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

Many white dwarfs accrete the debris of disrupted planetary bodies, detected through metal enrichment in their atmospheres, dusty or gaseous circumstellar discs, and photometric transits from debris. The composition of this accreted debris is diverse, yet it generally resembles Solar System bodies.

We report the discovery of a white dwarf accreting material 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. This composition is consistent with a candidate second-generation planet, formed from stellar material ejected during the giant phase. The key abundance signature distinguishing it from a first-generation planet is a high enrichment of s-process elements.

Photospheric helium and the absence of terrestrial rock-forming elements imply that the white dwarf is accreting from the escaped atmosphere of a photo-evaporating giant planet candidate. This is further supported by a detected sinusoidal photometric period of 4.399±0.026 days at an amplitude of 0.120±0.018%, which the authors attribute to thermal emission phase variability from a planetary day-night cycle or a transiting cometary tail of an evaporating gas giant.