Title: A giant planet undergoing extreme ultraviolet irradiation by its hot massive-star host,
Abstract: The amount of ultraviolet irradiation and ablation experienced by a planet
depends strongly on the temperature of its host star. Of the thousands of
extra-solar planets now known, only four giant planets have been found that
transit hot, A-type stars (temperatures of 7300-10,000K), and none are known to
transit even hotter B-type stars. WASP-33 is an A-type star with a temperature
of ~7430K, which hosts the hottest known transiting planet; the planet is
itself as hot as a red dwarf star of type M. The planet displays a large heat
differential between its day-side and night-side, and is highly inflated,
traits that have been linked to high insolation. However, even at the
temperature of WASP-33b's day-side, its atmosphere likely resembles the
molecule-dominated atmospheres of other planets, and at the level of
ultraviolet irradiation it experiences, its atmosphere is unlikely to be
significantly ablated over the lifetime of its star. Here we report
observations of the bright star HD 195689, which reveal a close-in (orbital
period ~1.48 days) transiting giant planet, KELT-9b. At ~10,170K, the host star
is at the dividing line between stars of type A and B, and we measure the
KELT-9b's day-side temperature to be ~4600K. This is as hot as stars of stellar
type K4. The molecules in K stars are entirely dissociated, and thus the
primary sources of opacity in the day-side atmosphere of KELT-9b are likely
atomic metals. Furthermore, KELT-9b receives ~700 times more extreme
ultraviolet radiation (wavelengths shorter than 91.2 nanometers) than WASP-33b,
leading to a predicted range of mass-loss rates that could leave the planet
largely stripped of its envelope during the main-sequence lifetime of the host
star.