Webb watches water clouds thicken and thin on the coldest known brown dwarf

As WISE 0855 turns, patches of its atmosphere rotate into view, and each has slightly different cloud cover and temperature ^[12]. Measured as brightness alone, those differences blur together. "Before JWST, we only had photometry, which mixed up the effects of clouds, chemistry and temperature all together," Miles said. "Now we can actually distinguish them." ^[3]

The design is a time series of spectra. One every 15 minutes for 11 hours comes to about 44 ^[15]. The telescope's medium-resolution spectrograph could follow changes in individual molecular features, which the release says no earlier observatory could do for an object this cold ^[5]. Following features one by one is how the team split the variation in two. Water clouds high in the atmosphere grew thicker and thinner as the object rotated ^[9]. Carbon monoxide and phosphine carried a separate, wavelike signal ^[13].

The gas signal has a familiar explanation. On Jupiter, convection dredges gases from deep, hot layers into the visible atmosphere, a process called disequilibrium chemistry that has also been seen in brown dwarfs before ^[14]. The team reads the carbon monoxide and phosphine swings the same way, as heat from the interior churning gas upward ^[13]. The spectra record the swings. Convection is the cause the team proposes, carried over from Jupiter, and I think it is a reasonable one. According to the release, watching the process vary in real time, molecule by molecule, is new ^[14].

Mark Marley, who heads the University of Arizona's Lunar and Planetary Laboratory and co-authored the work, described the difficulty of reading this light. "It's like looking at the world through a screen door, where the screen is filtering out some of the light," he said ^[4]. On WISE 0855 that screen changes as the object rotates ^[12].

The thing this doesn't tell you is how big the changes are. The release does not report how much the clouds thinned or how long WISE 0855 takes to rotate, so a reader cannot tell from it how much of a turn the 11 hours covered. The result also rests on one object in one observing window. It is posted on arXiv and accepted by The Astrophysical Journal ^[10]. The team calls it the first direct confirmation that water clouds on a body outside the solar system change thickness over time ^[1]. "This is the first time we've been able to confirm that water clouds are becoming thinner and thicker on a nearby world," Miles said ^[2].

WISE 0855 is close to a planet in the ways that matter here. It is about twice Jupiter's mass, nearly Jupiter's size and about 265 kelvin, colder than Earth's surface ^[11]. "Even though brown dwarfs are not true planets, they exhibit planet-like behavior," Miles said ^[16]. She argues that if the physics of convection, clouds and chemistry that governs Jupiter also holds on WISE 0855, it applies to the gas giant exoplanets astronomers are starting to study with JWST ^[17].