Life could survive and prosper in a hydrogen world: study

As new and more powerful telescopes blink in the years to come, astronomers will be able to point the megascopes at nearby exoplanets, observing their atmospheres to decipher their composition and look for signs of extraterrestrial life. But imagine that in our search we encountered strange organisms but did not recognize them as real life.

That is a perspective that astronomers like Sara Seager hope to avoid. Seager, the Professor of Planetary Sciences, Physics and Aeronautics and Astronautics of the Class of 1941 at MIT, is looking beyond a “earth-centered” view of life and is projecting a broader network of what kinds of environments beyond ours they could be really habitable.

In an article published today in the magazine. Astronomy of natureShe and her colleagues have observed in laboratory studies that microbes can survive and thrive in hydrogen-dominated atmospheres, an environment that is very different from Earth’s nitrogen and oxygen-rich atmosphere.

Hydrogen is a much lighter gas than nitrogen or oxygen, and an atmosphere rich in hydrogen would extend much further from a rocky planet. Therefore, it might be easier to detect and study with powerful telescopes, compared to planets with more compact atmospheres, similar to Earth.

Seager’s results show that simple life forms might inhabit planets with hydrogen-rich atmospheres, suggesting that once next-generation telescopes, such as NASA’s James Webb Space Telescope, begin to function, astronomers could Look first for signs of life on hydrogen-dominated exoplanets.

“There are a diversity of habitable worlds, and we have confirmed that Earth-based life can survive in hydrogen-rich atmospheres,” says Seager. “We should definitely add those kinds of planets to the options menu when we think about life on other worlds, and actually try to find it.”

Seager’s MIT co-authors on the document are Jingcheng Huang, Janusz Petkowski, and Mihkel Pajusalu.

Evolving atmosphere

On primitive Earth billions of years ago, the atmosphere looked very different from the air we breathe today. The infant planet still lacked oxygen, and was made up of a gas soup, which included carbon dioxide, methane, and a very small fraction of hydrogen. Hydrogen gas remained in the atmosphere for possibly billions of years, until what is known as the Great Oxidation Event, and the gradual accumulation of oxygen.

The small amount of hydrogen that remains today is consumed by certain ancient lines of microorganisms, including methanogens, organisms that live in extreme climates, such as deep in the ice, or inside the desert floor, and gobble up hydrogen, along with dioxide carbon, to produce methane. .

Scientists routinely study the activity of methanogens grown in laboratory conditions with 80 percent hydrogen. But there are very few studies exploring the tolerance of other microbes to hydrogen-rich environments.

“We wanted to demonstrate that life survives and can grow in a hydrogen atmosphere,” says Seager.

A hydrogen headspace

The team took to the laboratory to study the viability of two types of microbes in a 100 percent hydrogen environment. The organisms they chose were Escherichia coli bacteria, a simple prokaryote, and yeast, a more complex eukaryote, which had not been studied in hydrogen-dominated environments.

Both microbes are standard model organisms that scientists have long studied and characterized, helping researchers design their experiment and understand their results. Additionally, E. coli and yeast can survive with and without oxygen, a benefit to researchers, as they can set up their experiments with any of the organisms outdoors before transferring them to a hydrogen-rich environment.

In their experiments, they separately cultivated yeast and E. coli cultures, then injected the cultures with the microbes into separate bottles, filled with a “broth” or nutrient-rich culture that the microbes could feed. They then expelled the oxygen-rich air in the bottles and filled the remaining “headspace” with a certain gas of interest, such as a 100 percent hydrogen gas. They then placed the bottles in an incubator, where they were gently and continuously shaken to promote mixing between microbes and nutrients.

Every hour, a team member collected samples from each bottle and counted the live microbes. They continued taking samples for up to 80 hours. Their results represented a classic growth curve: at the beginning of the test, the microbes grew rapidly in number, feeding on the nutrients and populating the crop. Finally, the amount of microbes stabilized. The still prosperous population remained stable as new microbes continued to grow, replacing those that died.

Seager acknowledges that biologists do not find the results surprising. After all, hydrogen is an inert gas, and as such is not inherently toxic to organisms.

“It is not as if we have filled the upper space with a poison,” says Seager. “But seeing is believing, right? If no one has studied them, especially eukaryotes, in an environment dominated by hydrogen, you will want to do the experiment to believe it.”

It also makes clear that the experiment was not designed to show whether microbes can rely on hydrogen as an energy source. Rather, the point was more to demonstrate that a 100 percent hydrogen atmosphere would not harm or kill certain life forms.

“I don’t think it has occurred to astronomers that there might be life in a hydrogen environment,” says Seager, who hopes the study will foster cross-talk between astronomers and biologists, particularly in the search for habitable planets and extraterrestrial life. rises.

A world of hydrogen

Astronomers are not able to study the atmospheres of small rocky exoplanets with the tools available today. The few nearby rocky planets they have examined either lack an atmosphere or may simply be too small to detect with currently available telescopes. And although scientists have hypothesized that planets should harbor hydrogen-rich atmospheres, no working telescope has the resolution to detect them.

But if next-generation observatories select those hydrogen-dominated terrestrial worlds, Seager’s results show that there is a chance that life will thrive within them.

As for what a hydrogen-rich rocky planet would look like, it conjures up a comparison to Earth’s highest peak, Mt. Everest. Hikers trying to hike to the summit are left without air due to the fact that the density of all atmospheres decreases exponentially with height, and as a function of the descent distance for our nitrogen and oxygen dominated atmosphere. If a hiker were climbing Everest in an atmosphere dominated by hydrogen, a gas 14 times lighter than nitrogen, it could climb 14 times higher before running out of air.

“It’s a little difficult to understand, but that light gas only makes the atmosphere more expansive,” explains Seager. “And for telescopes, the larger the atmosphere compared to the bottom of a planet’s star, the easier it is to detect.”

If scientists ever have a chance to test such a hydrogen-rich planet, Seager imagines they could discover a surface that is different, but not unrecognizable from ours.

“We are imagining that if you drill at the surface, you would probably have hydrogen-rich minerals instead of what we call oxidized, and also oceans, since we believe that all life needs some kind of liquid, and you can probably still see a blue sky” says Seager. “We haven’t thought about the entire ecosystem. But it doesn’t necessarily have to be a different world.”