JWST sees ice clouds

Astronomers using the James Webb Space Telescope have found evidence that the nearby giant exoplanet Epsilon Indi Ab is wrapped in thick, patchy water-ice clouds. (arxiv.org) A planet’s atmosphere leaves fingerprints in infrared light, and the new JWST measurements show Epsilon Indi Ab is 0.88 ± 0.08 magnitudes brighter at 11.3 microns than at 10.6 microns, a sign of ammonia that is weaker than standard models predicted. Elisabeth Matthews and co-authors reported the result in a paper submitted March 9, 2026 and accepted by *The Astrophysical Journal Letters*. (arxiv.org) The team said the shallow ammonia feature could mean a low-metallicity or nitrogen-poor atmosphere, but they favored another explanation: thick water-ice clouds muting both the ammonia signal and the planet’s near-infrared glow. The same paper said cold giant exoplanets in the small JWST sample are repeatedly dimmer than expected between 3 and 5 microns. (arxiv.org) Epsilon Indi Ab is a directly imaged gas giant, which means Webb is collecting the planet’s own light rather than inferring it from a transit. NASA and the European Space Agency said in July 2024 that the planet lies about 12 light-years away and was already among the coldest exoplanets ever directly imaged. (webbtelescope.org, esa.int) That low temperature is the key to the new result. Matthews and colleagues described Webb as directly imaging solar-age giant exoplanets at roughly 200 to 300 kelvin for the first time, a regime where water-ice clouds are expected to condense and start reshaping the spectrum. (arxiv.org) A second March 2026 study on the same planet, led by Aniket Sanghi, extended Epsilon Indi Ab’s measured energy distribution from 4 to 25 microns and reported a temperature of about 275 kelvin. That paper said the data do not yet prove water-ice clouds, but the 25.5-micron point is brighter than cloud-free models predict and is better matched by a cloudy atmosphere. (arxiv.org) The planet’s mass is still being refined as more data come in. Matthews and co-authors reported 7.6 ± 0.7 Jupiter masses, while Sanghi and co-authors, using three decades of radial-velocity monitoring plus Gaia-Hipparcos and direct-imaging astrometry, reported 6.5 +0.7/-0.6 Jupiter masses. (arxiv.org, arxiv.org) Brown dwarfs have become the comparison set for this work because they are giant-planet lookalikes without a bright host star in the way. A January 2026 JWST retrieval study of 22 late-T to Y brown dwarfs found cloud-free models fit the warmer objects best, while colder dwarfs showed mixed or growing support for cloudy atmospheres. (arxiv.org) JWST results on the brown dwarf WISE 0855 pushed that picture further. A 2024 analysis of spectra from 0.8 to 22 microns found a changing water-abundance profile in the atmosphere, and a separate JWST observing program described objects colder than 350 kelvin as prime targets for water-ice clouds in their photospheres. (arxiv.org, stsci.edu) Ammonia itself is not the surprise; JWST had already detected it clearly in other cold worlds. A 2025 study reported a 12.5-sigma ammonia detection in the directly imaged companion GJ 504 b, which makes Epsilon Indi Ab’s weaker-than-expected ammonia band stand out as a cloud problem as much as a chemistry problem. (arxiv.org) For now, the Epsilon Indi Ab papers stop short of calling the clouds a settled detection. They argue that more JWST measurements near 10.6 and 11.3 microns, on this planet and other cold giants, will show whether muted ammonia and ice-cloud veils are common features of the coldest exoplanet atmospheres. (arxiv.org, arxiv.org)