Summary
An unusual mix of elements in the white dwarf HS 0209+0832, together with a repeating brightness signal, points to an evaporating giant-planet candidate. Researchers say its material may have formed from gas expelled by the star during an earlier giant phase.
The white dwarf HS 0209+0832 is accreting material with an unusual chemical signature, and its light varies on a 4.399-day cycle. In a study published in Nature Astronomy, researchers interpret the combined evidence as pointing to a giant-planet candidate losing material close to the white dwarf. The composition suggests the candidate may have formed from gas expelled by the star during an earlier giant phase, rather than from the disc that formed around the star when it was young.
The planet itself has not been observed directly. Instead, the case rests on the elements polluting the white dwarf’s atmosphere and a faint repeating signal in its brightness.
A chemical mix unlike rocky planetary debris
The researchers reanalysed ultraviolet and optical spectra of HS 0209+0832 from the Hubble Space Telescope, the Far Ultraviolet Spectroscopic Explorer and the Very Large Telescope. They identified copper and niobium among the metals in the white dwarf’s atmosphere, alongside elements including carbon, aluminium, silicon, calcium, titanium, nickel and zinc.
The mix differs sharply from familiar rocky material. Silicon is present only at trace levels, and iron was not detected; the estimated nickel-to-iron ratio in the accreted material is greater than 2.09, compared with about 0.05 in CI chondrites and bulk Earth. Niobium is especially enriched relative to calcium: the researchers report it at more than a thousand times its relative abundance in the Sun.
White dwarfs can pull in debris from planetary systems, leaving elements in their atmospheres. To estimate the composition of the material falling onto HS 0209+0832, the team accounted for how quickly different elements sink out of the atmosphere. Their analysis finds that radiative levitation—the upward support of some elements by stellar radiation—is negligible for several of the key elements, including calcium, titanium, nickel, copper, zinc and niobium.
Niobium and other elements made through slow neutron capture, or the s-process, provide a clue to the material’s origin. As a star reaches the asymptotic giant branch, a late stage of stellar evolution, it produces and expels material enriched in carbon and s-process elements. The researchers find that the observed pattern is consistent with a planet forming from some of that expelled material. Such a world is called a second-generation planet because it would form after the star’s original, main-sequence lifetime.
A possible evaporating planet close to the white dwarf
Observations from NASA’s Transiting Exoplanet Survey Satellite (TESS) show a sinusoidal brightness variation with a period of 4.399 ± 0.026 days and an amplitude of 0.120% ± 0.018%. The authors say this could be the changing thermal emission of a close-in planet as its day and night sides rotate into view. They also consider a comet-like tail of gas escaping from an evaporating planet as an explanation for the variability.
In the proposed picture, intense radiation from the hot white dwarf heats a nearby giant planet and drives gas from its atmosphere. Some of that material then falls onto the white dwarf, accounting for the unusual elements found in its atmosphere. The study’s calculations put a possible planetary orbit at about 0.04 astronomical units, though the brightness variation alone does not establish that orbit.
The periodic signal is detected in the combined TESS data and in two of the four observed sectors. The researchers did not find a significant periodicity in the other two sectors; their injection-recovery tests indicate that a signal of the measured amplitude could have been missed in those data. The planet interpretation therefore brings together two kinds of evidence—chemical enrichment and low-amplitude variability—while the variability could also arise from escaping material rather than the planet’s changing visible hemisphere.
If the interpretation is correct, HS 0209+0832 would show that planets can form from stellar ejecta after a star leaves the main sequence. The authors suggest that ultraviolet searches for high carbon and s-process abundances could help identify other candidates.