Astronomers using the James Webb Space Telescope have detected thick water ice clouds on Epsilon Indi Ab, a cold gas giant resembling Jupiter. The finding, led by researchers at the Max Planck Institute for Astronomy, reveals less ammonia than expected in the planet's atmosphere. This challenges existing models and advances techniques for studying distant worlds.
A team led by Elisabeth Matthews at the Max Planck Institute for Astronomy directly imaged Epsilon Indi Ab with JWST's Mid-Infrared Instrument (MIRI). The planet, orbiting the star Epsilon Indi A in the constellation Indus, has a mass of 7.6 times that of Jupiter but a similar diameter. It circles its star about four times farther than Jupiter does the Sun, with surface temperatures between 200 and 300 Kelvin, warmer than Jupiter due to residual formation heat. Using a coronagraph to block starlight, observations at 11.3 μm and prior 10.6 μm data showed reduced ammonia levels, pointing to patchy water ice clouds akin to Earth's cirrus formations. Elisabeth Matthews explained, 'JWST is finally allowing us to study solar-system analogue planets in detail. If we were aliens several light years away looking back at the Sun, JWST is the first telescope that would allow us to study Jupiter in detail.' Bhavesh Rajpoot, a PhD student at MPIA, noted the planet's mass and size. James Mang of the University of Texas at Austin added, 'What once seemed impossible to detect is now within reach, allowing us to probe the structure of these atmospheres, including the presence of clouds.' The results, published in The Astrophysical Journal Letters, underscore the need for better atmospheric models including clouds. Researchers plan further JWST observations and anticipate help from NASA's Nancy Grace Roman Space Telescope, launching in 2026-2027.