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Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4-minute periodic variation in the strength and shape of the Ca ii emission line profiles originating from the debris disc around the white dwarf SDSS J122859.93+104032.9. We interpret this short-period signal as the signature of a solid-body planetesimal held together by its internal strength.

The detection of a dust disc around G29-38 1 and transits from debris orbiting WD 1145+017 2 confirmed that the photospheric trace metals found in many white dwarfs 3 arise from the accretion of tidally disrupted planetesimals 4 . The composition of these planetesimals is similar to that of rocky bodies in the inner solar system 5 . Gravitationally scattering planetesimals towards the white dwarf requires the presence of more massive bodies 6 , yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a ≃ 27 750 K hot white dwarf that is accreting from a circumstellar gaseous disc composed of hydrogen, oxygen, and sulphur at a rate of ≃ 3.3 × 10 9 g s −1 . The composition of this disc is unlike all other known planetary debris around white dwarfs 7 , but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H 2 O and H 2 S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of ≃ 15 solar radii will undergo significant evaporation with expected mass loss rates comparable to the accretion rate onto the white dwarf. The orbit of the planet is most likely the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of spectroscopically detectable giant planets in close orbits around white dwarfs of ≃ 10 −4 . WD J091405.30+191412.25 (WD J0914+1914) was initially classified as a close interacting white dwarf binary on the basis of a weak Hα emission line detected in its spectrum obtained by the Sloan Digital Sky Survey (SDSS) 8 . Upon closer inspection of this spectrum, we identified additional emission lines of oxygen (O I at wavelengths 7,774Å and 8,446Å), and an emission line near 4,068Å that we tentatively identified as [S II]. The line flux ratios of the hydrogen and oxygen lines are extremely atypical for any white dwarf binary, casting doubt on the published classification.We obtained deep spectroscopy of this star using the X-Shooter spectrograph on the Very Large Telescope of the European Southern Observatory (see Fig. 1) which confirms the presence of [S II] (4,068Å), and contains additional emission lines of [O I] (6,300Å and 6,363Å) as well as a blend of O I and S I lines near 9,200Å.The double-peaked morphology of the Hα and the O I (8,446Å) emission lines (see Fig. 1) indicates an origin in a circumstellar gas disc 9 , reminiscent of several white dwarfs with dusty and gaseous planetary debris discs 10, 11 . However, the spectra of all known gaseous debris discs are dominated by the emission lines of the Ca II triplet (8,600Å), with weaker lines of Fe II, which are absent in the X-Shooter observations of WD J0914+1914. Moreover, none of the other gaseous debris discs around white dwarfs show Hα emission.