Far beyond our solar system, rocky planets with atmospheres dominated by hydrogen gas could potentially support life, indicates a paper published this week in Nature Astronomy. In an experiment involving sealed bottles and some very hardy microbes, scientists at MIT discovered that both yeast and E. coli can grow in an environment with an atmosphere composed purely of hydrogen—that’s a far cry from Earth’s nitrogen and oxygen-rich air that humans and many other lifeforms thrive in. This means that life might be found in a broader array of alien habitats than we’ve previously recognized.
When scientists envision habitable alien worlds, they imagine these places as having Earth-like atmospheres dominated by nitrogen or carbon dioxide like those found on Mars and Venus. Recently, though, researchers have begun to suspect that planets with other types of environments might also be able to support life. In particular, some scientists have zeroed in on hydrogen-rich atmospheres.
“Hydrogen-rich environments on Earth are incredibly rare and not well known,” Sara Seager, an astrophysicist and coauthor of the new research, told Popular Science in an email. To date, astronomers have yet to identify any terrestrial planets elsewhere in the universe with hydrogen-rich atmospheres, probably because the atmospheres surrounding rocky planets are generally more difficult to detect than those of gas giants. But it’s likely that these rocky, non-gaseous planets with hydrogen-rich atmospheres are out there, and more sophisticated telescopes are being developed that might begin spotting them in the next few years.
“Today we use Hubble and ground-based telescopes,” Seager said. But in the future, she said, scientists will rely on more powerful tools such as the James Webb Space Telescope, which is scheduled to be launched into orbit in 2021.
With this new technology, planets with hydrogen floating above their surfaces will actually be easier to find than their more Earth-like cousins—because hydrogen gas is so light, their atmospheres should expand much farther into space.
Suspecting that worlds with hydrogen-rich atmospheres were out there waiting to be discovered, Seager and her team wanted to show that life could thrive in these areas. “We decided to do a simple experiment to communicate clearly to astronomers that life can survive and thrive in hydrogen environments,” she said.
While hydrogen by itself isn’t harmful to life, nobody had ever grown microorganisms in a 100 percent hydrogen atmosphere before. For their experiment, Seager and her team decided to put yeast and E. coli—two organisms that can survive without oxygen but don’t at first glance seem well adapted for hydrogen-rich habitats—to the test.
The researchers grew colonies of yeast and E. coli inside bottles sealed off from the surrounding air. In addition to a nutritious broth for the microbes to feast on, the bottles contained a range of different atmospheres, including pure hydrogen, regular air, and other blends of gases likely to exist on other exoplanets.
Both the yeast and E. coli were able to survive and reproduce in the 100 percent hydrogen atmospheres, although more slowly than in air. The conditions Seager and her colleagues created in the lab don’t perfectly mimic what scientists would expect to find on an actual planet—its atmosphere would include some amount of other gases besides hydrogen, and it might have much stronger gravity or otherwise differ from Earth. However, the fact that simple bacteria as well as relatively complex microbes like yeast could eke out a living in these conditions improves the likelihood that planets cloaked in hydrogen could be inhabited by some form of life.
Now that researchers suspect that hydrogen-rich rocky worlds are out there, and that microorganisms can thrive on them, the next step is to figure out a way to identify alien life from far-away Earth. One way is to search for evidence of so-called biosignature gases. On Earth, lifeforms, like plants and photosynthetic bacteria, can produce these specific gases. Theoretically, identifying them on another planet would be a good indication that life exists there.
During Seager and her colleagues’ experiment, the growing E. coli discharged dozens of different gases, including some like ammonia that scientists would consider promising signs of life if their instruments spied them on a distant planet. This means future telescopes could perhaps detect the kind of life dwelling in a hydrogen-filled atmosphere. Now we just need to find these strange worlds.