This illustration shows WD 1856 b, a Jupiter-sized planet candidate orbiting a faint white dwarf star. Based on new observations from the James Webb Space Telescope, it may be among the coldest known exoplanets, with an estimated temperature of just 186 K (–87°C or –125°F). [Image credit: NASA’s Goddard Space Flight Center]
Prior estimates had estimated the exoplanet’s size to be about 13.8 times bigger than that of Jupiter.
Lead author Mary Anne Limbach, an assistant research scientist in the University of Michigan’s astronomy department, writes in the arXiv preprint that after JWST’s Mid-Infrared Instrument registered an unexpected glow, the team “attribute[s] this excess flux to the known giant planet in the system, making it the coldest exoplanet from which light has ever been directly observed.” They peg its brightness temperature at 186 ± 7 K and cap its mass at about six Jupiters.
In 2020, Scientific American chronicled the first hints of WD 1856 b, describing how NASA’s TESS telescope spotted a Jupiter-size object racing around a white dwarf every 34 hours. “WD 1856 b somehow got very close to its white dwarf and managed to stay in one piece,” noted Andrew Vanderburg, an assistant professor of astronomy at the University of Wisconsin-Madison, in a NASA article published at that time. Spitzer follow-ups showed no self-emitted infrared glow, bolstering the case that the oddball was a planet and not a failed star, yet its true mass and temperature remained out of reach.
Webb’s mid-infrared data settle the debate. In just 47 minutes of imaging across seven filters, the telescope logged a 5.7-sigma excess from the planet’s own heat, pinning its brightness temperature at 186 ± 7 K and narrowing its mass to 0.84–5.9 Jupiter masses. The result makes WD 1856 b the first intact planet confirmed inside a white dwarf’s “forbidden zone” and the first world with emitted light detected below 200 K. A second JWST pass in July 2025 aims to hunt for additional companions and refine the planet’s atmospheric profile.
The news is getting widespread coverage with popular-science outlets splashing the story across their homepages: Universe Today dubbed WD 1856 b “the coldest planet ever found,” while Space.com highlighted the world’s survival inside a “dead star’s forbidden zone.” ABC News led with the record −125°F reading, and Phys.org emphasized JWST’s role in clinching the direct-imaging temperature below 200 K.
