Life may not look for it on worlds that resemble our own
In the constellation Leo, a world more than eight times Earth's mass has offered astronomers something rare: a reason to reconsider where, and in what form, life might take hold across the cosmos. The James Webb Space Telescope has detected carbon-based molecules in the atmosphere of K2-18 b — a Hycean exoplanet with a hydrogen-rich sky and a possible ocean beneath — including a tentative trace of dimethyl sulphide, a molecule that, on Earth, is produced exclusively by living organisms. The finding does not confirm life, but it quietly dismantles the assumption that only Earth-like rocky worlds deserve our deepest attention. Humanity's search for company in the universe may need to widen its imagination.
- A molecule known only to be produced by life on Earth may have been detected in the atmosphere of a planet 120 light-years away — a signal faint enough to be a false positive, but too significant to ignore.
- The discovery forces a reckoning with decades of scientific habit: the hunt for extraterrestrial life has long favored small, rocky, Earth-like worlds, but K2-18 b belongs to an entirely different and far more common class of planet.
- Hycean worlds — hydrogen-wrapped, ocean-covered super-Earths — are now emerging as serious candidates for habitability, in part because their thick atmospheres make them unusually well-suited to the spectroscopic tools Webb wields.
- The dimethyl sulphide detection remains unconfirmed, and the planet's interior may harbor high-pressure ice and scalding ocean temperatures that would challenge any life as we know it.
- Future Webb observations are already being planned to either validate or rule out the presence of dimethyl sulphide, with results that could fundamentally reshape the scientific framework for searching for life beyond our solar system.
K2-18 b, a planet more than eight times Earth's mass orbiting a star in Leo, has long intrigued astronomers. But new observations from the James Webb Space Telescope have moved it from theoretical curiosity to the center of one of science's most profound questions: are we alone?
An international research team, working with data from Canadian and European instruments aboard Webb, found methane and carbon dioxide in the planet's atmosphere — molecules tied to biological processes on Earth. More striking still was a possible trace of dimethyl sulphide, a compound that, on our planet, is produced almost entirely by ocean phytoplankton. The detection is preliminary and will require further observation to confirm, but its implications are difficult to overstate.
What sets K2-18 b apart is its classification. It belongs to a category called Hycean worlds — larger than Earth, smaller than Neptune, with hydrogen-rich atmospheres and potentially vast liquid oceans. These planets have historically been overlooked in the search for life, with scientists favoring smaller, rocky, Earth-like candidates. But Hycean worlds are both more common in the galaxy and, as it turns out, more accessible to the kind of detailed atmospheric analysis Webb excels at.
Professor Nikku Madhusudhan of Cambridge, who led the research, was careful to temper excitement with caution. K2-18 b likely has a thick mantle of high-pressure ice, and any surface ocean could be too hot for life as we understand it. Yet the planet's existence — and what Webb has glimpsed within it — challenges old assumptions about where life might take root.
For now, K2-18 b remains an open question. But it is no longer a distant one.
Eight point six times the mass of Earth, orbiting a distant star in the constellation Leo, K2-18 b has spent years tugging at the curiosity of astronomers. Now, after observations through the James Webb Space Telescope, the planet has revealed something that shifts how scientists think about where life might exist beyond our solar system: carbon-based molecules floating in its atmosphere, and strong evidence of an ocean beneath.
The discovery came from an international team working with data collected by the Canadian and European instruments aboard Webb, the successor to the Hubble Space Telescope that first gave astronomers their initial hints about this world's atmospheric composition. What they found was methane and carbon dioxide—the kinds of molecules that, on Earth, are tied to biological processes. The planet, first spotted by NASA's Kepler spacecraft in 2015, had long seemed promising. It sits in the habitable zone, the orbital distance where liquid water could theoretically exist. But the new observations moved it from theoretical interest to concrete scientific focus.
What makes K2-18 b unusual is its category. It belongs to a class of worlds called Hycean exoplanets—planets larger than Earth but smaller than Neptune, wrapped in hydrogen-rich atmospheres and potentially covered by vast oceans. These worlds have not traditionally been the focus of the search for extraterrestrial life. Astronomers have historically concentrated on smaller, rocky planets similar to Earth, assuming those would be the most likely candidates. But Hycean worlds, as researchers now argue, may actually be easier to study. Their larger size and thicker atmospheres make them more amenable to the kind of detailed spectroscopic analysis that Webb performs so well.
The most intriguing finding, though, remains preliminary. The team detected possible traces of dimethyl sulphide in K2-18 b's atmosphere. On Earth, this molecule has a single source: life. It is produced almost entirely by phytoplankton in the oceans, released into the air in vast quantities. Finding it on another world would be extraordinary. But the detection is not yet confirmed. Future observations with Webb will need to verify whether the molecule is truly present in significant amounts, or whether the signal was a false positive.
Professor Nikku Madhusudhan of the University of Cambridge, who led the research team, emphasized that the discovery represents a fundamental shift in how scientists should approach the hunt for life beyond Earth. The presence of carbon molecules and the potential for water oceans do not guarantee habitability—K2-18 b's interior likely contains a thick mantle of high-pressure ice, and any ocean on its surface could be too hot to sustain life as we understand it. Yet the planet's very existence challenges old assumptions. Sub-Neptunes like K2-18 b are, in fact, the most common type of planet discovered so far in the galaxy. If life is to be found elsewhere, it may not look for it on worlds that resemble our own.
The coming months will bring more Webb observations, each one potentially narrowing the uncertainty around K2-18 b's atmosphere and what it might harbor. For now, the planet remains a question mark—but one that has moved from the margins of scientific inquiry to its center.
Citas Notables
The search for life on exoplanets should consider diverse habitable environments, not just smaller rocky planets, because larger Hycean worlds are significantly more conducive to atmospheric observations.— Prof Nikku Madhusudhan, University of Cambridge
We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, allowing us to work out the molecules that exist in its atmosphere.— Dr Subhajit Sarkar, Cardiff University
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter that we found these specific molecules—methane and carbon dioxide—on this particular planet?
Because on Earth, those molecules are tied to life and geological processes. Finding them together in an atmosphere, along with possible dimethyl sulphide, suggests conditions that could support biology. It's not proof, but it's a signal worth following.
But you said the ocean might be too hot to sustain life. So what are we actually looking at here?
We're looking at a world with the chemical ingredients and the right distance from its star. Whether those ingredients can actually cook up life is a separate question. The point is we now know where to look and what to measure.
Why haven't astronomers focused on these Hycean worlds before?
Habit, mostly. We've always assumed life would look like Earth—rocky, solid, familiar. These larger planets with thick atmospheres seemed less likely. But they're actually easier to study with our current tools, and they're far more common out there.
What happens next? Do we just wait for more telescope data?
Yes, but it's not passive waiting. The next observations are specifically designed to confirm whether dimethyl sulphide is really there. That molecule is the key. If it is, we have to take seriously the possibility that something alive might be producing it.
And if it's not there?
Then K2-18 b becomes a different kind of interesting—a world with the right chemistry but no sign of life. Either way, we learn something about what's possible in the universe.