Dissecting the atmosphere of a giant planet candidate orbiting a hot white dwarf

The study of white dwarfs accreting planetary material provides an unprecedented insight into the composition of exoplanets. Metals in the otherwise pure hydrogen or helium atmospheres of white dwarfs can be used to infer the abundances of the parent planetary bodies. So far, metals detected in over 1500 white dwarfs have unambiguously been linked to accretion of the debris of rocky bodies. However, to date, only one white dwarf has been associated with the accretion of a gas giant atmosphere. We present a second gas giant accreting candidate, HS 0209+0832. HS 0209+0832 is a hot white dwarf with an effective temperature of 35,500K, and traces of helium and metals detected in its hydrogen-dominated atmosphere. The photospheric abundances are utterly inconsistent with accretion of rocky material, presenting large depletions in iron and silicon but enhancements in calcium and nickel. HS 0209+0832 is also the only white dwarf with zinc detected in its photosphere. We demonstrate that HS 0209+0832 is in fact accreting a giant planet atmosphere, as evidenced by cloud-forming species detected in the white dwarf’s photosphere, including C, Ca, Ti and Ni, which have all previously been observed in transmission spectra of giant planets. Furthermore, metals such as Al and Zn have been predicted to be abundant in giant planets, both of which have been detected in this white dwarf. Because of the high temperature of HS 0209+0832, the atmosphere of a close-in giant planet is bombarded by extreme ultraviolet photons, driving a substantial outflow of the atmosphere, with some of the evaporated material subsequently being accreted by the white dwarf. We detect a 4.4 day period in TESS photometry of HS 0209+0832, which we interpret as the orbital period of the giant planet. We note that the photospheric helium abundance of the white dwarf is also variable, suggesting asymmetric accretion or a cometary tail from the giant planet. Studying white dwarfs evaporating giant planets represents a revolutionary way to infer their compositions, which is complementary to the traditional method of transmission spectroscopy. Through observation of hot white dwarfs, we can detect the traces of giant planet atmospheres and the abundances of cloud species.