn’t limit, or be too Earth-centric, in what we consider interesting.”

There’s a fun fact, too, if life exists on a planet with hydrogen skies. Organisms complex enough to have lungs and voice boxes would have squeaky, high-pitched voices.

“You know the effect where you inhale a helium balloon and your voice sounds like Mickey Mouse?” Kroll asks. Hydrogen does the same thing. “You could definitely travel to an alien planet, take one breath of air, say something with a squeaky voice, and put your helmet back on.”