Astronomers have watched Earth-like clouds drift and shift on a cold, distant world, the first time they've seen that beyond our solar system.
The world, WISE 0855, lies 7.5 light-years away in space. Astronomers classify it as a brown dwarf, something that is too large to be a gas giant planet but too small to burn like a star.
Though WISE 0855 doesn't have much in common with our planet, it does have clouds made of water like Earth's. Using NASA's James Webb Space Telescope, a team led by University of Arizona astronomer Brittany Miles observed those clouds thicken and thin in real time, while gases welled up from deep inside it.
An atmosphere is often the only part of a distant world astronomers can study, and much of what they hope to learn depends on interpreting it correctly: what it's made of, what's happening deep beneath its clouds, and how it compares with the worlds that orbit the sun. But clouds and weather constantly reshape the light that reaches a telescope, underscoring how important it is for astronomers to understand them or risk misreading the world underneath.
"Before JWST, we only had photometry, which mixed up the effects of clouds, chemistry, and temperature all together," Miles said in a statement. "Now we can actually distinguish them."
Photometry gauges how bright an object looks overall, so it lumps every source of change into a single number. A spectrum, on the other hand, spreads light into a rainbow-like readout that reveals which molecules the light passed through.
For 11 straight hours, Miles' team captured a fresh spectrum every 15 minutes. The results have been accepted for publication in The Astronomical Journal.
Mark Marley, director of the University of Arizona's Lunar and Planetary Laboratory and a co-author, likens the problem of trying to study a world wrapped in an atmosphere to looking at it through a screen door.
"We're learning about the world on either side of the screen, but we also have to understand the screen itself," he said.
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WISE 0855, about the size of Jupiter but double its weight, is an isolated, free-floating object in space. It doesn't orbit a star that shines light on it, but it does make its own light. Brown dwarfs radiate heat left over from their formation, and this one has cooled to about 17 degrees Fahrenheit — colder than any other known brown dwarf.
The chill lets water condense into clouds. By contrast, hotter worlds sometimes fill their skies with clouds of molten minerals and metals instead.
The observations also revealed a rhythmic pulse in two gases: carbon monoxide and phosphine, a compound of phosphorus and hydrogen. Heat from the brown dwarf's interior drives that pulse by hauling gases up from hotter layers below, the way a lava lamp lifts warm blobs from its base. Planetary scientists know this convection process from Jupiter and have spotted it in other brown dwarfs.
Brown dwarfs make handy targets for research. Most worlds beyond our solar system circle stars that outshine them so badly that a telescope struggles to pick them out. WISE 0855 has no blindingly bright star nearby to wash it out. And although it's some 44 trillion miles away from Earth, it actually sits relatively close to us in terms of the galaxy — close enough for Webb to watch it for hours.
What astronomers learn from this study may help them eventually read the atmospheres of smaller, harder-to-see planets, including ones that future telescopes could study directly. That WISE 0855 shares a behavior with Jupiter suggests that the same rules are governing worlds near and far, Miles said.
"There is a spectrum of behaviors, not a hard line between brown dwarfs and planets," she said. "Jupiter and this object look distinctly different, but they have similar weather patterns."