A Jupiter-sized world just 7.5 light-years away has revealed weather patterns and atmospheric processes that resemble those on Earth and Jupiter. Using the James Webb Space Telescope (JWST), Brittany Miles, assistant astronomer at the University of Arizona’s Steward Observatory, led a team that observed WISE 0855, the coldest known brown dwarf, for 11 hours, collecting a spectrum of its light every 15 minutes.
The observations provide the most detailed time-series portrait of this frigid object to date and offer the first direct confirmation that water clouds on another world change thickness over time, much like clouds on Earth.
The study found that WISE 0855’s atmosphere is shaped by at least two processes occurring simultaneously. High-altitude water clouds grow thicker and thinner as the object rotates, while convection carries chemical gases from deep within its atmosphere towards the surface. Separating these two signals, which older telescopes could not do, demonstrates the capabilities of JWST.
Previously, astronomers relied on photometric observations, which combined the effects of clouds, chemistry and temperature, making it difficult to distinguish between them. JWST’s spectroscopic observations have now enabled researchers to identify these processes separately.
Reading a cold world's atmosphere
Brown dwarfs occupy a middle ground between planets and stars. They are too massive to be classified as planets but too small to sustain the nuclear fusion that powers stars. Instead, they emit faint light from the residual heat left over from their formation.
WISE 0855, with a temperature of approximately 265 kelvins (-8°C or 17°F), is among the coldest known objects in this category. With roughly twice Jupiter’s mass and nearly the same size, it resembles a free-floating gas giant in several ways.
Understanding its atmosphere requires astronomers to account for how gases and clouds affect the light escaping into space. Mark Marley, director and department head of the University of Arizona’s Lunar and Planetary Laboratory and a co-author of the study, has highlighted the role of this atmospheric filtering in interpreting observations.
As WISE 0855 rotates, different regions of its atmosphere come into view. These patches have varying cloud cover and temperatures, creating a changing pattern of warmer and cooler areas.
JWST’s medium-resolution spectrograph was sensitive enough to track these variations across individual molecular features, a capability that previous observatories lacked for an object this cold.
Deep gases rise through convection
Alongside temperature variations linked to the brown dwarf’s rotation, the spectrograph detected a rhythmic, wave-like signal associated with carbon monoxide and phosphine. These gases fluctuate as heat from deep inside the object drives convection, carrying material upwards towards the observable atmosphere.
The process is familiar to planetary scientists. On Jupiter, convective mixing transports gases from deep, hot layers into the visible atmosphere. Known as disequilibrium chemistry, this process has previously been observed in brown dwarfs, but tracking its variations in real time, molecule by molecule, represents a significant advance.
The observations show that changes in cloud thickness and the movement of gases from deeper layers occur alongside one another, providing a more detailed picture of the brown dwarf’s atmospheric dynamics.
Planet-like weather across distant worlds
The findings could help scientists understand planetary atmospheres beyond WISE 0855. The underlying physics of convection, cloud formation and atmospheric chemistry that shapes Jupiter also operates in this cold, free-floating world more than seven light-years away.
If these processes follow the same physical principles across different environments, the findings could also inform studies of gas giant exoplanets, which astronomers are increasingly investigating with JWST.
Although brown dwarfs are not classified as planets, they display several planet-like characteristics. Their atmospheric behaviour also suggests that the distinction between brown dwarfs and giant planets is not always straightforward when comparing their physical processes.
The study builds on years of theoretical work, including atmospheric models developed by Marley and other researchers and tested against observations of Jupiter. The project brought together multiple contributors over several years to interpret the complex signals captured by JWST.
Miles and her team plan to conduct further observations with the telescope to better determine WISE 0855’s rotation and investigate the three-dimensional structure and movement of its atmosphere.
The findings offer a closer look at how clouds, convection and atmospheric chemistry interact on cold worlds, potentially providing new ways to understand the weather patterns of giant planets both within and beyond our solar system.