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.
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.