Discovery of a second-generation planet candidate accreting onto a white dwarf

Many white dwarfs accrete the debris of disrupted planetary bodies, detected in the form of metal enrichment in their atmospheres, dusty or gaseous circumstellar discs, and photometric transits from debris. The composition of accreted debris has been shown to be diverse yet overall closely resembles Solar System bodies. Here we report the discovery of a white dwarf accreting material that is unlike any Solar System object. The atmosphere of HS 0209+0832 is strongly enriched in trans-iron elements including zinc, copper and niobium but depleted in the canonical rock-forming elements silicon and iron. The composition of the accreted object is consistent with a candidate second-generation planet, formed from the stellar material ejected during the giant phase. The key abundance signature that differentiates it from a first-generation planet is the high enrichment of s-process elements. Photospheric helium and the absence of terrestrial rock-forming elements implies that the white dwarf is accreting from the escaped atmosphere of a photo-evaporating giant planet candidate. This is further supported by the detection of a sinusoidal photometric period of 4.399 ± 0.026 days at an amplitude of 0.120% ± 0.018%, which we attribute to thermal emission phase variability from a planetary day–night cycle or a transiting cometary tail of an evaporating gas giant. The discovery of this second-generation planet that is chemically distinct from first-generation material demonstrates that close-in planets around white dwarfs can form after the main sequence.