Epsilon Indi Ab orbits the star Epsilon Indi A in the southern constellation Indus. The planet has a mass of 7.6 Jupiter masses but a similar diameter to Jupiter. Its surface temperature sits between roughly 200 and 300 Kelvin, making it far colder than most exoplanets studied with JWST so far. It is slightly warmer than Jupiter because residual heat from its formation phase has not yet dissipated; over billions of years it will cool further.
Most exoplanet atmosphere studies using JWST rely on planets that transit their host star from the perspective of Earth, which selects for planets on close, hot orbits. Matthews and her colleagues instead used JWST's mid-infrared instrument MIRI to directly image Epsilon Indi Ab. A coronagraph blocked the glare of the central star, and the team compared images at 11.3 micrometers with earlier 10.6 micrometer observations taken in 2024. The ratio between these two channels probes the abundance of ammonia gas in the planet's upper atmosphere.
Given the temperature and mass of Epsilon Indi Ab, models predicted large quantities of ammonia gas. The photometric comparison showed less ammonia than expected. The best explanation the team found for the shortfall is the presence of thick but patchy water-ice clouds, analogous to high-altitude cirrus clouds in Earth's atmosphere. Such clouds would partially obscure the ammonia below, reducing the apparent signal. This was not a predicted outcome, and it directly challenged most published atmosphere models, which omit clouds because they complicate the calculations.
Co-author James Mang of the University of Texas at Austin said: "It's a great problem to have, and it speaks to the immense progress we're making thanks to JWST. What once seemed impossible to detect is now within reach, allowing us to probe the structure of these atmospheres, including the presence of clouds."
Bhavesh Rajpoot, a PhD student at MPIA who contributed to the study, noted that the planet's greater mass compared with Jupiter does not translate into a larger diameter: "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."
Matthews placed the result in the context of the broader JWST program: "JWST is finally allowing us to study solar-system analogue planets in detail. If we were aliens, several light years away, and looking back at the Sun, JWST is the first telescope that would allow us to study Jupiter in detail."
The water-ice clouds themselves may be directly observable in reflected starlight. NASA's Nancy Grace Roman Space Telescope, targeted for launch in 2026 to 2027 and in which MPIA is a partner institution, is well suited to that kind of observation. The Matthews team is also applying for additional JWST time to observe more cold Jupiter analogues and build the statistical foundation needed to understand how common such clouds are.
The study underlines that models used to interpret exoplanet observations must account for cloud layers, particularly for cold planets where condensation is expected to play a significant role. Theorists will need to incorporate clouds more routinely to match the quality of data now being produced. The results also demonstrate that direct imaging with JWST can reach planets at Jupiter-like separations from their stars, bridging the gap between the short-period hot planets that dominate transmission spectroscopy surveys and the cold long-period planets that more closely resemble those of the solar system.
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