Twenty-seven researchers, led by the University of Arizona’s Steward Observatory, used the James Webb Space Telescope for eleven uninterrupted hours to study WISE 0855, the coldest known object outside the Solar System. They report irregular water clouds that thicken and thin; the work is accepted by The Astronomical Journal and posted as a preprint. The free-floating brown dwarf lies just over seven light-years away, is almost Jupiter-sized, and has an effective temperature near 265 kelvin (about minus eight Celsius).
- Key Point: WISE 0855 is no planet; it is a Y-type brown dwarf orbiting no star.
- Key Point: Its water clouds are model-inferred H₂O condensates, not photographs.
- Key Point: Carbon monoxide varies most, reaching 10 percent peak-to-peak, while methane, ammonia and water bands move less.
A starless world, not a planet
Kevin Luhman discovered the object in 2014. Its mass is often misstated: Arizona’s release says about two Jupiter masses, other outlets three to ten, but the paper does not measure it, assuming a surface gravity and a one-Jupiter radius. The defensible description is a few Jupiter masses, undetermined by this study.
“Outside the Solar System” can suggest a planet around a star, and some English-language coverage made it one. First author Brittany E. Miles calls it “a nearby world,” not a planet, adding:
“Even though brown dwarfs are not true planets, they exhibit planet-like behavior,” Miles said.
What Webb’s long stare adds
Water-cloud hints appeared in 2014, and variability was already known: a 2016 Spitzer series with two 2015 epochs found that a few percent shift in cloud coverage sufficed. Webb synthetic photometry gives 4 to 6 percent peak-to-peak amplitudes, similar to Spitzer’s.
The paper’s actual novelty is the first spectroscopic time series for WISE 0855. NIRSpec observed in bright-object mode from 2.87 to 5.27 microns at resolution near 1,000 and signal-to-noise 80 to 100, separating chemistry, temperature and condensates for the first time. Miles said:
“Before JWST, we only had photometry, which mixed up the effects of clouds, chemistry, and temperature all together,” Miles said.
The clouds are inferred, not imaged: without water condensates, models cannot match the mean spectrum, and explaining the nearly static water-vapor bands requires varying cloud thickness and temperature.
Carbon monoxide modulates by up to 10 percent peak-to-peak; carbon dioxide and phosphine also vary, with CO and phosphine correlated. Methane-, ammonia- and water-dominated wavelengths move less than the overall light curve. The authors trace the CO and phosphine swings to temperature changes at depth—thinner cloud reveals deeper gas-rich layers—not to clouds; thermal shifts are about one kelvin.
Principal-component analysis assigns 60.5 percent of variability to the first eigenvector (apparent temperature or cloud cover) and 4.7 percent to the second (CO and phosphine). Yet 34.7 percent escapes both, implying complex behavior. Zinc sulfide, potassium chloride and sodium sulfide clouds are ruled out because they cannot explain the correlated molecules.
What the study cannot settle
Rotation is unresolved: significant power at 5.4–5.6 hours appears only above 3.7 microns, and at periods beyond the observing window; using 75 percent or less of the series yields none. Across three Spitzer-and-Webb epochs WISE 0855 has shown no consistent periodicity. Over twenty consecutive hours would be needed.
Cloud altitude is model-dependent: two families disagree, and one cannot reproduce small water-line variations, placing clouds lower. Water should condense at 1.2–2 bar; an earlier retrieval put a cloud at 10 bar.
The chemistry-only alternative dates to 2014. Clouds or disequilibrium chemistry fit the data broadly but imperfectly; earlier work matched the 4–5-micron region, cited as cloud evidence, with no clouds but required a pressure-temperature profile far from radiative-convective equilibrium. The authors read that deviation as a missing opacity source possibly due to clouds. No direct model fits the full 1–22-micron spectrum.
The advance is narrower than the water headline: Webb can now watch a starless object’s atmosphere change over time and separate chemistry from temperature and condensates. Patchy cloud cover and deeper carbon-rich gas surfacing when the deck thins fit the data better than a dominant water-cloud signal. The paper’s own limits—assumed mass, unknown rotation, model-dependent cloud height, an alternative chemistry-only fit—remain the strongest critique.