James Webb finds water‑ice clouds
- Astronomers reported on April 22 that new James Webb Space Telescope measurements of Epsilon Indi Ab, a giant exoplanet 12 light-years away, point to thick water-ice clouds in its cold atmosphere. - The team used Webb’s mid-infrared camera to compare 10.6- and 11.3-micron light, finding the planet 0.88 magnitude brighter at 11.3 microns and revising its mass to 7.6 Jupiter masses. - The result adds a nearby test case for how cold giant planets look in heat and reflected light, ahead of future direct-imaging work with NASA’s Roman telescope. (science.nasa.gov)
A planet is too far and too dim to photograph directly unless a telescope blocks most of its star’s glare. Webb has now used that trick to probe Epsilon Indi Ab, a giant world 12 light-years away, and the new data point to thick water-ice clouds in its atmosphere. (iopscience.iop.org) (science.nasa.gov) The result was published April 22 in The Astrophysical Journal Letters by Elisabeth C. Matthews and colleagues. They used the James Webb Space Telescope’s Mid-Infrared Instrument, or MIRI, to take a new coronagraphic measurement at 11.3 microns and combine it with earlier data. (iopscience.iop.org) (arxiv.org) Infrared light works like a heat map: colder planets glow most strongly at longer wavelengths that human eyes cannot see. Epsilon Indi Ab is cold by exoplanet standards, around 200 to 300 kelvins in the new paper, which is why Webb’s mid-infrared camera is useful here. (iopscience.iop.org) The team found the planet was 0.88 plus or minus 0.08 magnitude brighter at 11.3 microns than at 10.6 microns. That pattern points to ammonia in the atmosphere, but the ammonia signal came out weaker than cloud-free models predicted. (iopscience.iop.org) (arxiv.org) Their preferred explanation is thick water-ice clouds high in the atmosphere. In the models, those clouds mute the ammonia feature and also help explain why Epsilon Indi Ab looks unexpectedly faint at near-infrared wavelengths. (iopscience.iop.org) (mpia.de) The same paper also updates the planet’s orbit and mass. The authors refit the orbit to an eccentricity of 0.24, with uncertainties of plus 0.11 and minus 0.08, and estimate a mass of 7.6 plus or minus 0.7 times Jupiter’s mass. (iopscience.iop.org) That matters because Webb is only now getting direct measurements of giant planets as cool as the gas giants in our own solar system. NASA described Epsilon Indi Ab in 2024 as the closest exoplanet more massive than Jupiter and one of the coldest ever directly imaged. (science.nasa.gov) The cloud result also fits a broader pattern in a still-small sample of cold giant exoplanets. The paper says these worlds are repeatedly fainter than expected between 3 and 5 microns, which is consistent with the water-ice-cloud idea. (iopscience.iop.org) Webb has already shown it can read exoplanet atmospheres in several ways, from water vapor in WASP-96 b to carbon dioxide and sulfur dioxide in WASP-39 b. Epsilon Indi Ab extends that work to a colder, more Jupiter-like regime that is harder to observe directly. (science.nasa.gov) For now, the water-ice clouds are the team’s preferred explanation, not a final verdict. But the nearest super-Jupiter yet studied this way is starting to look less like a clear ball of gas and more like a cold, cloudy planet. (iopscience.iop.org)
