The search for life beyond Earth has led scientists to explore many suggestive mysteries, from plumes of methane on Mars to clouds of phosphine gas on Venus. But as far as we can tell, Earth’s inhabitants remain alone in the cosmos.

Now a team of researchers is offering what it contends is the strongest indication yet of extraterrestrial life, not in our solar system but on a massive planet, known as K2-18b, that orbits a star 120 light-years from Earth. A repeated analysis of the exoplanet’s atmosphere suggests an abundance of a molecule that on Earth has only one known source: living organisms such as marine algae.

“It is in no one’s interest to claim prematurely that we have detected life,” said Nikku Madhusudhan, an astronomer at the University of Cambridge and an author of the new study, at a news conference on Tuesday. Still, he said, the best explanation for his group’s observations is that K2-18b is covered with a warm ocean, brimming with life.

“This is a revolutionary moment,” Dr. Madhusudhan said. “It’s the first time humanity has seen potential biosignatures on a habitable planet.”

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The study was published Wednesday in the Astrophysical Journal Letters. Other researchers called it an exciting, thought-provoking first step to making sense of what’s on K2-18b. But they were reluctant to draw grand conclusions.

“It’s not nothing,” said Stephen Schmidt, a planetary scientist at Johns Hopkins University. “It’s a hint. But we cannot conclude it’s habitable yet.”

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