Astronomers have produced what amounts to a cloud record for an object 7.5 light-years from Earth, and it shows the clouds changing.
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 of its light every 15 minutes.
The object is about twice the mass of Jupiter and nearly the same size, at a temperature of roughly 265 Kelvin, around minus eight degrees Celsius, which makes it colder than the surface of the Earth.
The observations show water clouds high in its atmosphere growing thicker and thinner as the object rotates. It is the first time variable water clouds have been confirmed outside the solar system.
"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.
A second process shows up alongside the clouds. Carbon monoxide and phosphine from deep in the atmosphere are dredged upward by convection, producing a repeating, wavelike signal in the spectra.
The resolution of the instrument is what made the difference. "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."
Mark Marley, director and department head of the university's Lunar and Planetary Laboratory and a co-author, described the difficulty of reading an atmosphere from the light that escapes it. "The photons go through the atmosphere and escape to space," he said. "It's like looking at the world through a screen door, where the screen is filtering out some of the light."
The team argues that the processes resemble weather on Jupiter, which points to physics shared across large worlds. "Even though brown dwarfs are not true planets, they exhibit planet-like behavior," Miles said, adding: "There is a spectrum of behaviors – not a hard line between brown dwarfs and planets."
The findings are reported in The Astrophysical Journal. Miles plans further baseline observations with JWST to pin down the object's rotation.
The administrative description of what has been achieved is the impressive one. There is now a cloud observation, taken four times an hour for eleven hours, for a location 7.5 light-years away.
That is a shorter reporting interval than most terrestrial forecasters manage for places you can drive to.
Our analysts at the Department of Unnecessary Research note one complication, which is the delivery time. The light took 7.5 years to arrive. The clouds described are therefore seven and a half years old, and are being reported as the current state of the sky.
This does not make the observation less valuable. It makes it a different product. A forecast says what the weather will be and is checked afterwards. This says what the weather was, and cannot be checked at all, because the clouds have had seven and a half years to disperse and the nearest alternative observer is also us.
There is a further obstacle to practical use. The destination is 7.5 light-years away, the temperature is minus eight degrees, and the atmosphere contains phosphine. On this newspaper's reading the water clouds are the most hospitable detail on offer, and they are the part being reported as unstable.
The cloud report for WISE 0855 is thickening, with further thickening expected, and it is seven and a half years out of date.