It’s likely that the first time we uncover signs of life on a planet circling another star (an exoplanet), we’ll be looking at the gases in its atmosphere. With the rising number of known Earth-like planets, we may soon find gases linked with life on Earth in the atmosphere of an exoplanet.
But what if extraterrestrial life has a chemical that differs from ours? According to recent research published in Nature Astronomy, our greatest chance of finding evidence of life utilizing atmospheres is to widen our search beyond planets like our own to include those with a hydrogen atmosphere.
When an exoplanet passes in front of its star, we may study its atmosphere. The star’s light must pass through the planet’s atmosphere to reach us during transit, and part of it is absorbed along the way.
Working out what light is missing due to the transit by looking at the star’s spectrum (its light split down by wavelength) indicates which gases make up the atmosphere. One of the missions of the long-delayed James Webb Space Telescope is to document extraterrestrial atmospheres.
If we discover an atmosphere with a chemical composition that differs from what we anticipate, one of the most straightforward interpretations is that it is sustained by life activity. On Earth, this is the case. Methane (CH4), which naturally combines with oxygen to form carbon dioxide, is found in our planet’s atmosphere. Biological activities, on the other hand, keep the methane supply topped up.
Another way to look at it is that without photosynthetic microorganisms liberating oxygen from carbon dioxide during the so-called massive oxygenation event, which began around 2.4 billion years ago, oxygen would not exist at all.
Look beyond oxygen-rich environments.
The authors of the new study propose that we should begin looking into planets larger than Earth with hydrogen-dominated atmospheres. Because hydrogen and oxygen are a highly combustible combination, they may not contain any free oxygen.
In 1937, a fire destroyed the hydrogen-filled Hindenberg airship. On a world with an oxygen-free hydrogen atmosphere, such a fire would not be possible. Murray Becker/Associated Press photo
Hydrogen is the lightest of all the molecules and may quickly escape into space. A rocky planet with enough gravity to hold on to a hydrogen atmosphere must be a “super-Earth” with a mass between two and ten times that of the Earth.
The hydrogen might have been taken directly from the gas cloud in which the planet developed, or it could have been released later through a chemical process involving iron and water.
The density of a hydrogen-dominated atmosphere diminishes around 14 times slower as you ascend than it does in a nitrogen-dominated atmosphere like the Earth’s.
This results in a 14-fold larger envelope of the planet’s atmosphere, making it easy to see in spectrum data. The larger dimensions would also increase our chances of directly seeing such an environment with an optical telescope.
In the lab, hydrogen is breathed.
The authors conducted laboratory studies to show that the bacterium E. coli (billions of which dwell in your intestines) can survive and proliferate in the absence of oxygen in a hydrogen environment. They were able to show the same thing using a variety of yeast.
While this is intriguing, it does not contribute much to the case that life may thrive in a hydrogen environment. Many microorganisms under the Earth’s crust already survive by metabolizing hydrogen, and there is even a multicellular creature that spends its whole existence on the Mediterranean’s floor in an oxygen-free zone.
Spinoloricus is a microscopic multicellular creature that does not appear to require oxygen to survive. The scale bar is 50 micrometers in length.
The Earth’s atmosphere, which began without oxygen, is unlikely to have ever contained more than 1% hydrogen. However, it’s possible that early life had to metabolize by combining hydrogen and carbon to make methane rather than by combining oxygen and carbon to form carbon dioxide, as humans do.
Gases that have a biosignature.
However, the study did produce a significant discovery. The researchers demonstrated that E. coli products emit an “amazing variety” of gases when they are exposed to hydrogen.
In a hydrogen environment, several of these, such as dimethylsulfide, carbonyl sulfide, and isoprene, might be detectable “biosignatures.” This increases our chances of detecting life on an exoplanet – but only if we know what to search for.
However, metabolic activities that require hydrogen are inefficient compared to those that use oxygen. In the eyes of astrobiologists, however, hydrogen-breathing life is already a well-established idea. Some logically based science fiction, like David Brin’s Uplift novels, have featured sentient hydrogen breathers.
The authors of the current study also point out that molecular hydrogen may behave like a greenhouse gas at high concentrations. This might maintain a planet’s surface warm enough for liquid water, and so surface life, for longer than it would be otherwise.
The writers avoid discussing the possibility of life on huge gas planets like Jupiter. Nonetheless, by widening the pool of habitable planets to include super-Earths with hydrogen-rich atmospheres, scientists have effectively doubled the number of bodies we may investigate in search of the first signals of alien life.