  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 ~4,600K. 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 WD0032-317. Our analysis indicates a day-side
temperature of ~8,000K, and a day-to-night temperature difference of ~6,000K.
The amount of extreme-ultraviolet radiation (with wavelengths 100-912\r{A})
received by WD0032-317B is equivalent to that received by planets orbiting
close to stars as hot as a 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.
