The possibility of life on a distant planet is becoming increasingly remote.

In April, a team of scientists from the University of Cambridge claimed that a planet orbiting a distant star could support life . The announcement sparked intense debate among astronomers; many skeptics argued that the evidence was too ambiguous.

Now, a NASA-led team has made a new series of observations of the planet known as K2-18b, located 124 light-years from Earth. This time, they have a clearer image of the planet, and they have confirmed the presence of water, perhaps even in the form of a liquid ocean.

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However, the new observations failed to confirm the existence of life. In the original study, the Cambridge team suggested that K2-18b appeared to have a gas in its atmosphere that only living organisms produce on Earth. The NASA study found no convincing evidence of the presence of this gas.

Furthermore, the NASA team maintains that even if the gas were present on K2-18b, it could have been formed by simple chemical processes. What once seemed like a promising clue to life, a biosignature, may be nothing more than a mirage.

“ A key takeaway is that biosignatures are going to be difficult, regardless of what type of planet we’re talking about ,” explained Jacob Bean, an astronomer at the University of Chicago who was not involved in the study.

The NASA team, in collaboration with the Cambridge group leader, published their new results online last week. The scientists submitted the study to a scientific journal for publication.

Planets orbiting other stars are so dim that astronomers must decipher clues about them—for example, by observing how they block the light from their star as they pass in front of it.

Early studies of these exoplanets revealed that many are larger than Earth, but smaller than the giant planets in our outer solar system. With no clear analogues to known cases, researchers have struggled to imagine these unique exoplanets. Are they made of gas, or perhaps rocky worlds with liquid oceans?

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Starting in 2022, astronomers can use a powerful new instrument, the James Webb Space Telescope, to observe these distant worlds. In 2023 and 2024, a team of researchers led by Nikku Madhusudhan of the University of Cambridge made two observations of the exoplanet K2-18b.

Researchers used the space telescope to capture light emanating from K2-18b's sun as it traveled through the exoplanet's atmosphere. Molecules in the atmosphere absorbed certain wavelengths, creating subtle changes in the starlight.

In April, Madhusudhan and his colleagues said some of those changes were likely caused by a gas known as dimethyl sulfide, or DMS.

Earth has DMS in its atmosphere, but only living organisms, such as seaweed, produce it here. The Cambridge team said K2-18b could also have an ocean teeming with life that fills the atmosphere with DMS.

In three separate analyses, other researchers quickly challenged that claim. They argued that a DMS-like signal could have been an irrelevant element, or perhaps another molecule that doesn't influence the presence of life.

Everyone involved in the debate agreed that further observations of K2-18b would be useful. If DMS was indeed present in the atmosphere, its signal could be strengthened with more data.

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And that data was on its way. In 2024, a team led by Renyu Hu of NASA's Jet Propulsion Laboratory used the space telescope to make four observations of K2-18b as it passed in front of its star.

The researchers then analyzed the results to determine the exoplanet's chemical composition. In recent weeks, they partnered with Madhusudhan to analyze all the observations of K2-18b made so far and gain an even clearer picture.

In an interview, Hu said that some aspects of K2-18b were becoming clearer. “We can now conclusively state that the planet not only has methane in its atmosphere, but also carbon dioxide,” he noted.

Furthermore, observations reveal that up to half of the exoplanet's mass is made up of water. "It's a very water-rich world," Hu said.

Hu and his colleagues don't know exactly what form that water takes. Some of it could exist as ice in a rocky core. Others could ripple across the surface in the form of a liquid ocean. Despite that uncertainty, Bean said the new work made K2-18b "an exciting planet," one that "opens the door to studying a liquid water environment beyond our solar system for the first time."

However, the new study will be disappointing for those who had hoped for extraterrestrial life on K2-18b. “Our paper doesn’t provide conclusive evidence for the existence of DMS,” Hu said. “We don’t believe we have a detection.”

It's still possible that K2-18b harbors some DMS, but it would be at too low a level to clearly detect. And Hu and his colleagues have added a new twist to the debate about DMS and aliens. On a planet like K2-18b, they argue, DMS isn't a reliable biosignature.

The new observations show that K2-18b's atmosphere is profoundly different from Earth's. To see how the exoplanet's chemicals might react with each other, Hu and his colleagues created a computer model of the atmosphere. In their model, the planet accumulated a reservoir of DMS. The exoplanet didn't need algae or other organisms to produce the gas.

“Even if it were there,” Bean said, referring to the DMS, “that wouldn’t automatically mean there’s life.”

Hu remained optimistic about the space telescope's ability to reveal more details about exoplanets and even detect possible signs of life. But first, he cautioned, scientists will need to interpret the observations they are making now to understand how different exoplanets and Earth can be.

“It’s more complicated,” he said. “But it’s potentially achievable.” c. 2025 The New York Times Company.

By Carl Zimmer, The New York Times.