White dwarfs pack the mass of the Sun inside a radius that is 1% as small as the Sun’s. Any elements heavier than helium that land on a white dwarf are dragged beneath the surface by the star’s intense gravity. So when astronomers spotted evidence of silicon, iron, and other refractory elements in white dwarf spectra, they concluded that something must be replenishing those elements. Given that the sort of stars that become white dwarfs also host planetary systems, it’s likely that some white dwarfs are consuming their former asteroid belts, which offer a nearby supply of those heavier-than-helium elements.
But a few white dwarf spectra have recently been discovered to exhibit nitrogen and other elements that come from volatile species found on icy planets, such as Neptune, and on icy dwarf planets, such as the ones in the Kuiper belt. Because the orbits of those planetary bodies typically lie...
