The coldest known brown dwarf has weather, and Webb just watched its clouds change

Webb watched water clouds thicken and thin on WISE 0855, the coldest known brown dwarf, while deep gases changed on a rhythm of their own.

Clouds that build up and thin out over a few hours are ordinary weather on Earth. On the coldest brown dwarfs beyond the solar system, older telescopes couldn't separate clouds from temperature or chemistry, because all three blurred into one brightness reading.

The James Webb Space Telescope has now told them apart on WISE 0855, the coldest brown dwarf known. The findings show two kinds of weather at two different depths.

High in its atmosphere, water clouds appeared to grow thicker and thinner. Deeper down, gases carried up from the hot interior changed on a rhythm of their own.

Brittany Miles, an astronomer at the University of Arizona's Steward Observatory, led the team of 27 researchers.

"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.

Brown dwarfs never ignite

A brown dwarf is too heavy to count as a planet and too small to shine as a star. It glows dimly with heat left over from when it formed.

WISE 0855 is the coldest one anyone has found, at about 17°F (-8°C). That's roughly 15 degrees Fahrenheit below the point where water freezes.

About 7.4 light-years from Earth, the object has no star to orbit, so astronomers call it a free-floating world.

The object's temperature is in a useful range. It's cold enough for water to form clouds high in its atmosphere, but warm enough that ammonia stays a gas.

On Jupiter, a layer of ammonia clouds blocks the view of the water clouds underneath, but on WISE 0855 nothing does.

Astronomers have suspected clouds there since a 2014 study found that the object's colors matched models that included water clouds. Other teams later matched its brightness with cloud-free models, and the question stayed open.

Eleven hours of watching

On December 2, 2023, Webb watched WISE 0855 for 11 hours. Every 15 minutes it recorded a spectrum, which spreads the object's infrared light out by wavelength so astronomers can see which gases are absorbing which wavelengths.

NASA's Spitzer Space Telescope had already seen the object brighten and dim by a few percent during two sessions in 2015. But Spitzer measured only total brightness in two wide bands of light, so it couldn't say what was causing the change.

"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."

Light from deep inside WISE 0855 has to pass through its atmosphere before any telescope can collect it. Mark Marley, director of the University of Arizona's Lunar and Planetary Laboratory, was a co-author on the study.

"The photons go through the atmosphere and escape to space," Marley said. "It's like looking at the world through a screen door, where the screen is filtering out some of the light."

"We're learning about the world on either side of the screen - but we also have to understand the screen itself," he added.

Brown dwarf's cloud cover was changing

Across the 11 hours, the brightness changed by between 1 and 10 percent, depending on the wavelength. The smallest changes came at wavelengths where water vapor, methane, and ammonia absorb light strongly.

When the team compared the light with computer models of the atmosphere, every set of models matched better with water clouds than without them.

To copy the changes over time, the best models also needed the clouds to grow thicker and thinner while the temperature shifted by about 2°F (1°C).

The team suggested that cloudier patches and clearer patches were turning into view and back out as the object spun. One pattern of that kind accounted for about 60 percent of the variation that Webb recorded.

Gases from below vary too

A second pattern accounted for nearly 5 percent, and it showed up mostly in carbon monoxide and phosphine, a gas made of phosphorus and hydrogen.

Carbon monoxide produced the largest swings of anything in the data, up to 10 percent at some wavelengths, and phosphine rose and fell with it.

In the cold upper atmosphere, chemistry favors methane over carbon monoxide. Finding carbon monoxide there means warm gas is being carried up from hot layers far below. The same kind of mixing happens on Jupiter.

The team concluded that the gas changes started deep in the atmosphere, well below the water clouds, where the temperature itself was shifting. Where the clouds were thinner, Webb could see farther down into that gas-rich layer.

"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.

Winds may shape the pattern

Based on those big carbon monoxide swings and on what is known about Jupiter, the team thinks Webb sees WISE 0855 roughly side-on rather than looking down on a pole.

At that angle, the team expects long bands of wind circling the object, like Jupiter's stripes, to shape most of what Webb sees.

"Even though brown dwarfs are not true planets, they exhibit planet-like behavior," Miles said. "There is a spectrum of behaviors - not a hard line between brown dwarfs and planets."

"Jupiter and this object look distinctly different, but they have similar weather patterns. There are basic physics and chemistry that can be applied across all of these worlds," she added.

If that physics holds everywhere, Miles said, it also applies to giant planets around other stars, which astronomers are now studying with Webb.

Measuring a day there

Nobody knows how long a day lasts on WISE 0855. In Webb's data, the strongest repeats came every 5.4 to 5.6 hours, but longer periods fit the data too. Spitzer's two sessions didn't settle it either.

Neither family of computer models matched everything. One did better with carbon monoxide and phosphine but missed the small changes in water vapor, and the other did the reverse.

Miles and her colleagues wrote that observations longer than 20 hours will be needed to pin down the spin. Repeated looks with Webb could also show whether the deep changes come from bands of wind. The other possibility is a single storm that stays in one place.

The full study was published in the journal The Astrophysical Journal.

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