Planets with hydrogen-rich atmospheres could harbor life

Microbes can live and grow in an atmosphere of pure hydrogen, lab experiments show. The finding could widen the range of environments where astronomers seek signs of alien life.

“We’re trying to expand people’s view of what should be considered a habitable planet,” says exoplanet astronomer Sara Seager of MIT (SN: 10/4/19). “It seems to increase our chances that we may find life elsewhere.” Seager and her colleagues placed yeast and E. coli — both considered stand-ins for other single-celled organisms — in small bottles with some nutrient broth. The researchers displaced the air 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 air.

Every few hours, the researchers removed some of the microbes with a hypodermic needle to count how many were alive. The microbes had replicated in every atmosphere tested, the team reports May 4 in Nature Astronomy, thriving most in ordinary Earth air.

In addition, E. coli in particular produced several gases that already are considered potential biosignatures, or signs of possible life, if found in other planets’ atmospheres, including ammonia, methanethiol and nitrous oxide (SN: 4/18/16). “E. coli is such a simple organism, yet it produces an incredible array” of gases, says astrobiologist Giada Arney at NASA’s Goddard Space Flight Center in Greenbelt, Md., who was not involved in the experiments. “Knowing which gases can be produced by life is a necessary first step towards vetting them as possible detectable biosignatures on an exoplanet.”

But just seeking a hydrogen-rich atmosphere isn’t enough, says astrobiologist John Baross of the University of Washington in Seattle. A planet would also need to have the equivalent of the nutrient broth in the bottle for life to thrive — perhaps a liquid water ocean that exchanges chemicals with a rocky surface. Astrobiologists plan to search for signs of alien life by looking at starlight filtering through exoplanets’ atmospheres. If life on a rocky planet’s surface emits telltale gases, future telescopes like the James Webb Space Telescope could detect those emissions. It’s not clear whether rocky planets with pure hydrogen atmospheres exist. Based on what’s known about how planets form, pure hydrogen atmospheres should be rare, says planetary scientist Daniel Koll of MIT, who was not involved in the new work.

Because hydrogen is so light, an atmosphere of all or mostly hydrogen would be puffier and extend up to 14 times farther from the surface than Earth’s nitrogen-dominated atmosphere. That means more starlight would filter through the atmosphere on its way to Earthly telescopes, making it easier to probe those atmospheres for biosignatures.

(Science News)

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