Using the NASA/ESA/CSA James Webb Space Telescope, astronomers have detected water-ice clouds swirling through the atmosphere of Epsilon Indi Ab, a cold super-Jupiter that challenges existing models of giant planet atmospheres.
An artist’s impression of Epsilon Indi Ab with water clouds atop its ammonia-dominated atmosphere. Image credit: E.C. Matthews, MPIA / T. Müller, HdA.
Epsilon Indi A is a K5V star located approximately 12 light-years away from Earth in the southern constellation of Indus.
Also known as HD 209100 or HIP 108870, the star is between 3.7 and 5.7 billion years old.
The star is a bit less massive and a bit less hot than our Sun and is orbited by a gas-giant exoplanet several times the mass of Jupiter.
Known as Epsilon Indi Ab, the alien world has as a surface temperature of about 200 to 300 K (between minus 70 degrees Celsius and 20 degrees Celsius).
The reason the planet is slightly warmer than Jupiter (140 K, minus 133 degrees Celsius) is that there is still a lot of heat remaining from the planet formation phase.
Over the next billions of years, Epsilon Indi Ab will steadily cool down, eventually becoming colder than Jupiter.
“This planet has a considerably greater mass than Jupiter — the new study fixes its mass at 7.6 Jupiter masses — but the diameter is about the same as for its solar-system cousin,” said Bhavesh Rajpoot, a Ph.D. student at the Max Planck Institute for Astronomy.
Using Webb’s Mid-Infrared Instrument (MIRI), Rajpoot and colleagues obtained direct images of Epsilon Indi Ab.
They also estimated the amount of ammonia present in the planet’s atmosphere.
“For Jupiter, both ammonia gas and ammonia clouds dominate the upper layers of the atmosphere that are visible in observations,” they said.
“Given its properties, Epsilon Indi Ab was thought to have massive amounts of ammonia gas as well, although not ammonia clouds.”
“Surprisingly, the photometric comparison showed somewhat less ammonia than expected.”
The best explanation the astronomers found for this deficit was the presence of thick but patchy water-ice clouds, similar to the high-altitude cirrus clouds in Earth’s atmosphere.
“It’s a great problem to have, and it speaks to the immense progress we’re making thanks to Webb,” said Dr. James Mang, an astronomer at the University of Texas at Austin.
“What once seemed impossible to detect is now within reach, allowing us to probe the structure of these atmospheres, including the presence of clouds.”
“This reveals new layers of complexity that our models are now beginning to capture, and opens the door to even more detailed characterization of these cold, distant worlds.”
The findings appear in the Astrophysical Journal Letters.
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Elisabeth C. Matthews et al. 2026. A Second Visit to Eps Ind Ab with JWST: New Photometry Confirms Ammonia and Suggests Thick Clouds in the Exoplanet Atmosphere of the Closest Super-Jupiter. ApJL 1002, L5; doi: 10.3847/2041-8213/ae5823
