Could There Be Life Under a Hydrogen Sky?

In 2007, Sara Seager read a single line that changed the course of her career. It said: “Nothing would be more tragic in the American exploration of space than to encounter alien life and fail to recognize it.”

Ever since, Seager (MIT) has struggled with the same worry — concerned that we might observe life yet fail to identify it for the sheer reason that it doesn’t resemble life on Earth. “I still use this as a guiding principle,” she says.

So, she set out to demonstrate that life can persist in atmospheres quite different from our own — even those composed of 100% hydrogen. Her latest study, in Nature Astronomy, suggests that microorganisms can grow within the highly flammable gas. Although the finding is not necessarily surprising to many microbiologists, it does have implications for the search for life across the universe.  

Cultivating Life

For the most part, astronomers have only been able to investigate the atmospheres of large worlds — those as massive as Jupiter and Neptune. The puffy atmospheres of these giant planets make them far easier to detect than the slim atmospheres of rocky worlds. While new technology and tools are enabling astronomers to probe smaller and smaller worlds, the mantra still holds: The puffier, the better.

For that reason, astronomers will first be able to detect hydrogen-dominated atmospheres around rocky worlds before they can detect Earth-like atmospheres. So, Seager and her colleagues turned to the lab to see whether microorganisms could grow in such alien conditions.

The team took cultures of microbes — specifically E. coli and yeast, which are representative of simple and complex life forms, respectively — tossed them into a nutrient-rich solution, and flushed them with hydrogen gas. Then, every hour or so for the next few days, they counted the number of microbes within their testbed to see whether those numbers were increasing. “It’s like sourdough — it just kind of multiplies if you give it the right stuff,” Seager says.

At the end of the experiment, the team found that both microorganisms could reproduce normally in hydrogen, albeit at slightly lower rates than in air.

But Kenneth Nealson (University of Southern California), a microbiologist who was not involved in the study, argues that this is no surprise. When you place E. coli and yeast in a glucose broth, they will ferment — meaning that they will convert the sugar into alcohol, gases, or acids — and not respire. As such, they could grow in virtually no atmosphere.

“I don’t see that there is anything [here] that was not already well known by almost any microbial physiologist,” Nealson says. “I think this is a great example of the danger of interdisciplinary work.”

Seager, who is an astronomer and not a microbiologist, admits that she was not surprised either. Instead, she argues that one goal of this paper is to show the need for communication between the sciences, even if that means presenting simple, well-established microbial research to the wider astronomical community.

“Such communication is crucial if we are to find life elsewhere, as it requires inputs from multiple sciences that are understandable for all,” she says. “Our results are new for the astronomers and as such had to be shown in an astronomical context.”

On the Hunt for Hydrogen

With the experiments in hand, there is no need to focus on finding atmospheres that are particularly Earth-like. Instead, Seager will include ones that are overflowing with hydrogen in her search for life.

But Dirk Schulze-Makuch (Technical University Berlin), an astrobiologist who was not involved in the study, worries about this approach.

To begin, there likely aren’t a large number of rocky planets with hydrogen atmospheres since the element so easily escapes into space. “I don't think that there will be awfully many,” he says. “But if there are some, that enriches the possibilities of where you could have life.”

In addition, microbes find hydrogen tasty — so tasty that Schulze-Makuch worries that any life would deplete the hydrogen within the atmosphere. That happened to the early Earth. “So if I see a terrestrial planet that has a lot of hydrogen there, it would not really be a biosignature — but an anti-biosignature,” he says.

Although scientists can’t yet agree on the implications of a hydrogen-rich world, conversations like this will lay the groundwork for future detections of extraterrestrial life. Especially if these conversations include more microbiologists, as Nealson urges, since there is already a rich landscape of studies that show that life can utilize hydrogen as an energy source to not only grow, but thrive.