JWST spectra support the interpretation that patchy water clouds on the brown dwarf WISE 0855 change thickness. A study submitted to arXiv on September 17, 2026, analyzed 11 hours of NIRSpec observations and found variable gas signals alongside model-supported cloud changes.
What NIRSpec measured
The James Webb Space Telescope’s NIRSpec instrument collected a spectrum every 15 minutes across a wavelength range of 2.87–5.27 μm. The study describes WISE 0855 as the coldest known brown dwarf, with a temperature of 265 K.
| Study measurement | Reported value or result |
| Observing duration | 11 hours |
| Time between spectra | 15 minutes |
| Wavelength range | 2.87–5.27 μm |
| Spectral resolving power | Medium resolution, R ≈ 1,000 |
| Strongest variable spectral feature | Carbon monoxide (CO) absorption |
| CO variation | Up to 10% peak-to-peak at some wavelengths |
| Gas signals | CO and phosphine (PH₃) variations correlate |
| Cloud interpretation | Models support patchy water clouds with changing thickness |
Spectroscopy measures how light varies across wavelengths. Repeated spectra let researchers track changes in absorption features over time, rather than relying on a single brightness measurement. Earlier work had reported water-cloud signatures on WISE 0855; this study examines how cloud thickness changes.
Cloud and gas signals point to different pressure levels
The study’s atmospheric and structure-model comparisons support patchy water clouds. They place changes in cloud thickness at lower pressures than the variations in CO and PH₃ abundance. Because pressure generally falls higher in an atmosphere, the results associate the cloud changes with a different atmospheric level from the gas variations.
Principal-component analysis, a method for identifying patterns shared across measurements, found that CO and PH₃ varied together. The study reports CO absorption as the strongest changing spectral feature, with peak-to-peak modulation reaching up to 10% at some wavelengths. These gas signals and the modeled cloud changes are distinct parts of the atmospheric picture, not a direct image of the clouds.
Cloud formation depends on more than temperature
The study gives WISE 0855’s temperature as 265 K, or about 17°F. That is the object’s reported temperature, not a universal threshold for water clouds. Condensation depends on local temperature and pressure as well as the gas’s saturation vapor pressure.
For comparison, a planetary-atmosphere reference lists approximate cloud-layer temperatures for Jupiter and Saturn: ammonia at 150 K (about −190°F), ammonium hydrosulfide at 200 K (about −100°F), and water at 270 K (about 26°F). Saturn’s colder atmospheric profile places those layers deeper than Jupiter’s. These solar-system examples provide context for cloud formation, not measurements of WISE 0855’s cloud layer.
