Astronomers using the James Webb Space Telescope have tracked shifting water clouds on WISE 0855, a cold, free-floating brown dwarf located 7.5 light-years from Earth. The observations provide the first direct confirmation of changing water clouds beyond our solar system, revealing weather patterns driven by internal heat and convection.
Infrared Observations of WISE 0855 Reveal Thickening Water Clouds
Brittany Miles, a University of Arizona astronomer, led a research team that dedicated 11 hours at the James Webb Space Telescope to study WISE 0855, recognized as the coldest known brown dwarf. Operating without a nearby star to supply sunlight or wash out its faint emissions, the object radiates heat left over from its formation.
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That internal chill allows water to condense into high-altitude clouds. By capturing a fresh spectrum of light every 15 minutes, the researchers watched patches of these water clouds grow thicker and thinner as the brown dwarf rotated. While brown dwarfs initiate their existence in a manner comparable to stars, they ultimately fail to gather sufficient mass for continuous core atomic fusion and subsequent starlight ignition. Instead, they lack enough mass to sustain the hydrogen fusion that powers stars and gradually release heat left over from their formation.
This is the first time we’ve been able to confirm that water clouds are becoming thinner and thicker on a nearby world.
Brittany Miles, astronomer at University of Arizona Steward Observatory
Separating Clouds From Atmospheric Chemistry With Spectrography
Previous studies of distant celestial bodies relied on photometry, a method that measures overall brightness changes but lumps multiple atmospheric properties into a single reading. Because overall visual brightness is what photometry measures, it consolidates every variation factor into one singular metric. Conversely, a spectrum disperses light into a rainbow-like distribution that exposes the specific molecules the light traversed. Mark Marley, director and department head of the University of Arizona’s Lunar and Planetary Laboratory, compared the analytical challenge to viewing an environment through an obstructed barrier.
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.
Mark Marley, director and department head of the University of Arizona’s Lunar and Planetary Laboratory
To overcome that limitation, the team used Webb’s medium-resolution spectrograph to spread incoming light across individual molecular wavelengths. That breakdown allowed researchers to distinguish between cloud coverage, temperature fluctuations, and chemical shifts that older photometry techniques mixed together. Prior to the launch of James Webb, scientists and investigators relied exclusively on photometry methods, which conflated temperature, chemical composition, and cloud dynamics into indistinct data. Now, all of those properties can be distinguished and observed.
Convection Processes and Chemical Pulses Driven From Within
In addition to tracking cloud shifts, the spectral analysis revealed a rhythmic, wave-like pulse in two specific atmospheric gases: carbon monoxide and phosphine, a compound of phosphorus and hydrogen. As WISE 0855 spins on its axis, varying sectors of its terrain come into view, displaying distinct thermal states and cloud configurations.
Weighing about twice the mass of Jupiter and sharing a similar physical size, WISE 0855 exhibits weather behaviors that closely parallel our solar system’s largest gas giant despite having no host star. Comparable chemical mingling occurs within Jupiter’s atmosphere, a phenomenon that scientists have likewise documented in alternative brown dwarfs. Miles noted that the findings indicate universal rules govern bodies across vast distances.
There is a spectrum of behaviors, not a hard line between brown dwarfs and planets.
Brittany Miles, astronomer at University of Arizona Steward Observatory
Implications for Characterizing Distant Exoplanets
Its close proximity and lack of a blinding parent star make it an ideal laboratory for testing atmospheric models. Brown dwarfs possess certain investigative advantages because they generate their own thermal radiation and luminosity rather than being positioned adjacent to an intensely radiant star.
The techniques validated on this nearby object establish a baseline for analyzing smaller, harder-to-detect exoplanets that future space telescopes aim to examine directly. Insights gained from this current investigation could eventually assist researchers in analyzing the gaseous envelopes of diminutive, elusive planets, including candidates destined for direct observation by upcoming observatory instruments. Additional observations are expected to help astronomers refine estimates of the brown dwarf’s rotation period and map its atmosphere in three dimensions as researchers continue studying its complex weather cycles.