Planetary material is a major source of hydrogen in helium-atmosphere white dwarfs

Many white dwarfs with helium-dominated atmospheres show traces of hydrogen in their spectra. On average, the total mass of hydrogen within their convective envelopes increases with age, but there is no consensus on the origin of such hydrogen. Hypotheses include upward diffusion from hidden reservoirs deep within the envelope, or accretion, alongside heavier elements, of planetary material. We construct a simple model that includes both pathways, and test it using two independent samples of helium-atmosphere DB white dwarfs from the DESI and SDSS spectroscopic surveys, selected to have Gaia colours GBP − GRP > −0.25, corresponding roughly to 10 000 ≲ Teff ≲ 19 000 K. We find that at least 22 ± 2 per cent of the helium-atmosphere white dwarfs with detectable hydrogen must have accreted some of it from their planetary systems, via ices, hydrated minerals or atmospheres. We thus demonstrate that planetary hydrogen is a necessary ingredient to explain the spectral evolution of white dwarfs as a population, rather than only extreme hydrogen-enriched cases.