Space telescope spots possible signs of life on alien planet. Some are skeptical

An ocean world that’s teeming with microbes — and who knows what other kinds of life — is currently the best explanation for some chemical signatures that the James Webb Space Telescope has spotted in the atmosphere of a distant planet.

That’s according to Nikku Madhusudhan of the University of Cambridge’s Institute of Astronomy, who called his team’s new findings “astounding.”

“These are the first hints we are seeing of an alien world that is possibly inhabited,” he told reporters in a press briefing. “This is a revolutionary moment.”

It’s also a harbinger of future claims about possible signs of life beyond our solar system that should become increasingly common, as scientists take advantage of the James Webb Space Telescope’s uecedented ability to probe the atmosphere of small planets that orbit far away stars.

In this case, the purportedly tell-tale gases that appear to be in this planet’s atmosphere are either dimethyl sulfide, dimethyl disulfide, or some combination of the two. On Earth, these are only produced by life, particularly by marine microbes.

Madhusudhan, despite his enthusiasm, noted that the detection of these gases needs to be confirmed with more telescope observations. And other astronomers are skeptical.

“I think this is one of those situations where extraordinary claims require extraordinary evidence,” says Laura Kreidberg, an astronomer at the Max Planck Institute for Astronomy in Germany who was not part of the research team. “I’m not sure we’re at the extraordinary evidence level yet.”

She says scientists are still learning how to use this powerful new space telescope to analyze the make-up of alien atmospheres by studying the tiny amount of starlight that filters through them.

“I want to emphasize this is an insanely difficult measurement,” says Kreidberg.

Måns Holmberg, a researcher at the Space Telescope Science Institute who was part of the group announcing the findings, said the data they used would be publicly available within days, and he urged other astronomers to independently analyze it.

“For looking at things like this, it requires a dedicated community effort,” says Holmberg.

Even if the chemical signal they saw is confirmed, the researchers say, scientists will still have to figure out if these molecules could potentially be produced on this planet by previously unknown processes that don’t involve life.

“We should be cautious. I want that to be front and center,” says Holmberg. “Any claim of life on another planet requires a lot of justification, and I don’t think we’re there yet.”

Space phytoplankton?

The planet in question, K2-18b, was discovered in 2015 by NASA’s Kepler mission, which revealed that planets outside our solar system are so common that they outnumber stars.

K2-18b orbits a cool dwarf star that lies about 124 light years away, in the constellation of Leo. The planet is found in the “Goldilocks zone” around the star, where temperatures are not too hot and not too cold to have liquid water and, presumably, possible life.

The planet is smaller than Neptune but bigger than Earth, and belongs to a mysterious class of planets not found in our own solar system. Various observations have led some astronomers to suggest K2-18b could be a world completely covered by a deep ocean, with a hydrogen-rich atmosphere.

When researchers first used the James Webb Space Telescope to study this planet a couple of years ago, they reported detecting methane and carbon dioxide.

Intriguingly, they also saw hints of dimethyl sulfide. This sulfur-based molecule is seen as a potential indicator of life because on Earth, it’s only made by living things such as ocean phytoplankton.

The team announced this possible detection of dimethyl sulfide in 2023, but when other scientists followed up, it didn’t pan out.

“That was weak from the beginning,” says Holmberg, who said the instrument on the telescope that they used made it hard to distinguish this molecule from others like methane. “We didn’t have much statistical evidence.”

Now, though, the team has different and more robust data that they collected about a year ago, using another telescope instrument that Holmberg says is better suited to find the stuff.

And this time, the researchers saw signs that could be either dimethyl sulfide, a related chemical called dimethyl disulfide, or some mixture of both life-associated gases.

At a personal level, says Madhusudhan, “when you are seeing something like this, it’s like — this is a question humanity has been asking for thousands of years, and if you’re witnessing it for the first time, it is a shock to the system. And it takes a while to recover from that, from the enormity of it.”

He says the researchers were faced with “potentially one of the biggest landmarks in the history of science. And I know this sounds grand, and it’s not my intention to make it sound grand, but there’s no other way to put it.”

They spent months testing and working with the data “just trying to get rid of the signal,” but it persisted, he says, noting that this is the first time that potential indicators of life in a planet that lies in the habitable zone around a star have been discovered “in the history of our species.”

Still, he said, “we’re not currently claiming that it is due to life.”

Not a sure thing

Their work, reported in The Astrophysical Journal Letters, suggests the concentration of these gases in the planet’s atmosphere is thousands of times higher than on Earth.

A planet covered entirely in an ocean at higher temperatures could produce more biological productivity, says Madhusudhan, adding that what they saw is consistent with previous predictions about this kind of world.

Edward Schwieterman, an astrobiologist at the University of California, Riverside who wasn’t part of the research team that worked on these new results from K2-18b, says that to him this detection seems “tentative.”

“It is not a sure thing,” he says. And while he thinks the researchers were right to report it, he also thinks it would not be surprising “if the signal went away” when other groups reanalyzed the data.

If the gas or gases are really there, however, he says researchers will have to brainstorm to figure out if anything besides life might produce them at the rate required to sustain such a high level in an atmosphere that could look very different from Earth’s.

Schwieterman finds it incredible that scientists are now able to analyze the chemical make-up of distant planets’ atmospheres to search them for signs of life.

In the few years that the James Webb Space Telescope has been operating, he says, scientists have learned more about these planets’ atmospheres than in the entire three decades before.

“But because we are operating so close to the limits of our capabilities,” says Schwieterman, “we are going to have these problems of interpretation and potential false positives.”

And they will likely become more common in the years ahead, as more planets get studied in detail.

“For most planets where we think there’s a chance that they’re habitable, we’re still at a much earlier stage, just trying to figure out if they have any atmosphere at all, let alone what that atmosphere is made out of or whether it’s showing signs of life,” says Kreidberg.

Nikole Lewis, an exoplanetary scientist at Cornell University, says that for scientists who specialize in trying to learn about planets beyond our solar system, this phase after the launch of the James Webb Space Telescope is like the burst of new thinking that followed NASA’s Voyager mission.

The twin Voyager spacecraft flew by outer planets such as Jupiter and Saturn, revealing a huge amount of information about their moons and kicking off a serious search for life there.

“We have lots of people all over the country thinking about what life might look like in the oceans of Europa, what life might look like on the surface of Titan and all of that stuff,” says Lewis, noting that it takes a lot of theoretical and laboratory work to get a handle on the possibilities.

When it comes to planets beyond our solar system, she says, the field is entering a time when researchers need to let “the observational data sort of lead us in a way that we really need our theory and our laboratory work to catch up with.”

That’s why she thinks this new finding about planet K2-18b is “interesting and it’s certainly something that we need to keep following through. But I’m not running out of my front door crying, ‘Aliens!'”