🔭 This nearby giant exoplanet has mild temperatures and Earth-like clouds

At 11.9 light-years from us, i.e., our stellar neighborhood, a giant gaseous planet named Epsilon Indi Ab has surprised astronomers: its atmosphere contains water-ice clouds, similar to those on Earth, an unexpected phenomenon for a Super-Jupiter.

With a mass 7.6 times that of Jupiter and a similar diameter, it orbits four times farther from its star than our Jupiter does from the Sun. Its relatively mild surface temperature, between 200 and 300 kelvins (-73°C to +27°C), comes from the residual heat of its formation, making it warmer than Jupiter despite the distance from its star, categorizing it as a "cold Jupiter." Thanks to the James Webb Space Telescope, scientists have uncovered an unexpected atmospheric richness.


To probe this distant atmosphere, astronomers used an advanced technique: direct imaging with a coronagraph. JWST's MIRI instrument blocked the light from the host star, allowing the faint glow of the planet to be captured. Images taken at 11.3 micrometers revealed a lack of ammonia compared to predictions.

This absence is explained by the presence of thick water-ice clouds, which mask part of the gaseous ammonia. These clouds are reminiscent of Earth's cirrus, but their formation and distribution are still poorly understood.

Current models of giant gas planet atmospheres do not include water clouds. This discovery shows the need to improve them to better reflect reality. Researchers hope that the Nancy Grace Roman Space Telescope, scheduled for launch in 2026-2027, will make it easier to observe these reflective ice clouds. In the meantime, the team is requesting more observation time with JWST to study other exoplanets of this type.

Water-ice clouds in planetary atmospheres

On Earth, water-ice clouds form at high altitudes, such as cirrus. These clouds are composed of ice crystals and reflect sunlight. In the atmosphere of Epsilon Indi Ab, similar clouds have been detected.

These water-ice clouds are rare on hot gas giants because high temperatures prevent water from condensing. On a relatively cold but not too cold planet like Epsilon Indi Ab, water can condense to form clouds. Their presence affects the visibility of ammonia by blocking certain wavelengths.

Understanding the formation and distribution of these clouds is important for improving atmospheric models. Future observations will help determine their thickness, altitude, and changes. This knowledge will aid in interpreting signals from even smaller planets in the search for signs of life.