Discovery marks the first detection of variable water clouds outside of our solar system
A Jupiter-sized world just 7.5 light-years away is cloudy, chemically complex, and, it turns out, more like home than anyone expected.
Using the James Webb Space Telescope, Brittany Miles, assistant astronomer at University of Arizona Steward Observatory, led a team that spent 11 hours staring at WISE 0855, the coldest known brown dwarf, collecting a spectrum of its light every 15 minutes. The result is the most detailed time-series portrait ever taken of this frigid world – and the first direct confirmation that water clouds on another body are changing thickness over time, just like weather on Earth.
The study, published in The Astrophysical Journal, reveals that WISE 0855's atmosphere is shaped by at least two distinct processes playing out simultaneously: water clouds at high altitudes that grow thicker and thinner as the object rotates, and deep chemical gases being dredged upward by convection from far below. Untangling those two signals – previously impossible with older telescopes – is what makes JWST data so powerful.
"This is the first time we've been able to confirm that water clouds are becoming thinner and thicker on a nearby world," said Miles, a postdoctoral researcher at Steward Observatory. "Before JWST, we only had photometry, which mixed up the effects of clouds, chemistry, and temperature all together. Now we can actually distinguish them."
Brown dwarfs occupy a strange middle ground. Too massive to be planets, too small to ignite as stars, they glow dimly with leftover heat from their formation. WISE 0855, at roughly 265 Kelvin – colder than Earth's surface – sits at the very bottom of that category, blurring the line further. At about twice Jupiter's mass and nearly the same size, it looks and behaves, in many ways, like a free-floating giant planet.
Understanding its atmosphere requires looking at the world through what co-author Mark Marley, director and department head of the Lunar and Planetary Laboratory at U of A, describes as a kind of screen door. "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."