White dwarf planetary systems

The transition from the main-sequence to the white dwarf stage should only destroy the very inner-most part of their planetary-systems, and so white dwarfs should continue to be orbited by planets for many Gyr after their formation. We now see direct evidence for these planetary systems resulting from the disintegration of asteroids (and occasionally comets) which venture too close to the white dwarf. Once within about a Solar-radius of the white dwarf, gravitational tides, can shred an asteroid into dust, forming a debris disk which slowly accretes onto the white dwarf surface. These disks – heated by the nearby degenerate star – can be detected via their emission of infra-red flux. The chemical elements that make up the asteroids that have fallen onto the central star can also be detected, as they temporarily alter the composition of the white dwarf outer layers. Measuring the relative amounts of these elements (e.g. O, Si, Al, Mg, Ca, Fe, Ni), means we can directly determine the composition of extraSolar asteroids – the building blocks from which exoplanets are formed.