Astronomers using the James Webb Space Telescope have captured the first time-resolved spectra of WISE 0855, revealing that water clouds on the Jupiter-sized brown dwarf 7.4 light-years away are shifting in thickness in real time.
Researchers pointing the James Webb Space Telescope at WISE 0855 spent 11 hours collecting light spectra every 15 minutes, producing the most detailed time-series portrait ever recorded for an ultracool brown dwarf.
The observations offer the first direct confirmation that water clouds on another cosmic body are actively changing thickness over time.
Investigating WISE 0855 With Webb NIRSpec
Located about 7.4 light-years away, WISE 0855 sits at the absolute bottom of the brown dwarf category. Weighing in at roughly twice the mass of Jupiter while sharing nearly identical dimensions, the object glows faintly with residual heat from its formation rather than nuclear fusion. At a frigid temperature of approximately 265 Kelvin—cooler than Earth’s surface—it occupies a rare middle ground cold enough to condense water into high-altitude clouds yet warm enough to keep ammonia in a gaseous state.
Before the Webb mission, researchers relied primarily on overall brightness measurements that blended the distinct effects of clouds, chemistry, and temperature into a single ambiguous signal. That limitation ended when the team utilized the telescope’s near-infrared spectrograph to split incoming light into its constituent wavelengths, recording a detailed spectrum every 15 minutes on 2 December 2023.
Miles noted that earlier photometric tools made it impossible to separate cloud interference from thermal and chemical signatures. The new medium-resolution data allowed researchers to isolate individual molecular features and track how they shift as the object rotates.
Water Clouds and Chemical Gases Obscure Atmospheric Data
The data revealed an atmosphere governed by two simultaneous phenomena: high-altitude water clouds shifting in density and deep chemical gases moving upward through convective mixing. Co-author Mark Marley likened the observational challenge to looking through a screen door.
“The photons go through the atmosphere and escape to space. 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.”
Dr. Mark Marley, astronomer at the University of Arizona
As the brown dwarf rotates, distinct patches of cloud cover and varying temperatures drift into view. These chemical changes stem from deep internal heat pushing warmer material upward, mirroring the convective processes that drive weather on Jupiter.
Implications for Planetary Weather Models
While atmospheric models incorporating water clouds match the observed spectrum far better than cloud-free simulations, questions remain. Carbon monoxide and carbon dioxide levels point to varying degrees of mixing that do not fit basic theoretical models, and measured amounts of phosphine came in lower than expected.
Researchers acknowledge that settling whether these cycles stem from rotation, wind bands, or atmospheric waves will require extended observation campaigns exceeding 20 hours across multiple dates.