Astronomy Picture Of The Day: “Hycean world”

 

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Imagine this: 124 light-years away, in the constellation Leo, a strange world quietly orbits its dim red star. Its name is K2-18 b. It’s not a rocky Earth-like planet, and it’s not a gas giant like Jupiter. It’s something in between, what astronomers now call a “Hycean world”: a deep, planet-wide ocean of liquid water hidden beneath a thick, hydrogen-rich atmosphere.And now comes the part that makes your skin tingle.This year, the James Webb Space Telescope peered into that atmosphere and caught a scent that, on Earth, means only one thing: life in the sea.Floating among the clouds of K2-18 b are molecules of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS). Here on Earth, these sharp, slightly cabbage-like gases are produced almost exclusively by marine phytoplankton: those microscopic algae that give the ocean its unmistakable “beach-on-a-hot-day” smell. No known abiotic chemical process can pump out DMS in these quantities. On our planet, it’s practically a signature scribbled by biology itself.We’d already spotted methane and carbon dioxide in K2-18 b’s air (the classic cocktail of a living world), but now we’ve found the exact compounds that scream: “Something down there is breathing, eating, and multiplying.”“This is a transformational moment,” said Nikku Madhusudhan, the Cambridge astronomer who led the study. “We’re moving from ‘maybe’ to ‘this really looks like it.’”Of course, the scientists are still being careful. More observations are needed: more spectra, more data, future telescopes. But for the first time in human history, we have a serious candidate for a truly inhabited exoplanet, where, beneath a crimson sky lit by a cool red dwarf, a global dark ocean might be swaying with clouds of something eerily similar to plankton.And 124 light-years away, it might smell like the sea. Madhusudhan, N., et al. (2025). James Webb Space Telescope Observations of K2-18 b. Nature Astronomy