Possible signs of life found on distant planet K2-18b, say scientists

By Carl Zimmer  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. “It’s the first time humanity has seen potential biosignatures on a habitable planet.” 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.”  If there is extraterrestrial life on K2-18b, or anywhere else, its discovery will arrive at a frustratingly slow pace. “Unless we see ET waving at us, it’s not going to be a smoking gun,” said Christopher Glein, a planetary scientist at the Southwest Research Institute in San Antonio.  Canadian astronomers discovered K2-18b in 2017, while looking through ground-based telescopes in Chile. It was a type of planet commonly found outside our solar system, but one without any analog near Earth that scientists could study closely for clues. These planets, known as sub-Neptunes, are much bigger than the rocky planets in our inner solar system, but smaller than Neptune and other gas-dominated planets of the outer solar system.  In 2021, Madhusudhan and his colleagues proposed that sub-Neptunes were covered with warm oceans of water and wrapped in atmospheres containing hydrogen, methane and other carbon compounds. To describe these strange planets, they coined a new term, “Hycean,” from a combination of the words “hydrogen” and “ocean.”  The launch of the James Webb Space Telescope in December 2021 allowed astronomers a closer look at sub-Neptunes and other distant planets.  As an exoplanet passes in front of its host star, its atmosphere, if it has one, is illuminated. Its gases change the color of the starlight that reaches the Webb telescope. While inspecting K2-18b, Madhusudhan and his colleagues discovered it had many of the molecules they had predicted a Hycean planet would possess. Researchers want to wait to see what the Webb telescope finds as it continues to examine K2-18b; provocative early findings sometimes fade in the light of additional data.  ©2025 The New York Times News Service