Around the star Epsilon Indi, visible to the naked eye from the Southern Hemisphere, orbits a planet the size of Jupiter. The temperature there is like a good freezer: minus 200 degrees Celsius. The James Webb Space Telescope has for the first time directly imaged it and measured the light the planet emits in the thermal range.
The planet's light at a wavelength of 11.3 micrometers turned out to be noticeably brighter than at 10.6 micrometers. Such a jump is a fingerprint of ammonia in the atmosphere. But the signal was fainter than expected. Astronomers suspect the culprit is clouds of water ice. They act like a thick curtain on a window — hiding part of the light and preventing us from seeing what lies deeper.
If icy clouds do exist on this planet, they are likely common on other cold giants as well. If so, our previous atmospheric models, which assume clear skies, will need a rethink. Such clouds can alter temperature, chemistry, and even how the planet appears through a telescope.
Now scientists want to measure the light of several more cold giants at the same wavelengths. If the ammonia signature is muted in all of them, then icy clouds are not the exception but the rule. And then the question 'why are they like this?' becomes a new mystery.
as written by the authors arXiv:2603.08780
With JWST, we are directly imaging cold (~200-300K), solar-age giant exoplanets for the first time. At these temperatures many molecular features appear and water-ice clouds may condense and affect the emission spectrum; early photometric measurements of cold giant planets are already showing some tension with the predictions of cloud-free, solar-metallicity atmosphere models. Here we present new JWST/MIRI coronagraphic observations of the cold giant exoplanet Eps Ind Ab at 11.3um. Together with archival data, we use these new observations to study the atmosphere of this cold exoplanet, and we also re-fit its orbit, finding an updated mass of $$7.6\pm0.7$$ Mj and an eccentricity of $$0.24^{+0.11}_{-0.08}$$. The planet is significantly brighter (by $$0.88\pm0.08$$ mag) at 11.3um than at 10.6um, indicating the presence of ammonia. However, this ammonia feature is shallower than expected. This could indicate a low-metallicity or nitrogen-depleted atmosphere, but our preferred explanation is the presence of thick water-ice clouds that suppress the ammonia feature and the near-IR emission of Eps Ind Ab. Photometry of the small but growing sample of cold, giant exoplanets demonstrates that they are consistently fainter than expected between 3 to 5um, consistent with the water-ice cloud hypothesis. 10.6um and 11.3um photometry of this cold exoplanet sample would be valuable to determine whether the suppressed ammonia feature is universal, and to frame a new open question about the underlying physical cause.
The temperature on Eps Ind Ab is about 200–300 Kelvin, or from −73 to +27 °C, like a Siberian winter or a refrigerator.
In Stanisław Lem's novel Solaris, the planet's ocean hides its secrets beneath clouds — here, icy clouds also conceal the atmosphere's composition from us.