# A giant planet undergoing extreme ultraviolet irradiation by its hot massive-star host

## TL;DR

This paper reports the discovery and analysis of KELT-9b, a highly irradiated giant exoplanet orbiting a very hot star, with extreme ultraviolet exposure potentially causing significant atmospheric mass loss.

## Contribution

It presents the first detailed characterization of KELT-9b, the hottest known transiting exoplanet, and discusses its atmospheric properties and potential for ablation due to intense UV irradiation.

## Key findings

- KELT-9b's day-side temperature is approximately 4600K.
- The host star's temperature is about 10,170K, near the boundary between A and B types.
- KELT-9b receives roughly 700 times more extreme UV radiation than WASP-33b.

## 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.