2023NatAs...7.1329H

Planets orbiting close to hot stars experience intense extreme-ultraviolet radiation, potentially leading to atmosphere evaporation and to thermal dissociation of molecules. However, this extreme regime remains mainly unexplored due to observational challenges. Only a single known ultra-hot giant planet, KELT-9b, receives enough ultraviolet radiation for molecular dissociation, with a day-side temperature of ∼4,600 K. An alternative approach uses irradiated brown dwarfs as hot-Jupiter analogues. With atmospheres and radii similar to those of giant planets, brown dwarfs orbiting close to hot Earth-sized white dwarf stars can be directly detected above the glare of the star. Here we report observations revealing an extremely irradiated low-mass companion to the hot white dwarf WD 0032-317. Our analysis indicates a day-side temperature of ∼8,000 K, and a day-to-night temperature difference of ∼6,000 K. The amount of extreme-ultraviolet radiation (with wavelengths 100-912 Å) received by WD 0032-317B is equivalent to that received by planets orbiting close to stars as hot as late B-type stars, and about 5,600 times higher than that of KELT-9b. With a mass of ∼75-88 Jupiter masses, this near-hydrogen-burning-limit object is potentially one of the most massive brown dwarfs known.