Scientists stunned as JWST finds ice clouds on a giant alien planet

The search for planets beyond our solar system has evolved over decades. Scientists ultimately hope to detect signs of life on distant worlds, possibly within the next few decades. Early efforts, from 1995 through about 2022, focused mainly on discovering new exoplanets. Researchers relied on indirect techniques that could reveal a planet's mass, size, or sometimes both.

The launch of the James Webb Space Telescope (JWST) in 2022 marked the beginning of a new phase. For the first time, astronomers could study the atmospheres of many exoplanets in detail, gaining insights into their composition and structure. Even so, this stage is still a step away from directly searching for life, which will likely require more advanced telescopes in the future.

The latest research pushes these techniques further, although it does not yet target Earth like planets. Elisabeth Matthews (Max Planck Institute for Astronomy), the study's lead author, explains: "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. For studying Earth in detail, we would need much more advanced telescopes, though."

Why Jupiter Like Exoplanets Are Hard to Study

Despite JWST's capabilities, studying planets similar to Jupiter has been difficult. Most gas giants observed so far are much hotter than Jupiter. This is because the most common method of studying exoplanet atmospheres requires the planet to pass in front of its star from Earth's perspective. Planets closer to their stars are more likely to align this way, but they are also much hotter.

To get around this limitation, Matthews and her team used a different approach. Their work provides one of the closest looks yet at a true Jupiter analogue, and it revealed an unexpected feature.

Using JWST's mid infrared instrument MIRI, the team directly imaged Epsilon Indi Ab. This planet orbits the star Epsilon Indi A in the constellation Indus (in the southern sky). According to Bhavesh Rajpoot, a PhD student at MPIA who contributed to the research, "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."

A Cold Giant With Lingering Heat

Epsilon Indi Ab orbits about four times farther from its star than Jupiter does from the Sun. Its host star is slightly smaller and cooler than the Sun, which keeps the planet's temperature relatively low. Its surface temperature is estimated to be between 200 and 300 Kelvin (between -70 and +20 degrees Celsius).

Even so, the planet is warmer than Jupiter, which has a temperature of about 140 K. Scientists believe this extra warmth comes from heat left over from the planet's formation. Over billions of years, Epsilon Indi Ab is expected to cool and eventually become even colder than Jupiter.

To observe the planet, astronomers used a coronagraph on the MIRI instrument to block out the bright light from the host star. This allowed them to detect the faint glow of the planet itself. They captured images using a filter at 11.3 μm, which sits just outside a wavelength associated with ammonia molecules NH3. By comparing these observations with earlier images taken at 10.6 μm in 2024, the team was able to estimate how much ammonia is present. (Incidentally, both the mechanical filter wheels placing the coronagraph and the filter in front of the MIRI camera were constructed at MPIA, one of the German contributions to the JWST.)

Evidence Points to Water Ice Clouds

In Jupiter's atmosphere, ammonia gas and ammonia clouds dominate the visible upper layers. Based on its properties, Epsilon Indi Ab was expected to contain large amounts of ammonia gas as well, but not ammonia clouds. Instead, the observations revealed less ammonia than predicted.

The most likely explanation is the presence of thick but uneven water ice clouds, similar to cirrus clouds high in Earth's atmosphere -- an unexpected complication.

Astronomers typically interpret such data by comparing observations with computer models of planetary atmospheres. However, many existing models do not include clouds because they are difficult to simulate. This discovery highlights the need to improve those models. James Mang (University of Texas at Austin), a co author of the study, notes: "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. This reveals new layers of complexity that our models are now beginning to capture, and opens the door to even more detailed characterization of these cold, distant worlds."

Looking Ahead With Future Telescopes

Future observations could provide even clearer views of these clouds. NASA's Nancy Grace Roman Space Telescope, where MPIA is a partner, is expected to launch in 2026-2027 and should be well suited for directly detecting reflective water ice clouds.

In the meantime, Matthews and her colleagues are seeking additional JWST observation time to study more cold Jupiter like planets. As researchers continue refining their techniques, they are building the foundation for studying Earth like worlds in the future and, ultimately, searching for signs of life beyond our solar system.

Background Information

The results described here have been published as E. C. Matthews et al., "A second visit to Eps Ind Ab with JWST: new photometry confirms ammonia and suggests thick clouds in the exoplanet atmosphere of the closest super-Jupiter" in the Astrophysical Journal Letters.

The MPIA researchers involved are Elisabeth Matthews and Bhavesh Rajpoot, in collaboration with James Mang and Caroline Morley (University of Texas at Austin), Aarynn Carter and Mathilde Mâlin (Space Telescope Science Institute), and others.