In April, a team of scientists based at the University of Cambridge claimed that a planet orbiting a distant star bore a possible signature of life. The announcement kicked up a fierce debate among astronomers, with many skeptics arguing that the evidence was too ambiguous.
Now a NASA-led team has made a new set of observations of the planet known as K2-18b, which lies 124 light-years from Earth. They have provided a clearer picture of the planet — confirming the presence of water, perhaps even as a liquid ocean.
But the new observations have failed to confirm evidence for life. In the original study, the Cambridge team claimed that K2-18b appeared to have a gas in its atmosphere that on Earth is produced only by living things. The NASA study did not find strong evidence for that gas.
What’s more, the NASA team argues that even if the gas was on K2-18b, it might have formed through mere chemistry. What once seemed like a promising clue of life — a biosignature — might be a mirage.
“A key takeaway is that biosignatures are going to be hard, no matter what kind of planet we are talking about,” said Jacob Bean, an astronomer at Cornell University, who was not involved in the study.
The NASA team, working with the leader of the Cambridge group, posted its new results online last week. The scientists have submitted the study to a scientific journal for publication.
Planets orbiting other stars are so dim that astronomers must piece together clues about them, for instance, by observing how they block light from their star as they pass in front of it.
Early studies on such exoplanets revealed that a lot of them are bigger than Earth but smaller than the giant planets in our outer solar system. Without any clear analogies close to home, researchers have struggled to envisage these strange exoplanets. Are they made of gas, or are they rocky worlds with liquid oceans, perhaps?
Since 2022, astronomers have been able to use a powerful new instrument — the James Webb Space Telescope — to peer at these distant worlds. In 2023 and 2024, a team of researchers led by Nikku Madhusudhan at the University of Cambridge, made two observations of the exoplanet K2-18b.
The researchers used the space telescope to capture light that came from K2-18b’s sun and then traveled through the exoplanet’s atmosphere. Molecules in the atmosphere absorbed certain wavelengths, creating subtle shifts in the starlight.
In April, Dr. Madhusudhan and his colleagues argued that some of those shifts were most likely caused by a gas known as dimethyl sulfide, or DMS.
Earth has DMS in its atmosphere, but here it’s produced only by living organisms, such as marine algae. The Cambridge team suggested that K2-18b might also have an ocean full of life pumping DMS into the atmosphere.
In three separate analyses, other researchers swiftly challenged that claim. They argued that a signal that looked like DMS could have been meaningless noise or perhaps another molecule that has no bearing on the presence of life.
Everyone involved in the debate agreed that more observations of K2-18b would help. If DMS was indeed present in the atmosphere, its signal might grow stronger with more data.
And that data was on its way. In 2024, a team led by Renyu Hu at 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 pored over the results to figure out the chemical makeup of the exoplanet. In recent weeks, they teamed up with Dr. Madhusudhan to analyze all the observations of K2-18b made so far to gain an even clearer picture.
In an interview, Dr. Hu said that some aspects of K2-18b were coming into focus. “Now we can conclusively say that the planet has not just methane in its atmosphere but also carbon dioxide,” he said.
Additionally, the observations reveal that as much as one-half of the exoplanet’s mass is made of water. “That’s quite a water-rich world,” Dr. Hu said.
Dr. Hu and his colleagues can’t say exactly what form that water takes. Some of it may exist as ice in a rocky core. Some of it may slosh around on the surface as a liquid ocean. Despite that uncertainty, Dr. Bean said that 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.”
But the new study will come as a disappointment for anyone hoping for alien life on K2-18b. “Our paper does not provide any conclusive evidence for the existence of DMS,” Dr. Hu said. “We do not think we have a detection.”
It is still possible that K2-18b harbors some DMS, but it would be at a level too low to detect clearly. And Dr. Hu and his colleagues have added a new twist to the debate about DMS and extraterrestrials. On a planet like K2-18b, they argue, DMS is not a reliable biosignature.
The new observations demonstrate that K2-18b’s atmosphere is profoundly different from Earth’s. To see how the exoplanet’s chemicals might react with each other, Dr. Hu and his colleagues created a computer model of the atmosphere. In their model, the planet built up a supply of DMS. The exoplanet did not need algae or other organisms to produce the gas.
“Even if it was there,” Dr. Bean said, referring to DMS, “it wouldn’t automatically mean life.”
Dr. Hu remained optimistic that the space telescope could reveal more details about exoplanets and perhaps even find a possible signature of life. But first, he cautioned, scientists will need to make sense of the observations they are now making, to understand just how different from Earth exoplanets can be.
“It’s more challenging,” he said. “But it is potentially achievable.”
Carl Zimmer covers news about science for The Times and writes the Origins column.
The post Hints of Life on Exoplanet Recede Even Further appeared first on New York Times.
