Exciting rumors have been swirling in the halls of astrobiology. The James Webb Space Telescope, which has been scrutinizing the cosmos in unprecedented detail since its deployment in 2022, has been on a tear lately, and folks in the know say it might finally have detected life beyond Earth. That’s the buzz, anyway. Says astrophysicist Rebecca Smethurst, as reported by The Spectator, “I think we are going to get a paper that has strong evidence for a biosignature on an exoplanet very, very soon.”
In other words: Awesome! But also: Calm down. “Strong evidence for a biosignature” is a long way from proof of life on other planets. A biosignature is basically a signal that’s consistent with life but that may also be produced by something else. It’s intriguing but not incontrovertible evidence. And given the many uncertainties surrounding a discipline still in its infancy, the public should not get its hopes up. “So many people want this to be the year. There will definitely be claims,” says Sara Seager, an MIT professor of astrophysics. “There won’t be any robust findings.”
One reason it’s hard to pin down unequivocal evidence of life is that we don’t really know what life is. Here on Earth, biology involves DNA and carbohydrates and requires liquid water, but the chemistry could be different on other worlds. Maybe life could use liquid methane instead of water, or silicon instead of carbon. So, in its most fundamental formulation, what is life all about, and how do we know what to look for?
One idea is that life must always exist far from chemical equilibrium. Earth’s atmosphere, for instance, contains both oxygen and methane. Left on their own, these molecules would react to form carbon dioxide and water, and in time the oxygen and methane would vanish. “The only reason they are present and coexist is that they keep being replenished by some process,” says NASA astrobiologist Marc Neveu. “In our case, it turns out to be life.”
So if you were looking at Earth from far away, the presence of methane and oxygen together would be a clue that life is operating here. But it wouldn’t be proof because these gases can be generated by non-biological causes.
Just because you can’t prove your evidence is incontrovertible, however, doesn’t mean your conclusion is wrong. By that measure, we might already have found alien life. That is to say, multiple pieces of evidence exist that we may someday recognize as the first glimmer of fact that we are not alone.
2004: Life on Mars
In December 2004, scientists reported that a spectrometer aboard the European Space Agency’s Mars Express orbiter had measured a tiny but detectable quantity of methane gas in the Red Planet’s atmosphere. That same year, astronomers using ground-based telescopes reported that they too had found signs of methane. On Earth, the gas is produced mostly by living things (including, famously, cow farts) but can also be produced as a result of seismic activity. “It’s widely thought that there are underground streams of liquid water,” says Neveu. “It’s certainly possible that there are microbes in this environment.”
2023: Life on Europa
Jupiter and Saturn, the largest planets, are too cold and too far from the Sun to harbor life as we currently know it. But several of their icy moons are believed to harbor oceans below their frozen surface. Kept warm by tidal forces, these oceans may be able to support life. Particularly interesting is Jupiter’s moon Europa. Last year, the Webb telescope detected carbon dioxide within a geologically young area of the surface, suggesting it might have come from an ocean.
2023: Life Beyond the Solar System
Astronomers have long assumed that stars outside our solar system have planets around them, but not until 1992 were the first ones discoveredorbiting neutron stars. Now we know of more than 5,000, but we’ve had no evidence of whether any harbor life. Then, last fall, a team of U.S. and U.K. astronomers released a preprint saying they had detected methane, carbon dioxide, and maybe dimethyl sulfide, or DMS, around a star called K2-18b, which lies 124 light-years away. On Earth, DMS is produced by living organisms.
It’s an extremely tenuous finding, however. Exoplanets are so far away and so dim compared with the star they orbit that even the mighty Webb can’t image them directly. Instead, it waits for the planet to pass in front of the star and then detects a change in the intensity of the light received — in this case, on the infrared part of the spectrum, just beyond the range of human perception. Seagal is skeptical, saying that what the authors took to be a sign of DMS is really just a misinterpretation of the data. “There’s no signal,” she maintains.
Identifying biosignatures is nice, but it would be far more exciting to establish once and for all that extraterrestrial life definitely exists. One way to do that would be to detect a so-called technosignature, a gas that can be produced only artificially through the application of a sufficiently advanced technology. Nitrogen trifluoride, for instance, which is used on Earth in manufacturing, is not known to result from any known natural process. “It would be a more robust finding,” says Seager.
Failing that, the only way to prove the existence of a living thing is to go out and find it and study it up close — watch it metabolize, grow, and reproduce. That’s a tall order. Life on Venus, if it exists, is probably wafting around in the cooler reaches of the planet’s acid atmosphere, while Mars’s methane-belching microbes could be buried miles deep underground. The hardest of all to access would be any life sheltering around distant stars, the nearest of which would take at least 70,000 years to reach using current technology.
The oceans under icy moons may be easiest to get into. This October, NASA is planning to launch a probe called the Europa Clipper, which will swing through the clouds of ice grains Europa ejects through its geysers and see if it can detect the kinds of molecules we might expect if life exists in its subsurface oceans. If the signs are encouraging, they will likely light a fire to send a spacecraft to take a closer look. Last year, researchers in Germany announced they were working on a probe that could melt its way through the crust of an icy moon and explore the ocean below.
Seager is optimistic that the search for extraterrestrial life will ultimately be successful — just not perhaps with the kind of speed people would like to see. She cites the example of the discovery of exoplanets, which seemed exotic and somewhat dubious when the first hints of them were uncovered three decades ago. By analogy, the signs of alien life that first appear will start out being doubtful and rare before becoming ironclad and common.
Neveu agrees that it will take patience. “I think it’s going to be a gradual process,” he says. “Maybe it’s already started.”
Note: This article originally ran on March 31, 2024 in New York magazine.