This artist's concept shows an exoplanet and debris disk orbiting a polluted white dwarf.

White dwarfs are dim, dense remnants of stars similar to the Sun that have exhausted their nuclear fuel and blown off their outer layers. By "pollution," astronomers mean heavy elements invading the photospheres -- the outer atmospheres -- of these stars.

The leading explanation is that exoplanets could push small rocky bodies toward the star, whose powerful gravity would pulverize them into dust. That dust, containing heavy elements from the torn-apart body, would then fall on the star.