The first time we seek indications of life on a planet orbiting another star (an exoplanet), we’ll probably look at the gases in its atmosphere. We may soon uncover gases associated with life on Earth in the atmosphere of an exoplanet, as the number of known Earth-like planets grows.
But what if alien life has a different chemistry than we do? According to a new study published in Nature Astronomy, expanding our search beyond planets like our own to include those with a hydrogen atmosphere increases our chances of discovering evidence of life using atmospheres.
We can analyze an exoplanet’s atmosphere as it passes in front of its star. During transit, the star’s light must travel through the planet’s atmosphere to reach us, and some of it is absorbed.
Looking at the star’s spectrum (light divided down by wavelength) to see what light is missing owing to the transit reveals which gases make up the atmosphere. Documenting alien atmospheres is one of the long-delayed James Webb Space Telescope’s tasks.
One of the most basic interpretations of an environment with a chemical makeup that varies from what we expect is that it is supported by biological activity. This is the situation on Earth. Our planet’s atmosphere contains methane (CH4), which naturally interacts with oxygen to generate carbon dioxide. Biological activity, on the other hand, ensures that the methane supply is never depleted.
Another way to look at it is that oxygen would not exist at all if photosynthetic bacteria had not liberated oxygen from carbon dioxide during the so-called huge oxygenation event, which occurred roughly 2.4 billion years ago.
Take a look outside of oxygen-rich areas.
The authors of the current research suggest that we start looking at planets with hydrogen-dominated atmospheres that are bigger than Earth. Because hydrogen and oxygen are such a flammable combination, there may be no free oxygen present.
The hydrogen-filled Hindenberg airship was destroyed by fire in 1937. Such a fire would not be conceivable on a globe with an oxygen-free hydrogen atmosphere.
Hydrogen is the lightest molecule in the universe, and it can easily escape into space. A rocky planet with adequate gravity to maintain a hydrogen atmosphere would have to be a “super-Earth” with a mass of two to ten times that of Earth.
The hydrogen might have come straight from the gas cloud in which the planet formed, or it could have come from a chemical reaction involving iron and water.
When compared to a nitrogen-dominated atmosphere like the Earth’s, the density of a hydrogen-dominated atmosphere decreases 14 times slower as you climb.
As a consequence, the planet’s atmosphere has a 14-fold bigger envelope, which is seen in spectrum data. With greater dimensions, we’d have a better chance of observing such an environment directly with an optical telescope.
Hydrogen is inhaled in the lab.
The scientists used laboratory tests to prove that the bacteria E. coli (which may be found in billions in your intestines) can live and thrive in a hydrogen environment without oxygen. Using a variety of yeast, they were able to demonstrate the same phenomenon.
While this is fascinating, it adds nothing to the argument that life may flourish in a hydrogen atmosphere. Many microbes living deep below the Earth’s crust already live by metabolizing hydrogen, and there is even a multicellular species that spends its whole life on the Mediterranean’s floor in an oxygen-free environment.
Spinoloricus is a small multicellular species that seems to exist without the need for oxygen. The length of the scale bar is 50 micrometers.
It’s improbable that the Earth’s atmosphere, which started without oxygen, ever held more than 1% hydrogen. Early life may have had to metabolize by combining hydrogen and carbon to generate methane rather of mixing oxygen and carbon to form carbon dioxide, as humans do.
Gases with a unique biosignature.
The research did, however, provide a notable finding. When E. coli products are exposed to hydrogen, they generate a “astonishing range” of gases, according to the researchers.
Several of these, such as dimethylsulfide, carbonyl sulfide, and isoprene, may be identifiable “biosignatures” in a hydrogen environment. This enhances the likelihood of discovering life on an exoplanet — but only if we know what to look for.
However, metabolic processes requiring hydrogen are less efficient than those requiring oxygen. However, astrobiologists consider hydrogen-breathing life to be a well-established concept. Sentient hydrogen breathers have appeared in some rationally based science fiction, such as David Brin’s Uplift books.
At high quantities, molecular hydrogen may function as a greenhouse gas, according to the authors of the present research. This might keep a planet’s surface warm enough for liquid water, and so surface life, for longer than it otherwise would.
The authors do not address the likelihood of life on massive gas planets like Jupiter. Nonetheless, scientists have essentially quadrupled the number of bodies we may explore in search of the first signs of extraterrestrial life by expanding the pool of habitable planets to include super-Earths with hydrogen-rich atmospheres.