SPHEREx recorded the coldest known brown dwarf. Model-inferred radii spread up to fourfold

The telescope measures what the molecules remove

Brown dwarfs form from collapsing clouds of gas, but they never become massive enough to sustain ordinary hydrogen fusion. They glow with heat left from formation, then cool over time. Nearby examples can be studied without the glare of a host star.

SPHEREx does not resolve one into continents, storms or cloud bands. It measures brightness in 102 channels spanning wavelengths from visible deep red to infrared light near 5 microns. When a molecule absorbs part of that light, it leaves a dip in the spectrum.

Across the sequence, researchers traced signatures from water, methane, carbon dioxide and carbon monoxide. Water vapor in Earth's atmosphere blocks some of the same infrared wavelengths, so observing from orbit opens bands that are difficult to measure from the ground.

The hardest handoff sits between clouds and methane

Hotter L dwarfs can carry thick clouds made from minerals that condense at temperatures far above water's freezing point. As the objects cool into the T class, those clouds change and retreat below view while methane becomes more prominent.

The researchers compared the observations with five model families. Near the L-to-T transition, a model could reproduce the near-infrared peaks yet miss the window around 4 microns. The largest deviations appeared around features associated with carbon dioxide and carbon monoxide.

No single model family won across every object and wavelength. Different grids use different assumptions about clouds, chemistry, vertical mixing and temperature structure. They broadly captured the chemical trend, but the cloudy transition exposed places where those assumptions pulled the predicted spectrum away from the observation.

A precise-looking number moved with the model

One late-T object made the consequence visible. The best-fitting model temperatures for WISE J0005 ranged from roughly 550 to 650 kelvin. The radii inferred from those fits differed by about 50 percent.

The colder WISE J0855 pushed the uncertainty farther. Its reliable flux was confined mainly to wavelengths longer than about 3.9 microns, leaving the models less information to work with. The resulting radius estimates spanned a factor of roughly three to four.

Neither object swelled or shrank between calculations. Temperature and radius were outputs of models fitted to the measured light, not direct measurements of a solid edge.

The colored dots are observations, not portraits

NASA's collage is built from authentic SPHEREx observation tiles. The white, orange and red points do not show surfaces or bands of weather. They are unresolved sources recorded through the telescope's channels and arranged with labels for spectral type, distance and estimated temperature.

WISE J0855 has one physical radius. The threefold to fourfold spread belongs to the models, and that is precisely what its sparse spectrum revealed.