Water clouds are growing thicker and thinner on a frigid world just 7.5 light-years from Earth, giving astronomers a direct view of changing weather beyond the solar system.
A team led by Brittany Miles at the University of Arizona’s Steward Observatory used the James Webb Space Telescope to observe WISE 0855, the coldest known brown dwarf, for 11 hours.
Collecting a spectrum every 15 minutes, the researchers distinguished shifting cloud cover from gases rising out of deeper atmospheric layers.
Watching water clouds change
WISE 0855 sits near the boundary between brown dwarfs and giant planets. Brown dwarfs lack sufficient mass to ignite as stars and instead emit faint light from heat left over after their formation.
At about twice Jupiter’s mass and nearly the same size, WISE 0855 resembles a free-floating giant planet. Its temperature is roughly 265 kelvins, or 17°F, placing it at the coldest end of the known brown dwarf population.
“This is the first time we’ve been able to confirm that water clouds are becoming thinner and thicker on a nearby world,” University of Arizona researcher Brittany Miles said.
“Before JWST, we only had photometry, which mixed up the effects of clouds, chemistry, and temperature all together. Now we can actually distinguish them,” she added.
As the object rotates, regions with different cloud cover and temperatures come into view.
Webb’s medium-resolution spectrograph tracked these variations across individual molecular features, allowing the team to separate atmospheric signals that earlier observations mixed together.
Deep gases reveal atmospheric mixing
The observations revealed at least two processes operating simultaneously. Water clouds at high altitudes changed thickness, while convection carried chemical gases upward from deeper layers.
Alongside temperature variations associated with rotation, the instrument detected a rhythmic, wavelike signal linked to carbon monoxide and phosphine. Heat inside the brown dwarf churns these gases upward, connecting its deeper atmosphere with the layers astronomers can observe.
A similar process occurs on Jupiter, where convective mixing brings gases from hot, deep layers into the visible atmosphere.
Known as disequilibrium chemistry, this behavior has previously been observed in brown dwarfs. Tracking its variations in real time across individual molecules adds another dimension.
“We’re seeing water clouds getting thicker and thinner, and deep gases rising and falling, and we can actually watch them change in real time,” Miles said.
A connection to giant planets
The findings also offer a way to understand atmospheres beyond WISE 0855. Clouds, convection and chemistry link this cold object’s behavior to processes operating on Jupiter.
“Even though brown dwarfs are not true planets, they exhibit planet-like behavior,” Miles said.
Those similarities could help researchers interpret gas giant exoplanets observed with Webb, despite differences between individual worlds.
Miles is looking toward additional baseline observations to clarify WISE 0855’s rotation and the three-dimensional movement of its atmosphere. Longer monitoring could sharpen the picture of how its clouds and gases change together.
The research is available on the arXiv preprint server and has been accepted for publication in The Astrophysical Journal.