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In April, scientists captured global attention by announcing they'd found a molecule in a faraway planet's atmosphere that could signal life there.

But a new analysis by University of Chicago scientists adds to the growing skepticism around the finding. Reviewing data from multiple observations of the planet, they found it cannot be called a conclusive detection. What's more, they found that other molecules, not just those possibly indicating signs of life, could explain the readings—putting caution signs around the claim.

"We found the data we have so far is much too noisy for the proof that would be needed to make that claim," said Rafael Luque, a postdoctoral researcher at UChicago and first author on a paper detailing their findings, which is submitted to Astronomy & Astrophysics Letters and is on the arXiv preprint server. "There's just not enough certainty to say one way or the other."

A molecular puzzle

The April 16 announcement, from a team led by Cambridge researchers, focused on a planet known as K2-18b, which is 124 light-years away from Earth. The group had analyzed readings taken by the James Webb Space Telescope, and concluded they confirmed the presence of either dimethyl sulfide or dimethyl disulfide—two molecules that, on Earth, are only associated with the presence of life.

But UChicago astrophysicists wanted to reexamine the data, mindful that extraordinary claims demand extraordinary evidence.

A lot of educated guesswork has to go into interpreting the data from telescopes. These planets are extremely far—this one is many light-years away—and too faint to observe directly, which means scientists have to look for indirect clues.

In this case, the Webb Telescope waits until the planet crosses in front of its star, then picks up the starlight that filters through the planet's atmosphere. As the light passes through the planet's atmosphere, different amounts of light are blocked at different wavelengths, depending on what molecules are present.

Study co-author Michael Zhang explained that when working with readings this faint, it's very difficult to uniquely identify a particular molecule.

"Anything with a carbon bonded to three hydrogens will show up at a particular wavelength," he said. "That's what dimethyl sulfide has. But there are countless other compounds that contain a carbon and three hydrogens, and would exhibit similar features in Webb's data. So, even with much better data, it'll be hard to be sure that dimethyl sulfide is what we're seeing."

Their analysis concluded that multiple other molecules could fit the bill for what the telescope saw. For example, another molecule that has a similar profile is ethane, a gas that has been found in many planets' atmospheres, such as Neptune—which definitely doesn't indicate life.

Co-author Caroline Piaulet-Ghorayeb said researchers usually favor the simplest explanation when reviewing data: "We should only introduce exotic molecules in the interpretation after ruling out molecules that we would expect to be in the atmosphere."

In this case, if the signature could be dimethyl sulfide or ethane—a molecule we've seen around planets in our own solar system—they assume the answer that's more common, not the most exciting.

'We don't want that to be overshadowed'

Another caution is that the analysis reported in April was based only on one set of observations.

Telescopes, including both Webb and Hubble, have taken multiple passes at observing this planet. If you include the data from all these passes, the team said, the evidence for dimethyl sulfide looks much weaker.

The authors said their report aims to provide a fuller view of the findings.

"Answering whether there is life outside the solar system is the most important question of our field. It is why we are all studying these planets," Luque said. "We are making enormous progress in this field, and we don't want that to be overshadowed by premature declarations."

In addition to Luque, Piaulet-Ghorayeb and Zhang, study co-authors included grad student Qiao Xue; postdoctoral researchers Michael Radica, Maria Steinrueck and Dominic Samra; and Prof. Jacob Bean.