How lightning on exoplanets could make it harder to find alien life

We’re used to thunder and lightning here on Earth. But what might they be like on another planet? We know other worlds in the solar system have lightning strikes, for example, high in the clouds of Jupiter or during dust storms on Mars. Now, astronomers are thinking about lightning on planets beyond the solar system–and its effects on the signs of life on those planets.

In a new research paper accepted to the journal Astronomy and Astrophysics, a team of astrobiologists investigated how lightning might change some of the biosignatures—chemical signs of life—we could look for on other worlds. Overall, the results are nuanced, just like the complex atmosphere we experience here on Earth. Lightning, it turns out, can amplify some biosignatures while masking others. This both offers clues to find extraterrestrial life and complicates our observations.

Lightning appears to us as a bright flash, usually during a big rainstorm, and it’s caused by electricity in the atmosphere discharging between clouds or to the ground. It also “influences the chemistry of planetary atmospheres, including, as we all know, on Earth,” explains co-author Edward Schwieterman, an astrobiologist at the University of California, Riverside. Lightning even may have played a role in how life got started on our planet—astrobiologists think it could have brought together some of the molecules that eventually became amino acids in our bodies.

How much lightning happens on a planet depends on a whole array of factors, including how much water is in the atmosphere and how hot (or cold) it is. And if there’s enough lightning, it might significantly change what’s going on in the atmosphere. This is particularly critical for astrobiologists planning to look for specific chemicals that indicate that there is alien biology happening on planets beyond the solar system.

“A lot of the work that people do on exoplanet atmospheric biosignatures is very theoretical, based purely on computational modeling,” adds Nick Wogan, an astronomer at NASA Goddard not affiliated with the new work. This research team, however, combined both hands-on lab experiments and computer simulations to study how many different chemicals are produced and affected when lightning strikes. They found that the outcome depends on the type of atmosphere and how much lightning is happening.

What signs of life are astrobiologists looking for?

A few particularly interesting chemicals for biology are ammonia, methane, ozone and nitrous oxide. Scientists don’t know of a way ammonia can be made without biological processes, and nitrous oxide is a common product of bacteria’s metabolisms. Methane is very commonly produced by life here on Earth, including when humans burn fossil fuels and cows burp, and ozone is a good sign that an atmosphere contains oxygen, which we humans famously need to breathe. If these chemicals are detected on a faraway planet, there is potential that life may exist on that world, marking it as a major target of interest for further research.

In the new study, the team showed that lightning can’t make enough ammonia, methane, or nitrous oxide on its own, making those chemicals even more reliable signs of life. They also showed that it can’t make carbon monoxide—sometimes called an anti-biosignature, because its presence means life probably doesn’t exist there—meaning we won’t discount an inhabited planet as dead and full of carbon monoxide just because it has lightning on it.

[ Related: How scientists decide if they’ve actually found signals of alien life ]

However, lightning might make it harder to spot ozone. That is, a planet with only a few times more lightning than Earth might have its ozone signatures hidden by the effects of lightning.

When will we see these signs?

Actually observing chemistry and lightning on an Earth-like rocky exoplanet is still a goal for the future, and something we haven’t really done yet. “We have yet to identify the presence of an atmosphere on a temperate rocky planet like Earth, though few planets have been examined so far,” explains Schwieterman. Most of the rocky planets we have studied so far are close enough to their stars that their atmospheres are gone, burned off by the intense radiation of their suns.

Astronomers are, however, making tangible progress towards observing truly Earth-like planets. The Habitable Worlds Observatory, planned for launch in the 2040s, will be designed to look for biosignatures. Research to understand atmospheres and their many factors—including lightning—“will be key when future telescopes begin searching for biosignature gases on Earth-sized exoplanets,” adds Wogan.

“Overall, the effect on biosignatures by lightning seems to be not too much to worry about,” says lead author Patrick Barth, astronomer at the University of Stuttgart. Lightning, he adds, is just “another piece of information that you have to take into account when you’re observing an exoplanet.”

Update 03/11/24 8:55am: The missing word “enough” has been added to sentence beginning with, “In the new study, the team showed that lightning…”