JWST Finds Ice Clouds on Giant Alien Planet, Scientists Stunned by the Concept of 'Weather'

In a development that has astronomers adjusting their models and possibly their expectations, the James Webb Space Telescope has spotted water ice clouds on a distant gas giant. The discovery, led by Elisabeth Matthews at the Max Planck Institute for Astronomy (MPIA), was made on the planet Epsilon Indi Ab, a Jupiter analogue that is apparently more complex than we gave it credit for.

This marks a significant step in the long, slow march from just finding exoplanets to actually understanding them. For decades, from 1995 through about 2022, the game was simply discovery via indirect methods. JWST's launch in 2022 changed that, allowing for detailed atmospheric study, though we're still a few telescopes away from checking for alien lawn signs.

As lead author Elisabeth Matthews put it with admirable perspective: "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." The implication being, of course, that studying Earth in detail would require tech we haven't invented yet, which is probably for the best.

Studying true Jupiter-like planets has been tricky because the easiest method requires them to pass in front of their star, which favors hot, close-in gas giants. Matthews's team got around this by directly imaging Epsilon Indi Ab using JWST's mid-infrared instrument MIRI. The planet, orbiting star Epsilon Indi A in the constellation Indus, is a beefy fellow with a mass of 7.6 Jupiters but a similar diameter. It orbits about four times farther from its slightly cooler star than Jupiter does from the Sun, giving it a surface temperature between 200 and 300 Kelvin (-70 to +20 °C). That's warmer than Jupiter's 140 K, a leftover heat from its formation that will slowly dissipate over billions of years.

The team used a coronagraph on MIRI to block the star's light and captured images at 11.3 μm, just outside a wavelength associated with ammonia. By comparing these to 2024 images at 10.6 μm, they could estimate ammonia levels. The MPIA-built hardware deserves a nod here.

The surprise? Jupiter's upper atmosphere is dominated by ammonia gas and clouds. Epsilon Indi Ab was expected to have lots of ammonia gas, but not clouds. Instead, observations showed less ammonia than predicted. The leading explanation? Thick, patchy water ice clouds, like Earth's cirrus clouds, are getting in the way.

This presents a delightful problem for astronomers, whose computer models often simplify things by ignoring clouds because they're hard to simulate. Co-author James Mang of the University of Texas at Austin called it "a great problem to have," noting JWST is revealing complexity that models are now "beginning to capture."

The future looks cloudy in the best way. NASA's Nancy Grace Roman Space Telescope, launching in 2026-2027 with MPIA as a partner, should be great at directly detecting reflective water ice clouds. Meanwhile, Matthews's team is seeking more JWST time to study other cold Jupiter analogues, building the foundation for the ultimate goal: studying Earth-like worlds and, one day, looking for signs that we're not alone.

The findings are published in the Astrophysical Journal Letters by E. C. Matthews et al. The MPIA researchers involved are Elisabeth Matthews and Bhavesh Rajpoot, collaborating with James Mang and Caroline Morley (University of Texas at Austin), Aarynn Carter and Mathilde Mâlin (Space Telescope Science Institute), and others.