In April, a team of astronomers announced that they might — just might — have found signs of life on a planet over 120 light-years from Earth. The mere possibility of extraterrestrial life was enough to attract attention worldwide. It also attracted intense scrutiny from other astronomers.
Over the past month, researchers have independently analyzed the data, which suggested that the planet, called K2-18b, has a molecule in its atmosphere that could have been created by living organisms. Three different analyses have all reached the same conclusion: They see no compelling evidence for life on K2-18b.
“The claim just absolutely vanishes,” said Luis Welbanks, an astronomer at Arizona State University and an author of one of the studies.
The debate has less to do with the existence of alien life than with the challenge of observing distant planets. We can see a nearby planet like Jupiter because it reflects enough sunlight to become visible to the naked eye. But a planet like K2-18b is so far away that it becomes invisible not just to the naked eye but to conventional telescopes.
Astronomers have devised a series of increasingly sophisticated tricks to glean information about distant planets. They can measure the wobble of stars and the gravity of planets orbiting them. In 2010, researchers caught a glimpse of GJ 1214b, a planet 48 light-years away, as it passed in front of the star it orbits. When the star’s light shined through the planet’s atmosphere, certain wavelengths were absorbed indicating that GJ 1214b might have an atmosphere rich in water vapor.
In 2022, astronomers began using a powerful new tool to peer at distant planets this way. They pointed the James Webb Space Telescope at faraway solar systems and began detecting exquisitely faint patterns in starlight, clues to the complexity of exoplanet atmospheres.
The following year, Nikku Madhusudhan, an astronomer at the University of Cambridge, and his colleagues zeroed in on K2-18b as it passed in front of its own star, using instruments on the Webb telescope that are exquisitely sensitive to near-infrared light. As K2-18b passed in front of the star, the starlight underwent a subtle shift — caused by a planetary atmosphere containing hydrogen, carbon dioxide and methane, the researchers concluded.
They also found suggestive hints of a fourth gas, dimethyl sulfide — which could be a very big deal. On Earth, the only source for dimethyl sulfide in the atmosphere is life. Photosynthetic microbes in the ocean produce the molecule as a defense against ultraviolet light from the sun. The molecule escapes their cells and ends up in the air.
But the signal was so faint that it was hard to be certain that it was real. So Dr. Madhusudhan’s team arranged to look again at K2-18b, in 2024. This time, they used a different instrument on the space telescope, which looks at longer wavelengths of mid-infrared light.
In the team’s second search, they again found a signature of dimethyl sulfide, this one seemingly even stronger than the first. In April, Dr. Madhusudhan and his colleagues described their results in a paper published in Astrophysical Journal Letters. Speaking at a news conference the day before, Dr. Madhusudhan said there was only “a three-in-a-thousand chance of this being a fluke.”
Rafael Luque, an astronomer at the University of Chicago, characterized Dr. Madhusudhan as a world expert on exoplanets. “Madhu has been a pioneer in the field,” he said. “I have the utmost respect for that team.”
Nevertheless, Dr. Luque and his colleagues decided to take a look at the data for themselves.
For their own analysis, the scientists combined all the observations of K2-18b in both the near-infrared and mid-infrared wavelengths. On Monday, they reported that this combined data contained strong signals of hydrogen, carbon dioxide and methane, but no clear evidence of dimethyl sulfide.
The critics argue that the new mid-infrared observations were much weaker than those in near-infrared. On its own, they say, the mid-infrared light could fool researchers with faint noise masquerading as a real signal of dimethyl sulfide.
“I can just say straight up there is no statistically significant signal in the data that were published a month ago,” said Jacob Bean. Dr. Bean, an astronomer at the University of Chicago who discovered GJ 1214b’s atmosphere, worked with Dr. Luque on Monday’s study.
Dr. Welbanks, a former student of Dr. Madhusudhan’s, and his colleagues analyzed the K2-18b data in a different way. If the mid-infrared signal was genuine, did it have to come from dimethyl sulfide?
The team considered 90 different molecules that could plausibly be produced on a planet like K2-18b. Those molecules didn’t have to be produced by life, however; chemical reactions driven by sunlight could be enough.
The researchers concluded that the mid-infrared signal might have been produced by 59 of the 90 molecules. The strongest candidate in their analysis was not dimethyl sulfide but propyne, a gas that welders use as fuel.
Dr. Welbanks and his colleagues aren’t claiming that propyne is actually present on K2-18b. They simply argue that the faint light from the planet’s atmosphere can create ambiguous patterns that might be the result of one of many gases. Such scanty data certainly isn’t sufficient to consider any planet to be a possible home for life.
Last week, Dr. Madhusudhan and his colleagues responded to Dr. Welbanks’s team with a study of their own. They examined 650 possible molecules that might be in K2-18b’s atmosphere; dimethyl sulfide ended up among the molecules at the top of the list. “We’re exactly where we left off a month ago; it’s a good candidate,” Dr. Madhusudhan said.
Dr. Welbanks said that the new study by Dr. Madhusudhan simply provided more evidence that dimethyl sulfide does not stand out compared to other possible molecules on K2-18b. “In effect, this is a self-rebuttal,” he said.
It’s possible that the debate over K2-18b could be resolved within months. Last year Renyu Hu, an astronomer at the Jet Propulsion Laboratory, and his colleagues made more near-infrared observations of the planet. They are now preparing their results. “It will include substantially more data than previously published,” Dr. Hu said.
Dr. Bean said that the new observations could dispel much of the confusion about K2-18b. “The science is definitely working,” he said. “It’s going to play out pretty quickly, and I think we’ll have some clarity.”
Carl Zimmer covers news about science for The Times and writes the Origins column.
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