Scientists have designed a virtual model of an alien cell and dropped it onto a hypothetical exoplanet to see how it behaves, an approach that could help astronomers recognize signs of extraterrestrial life on distant worlds, according to a recent study published in Monthly Notices of the Royal Astronomical Society.
The results revealed new insights about how microbes that produce methane gas, known as methanogens, might evolve on different exoplanets, such as Earth-sized ocean worlds or a class of hypothetical giant “Hycean planets” that host global oceans and hydrogen-rich atmospheres.
Predicting the impacts that methanogens might have on their environments could help scientists recognize potential signs of life, known as biosignatures, in the skies of worlds outside of our solar system.
Whereas many past studies have speculated about what forms of life might evolve on a particular type of exoplanet, this approach flips the idea on its head by starting with a general cell model and then considering how it might evolve in different biospheres.
“This is a kind of plausible life that's very unconstrained by Earth-based conditions,” said Arwen Nicholson, an astrophysicist at Exeter University who led the study, in a call with 404 Media. “We can put it into different environments, and then see: What other effects does it have?”
The idea, she explained, is to simplify one aspect of the hugely complex search for alien life .
“This is a generalized idea of what biology would roughly do that we can then put in different places to try and start getting biosignature predictions ideally for different planets, or to understand what we're seeing from the telescopes,” said Nicholson.
It’s possible that the first lifeforms to emerge on Earth were methanogens that thrived some four billion years ago when our planet was far more depleted in oxygen. These microbes still exist in a wide variety of ecological niches today. Unlike photosynthetic life, which requires sunlight for energy, methanogens can run on chemical energy in a process known as chemosynthesis. As a result, methanogens are adaptable and provide a useful model for considering the emergence of life in a diverse array of habitats, including dark regions that receive no direct starlight.
“We do know that the Earth's early biosphere did have this big component of it that was producing methane and consuming methane,” Nicholson said. “That gives us a starting point to look at other planets. It's a reaction we know works. It works on Earth, and it's something that is thought to work in environments that are very different to Earth.”
To that end, the team developed a baseline model of a spherical methanogen microbial cell and tinkered with different cell sizes, life cycles, and environmental effects that might evolve in various alien biospheres.
The results revealed a host of insights, including that a planet’s levels of hydrogen and methane are sensitive to the microbe’s particular features. In this way, “life will act to ‘erase’ abiotic aspects of their environment” by transforming the composition of gasses in the atmosphere in ways that could be detectable to astronomers, according to the study.
The team also speculated that resource competition on alien worlds would likely favor methanogens that are smaller and longer-lived, which could outcompete similar species that are shorter-lived, larger, or require more energy. But while these clues may help scientists spot likely biosignatures in the future, it will be a challenge to actually predict what sort of lifeform is producing those signals.
“You could have different life forms in your ocean and they would look the same in terms of a biosignature,” Nicholson said. “You could have a single film of some algae covering the whole ocean, or a little amoeba, or maybe something else. I thought that was quite interesting. It almost frees us up from making too many assumptions of what alien life would look like, because how are we going to know?”
“We can use spherical microbes as a representation, but even if we found a biosignature, even if it all made sense, and we found life, we still wouldn't actually know what they look like,” she added.
Nicholson and her colleagues plan to continue this line of research by modeling simplified cells of photosynthetic life and predicting their impact on different exoplanet environments. Ultimately, these studies will help scientists distinguish between genuine biosignatures and complex atmospheric chemistry that arises from geological activity.
It’s also worth preparing for the rather excruciating possibility that we find clear biosignatures on some farflung exoplanet and are never able to learn much more about them. In science fiction, we are used to aliens showing up on Earth or sending us obviously artificial messages. But it’s more likely that we would discover gassy signs of life in a distant world’s skies and then have to accept that we might never learn its specific nature.
“We don't live in sci-fi,” said Nicholson. “Sci-fi is super cool. I love it too. But the reality is, we're looking at a habitable planet like our own. If we find anything, they will be life forms embedded in their world, just like we're embedded in our world.”
“It's a super exciting time to be in astrobiology again with all of this data, and it's super neat that people are so interested in it as well,” she concluded.