My conception of a white dwarf with a dusty disk. The dust must sublimate and will accrete onto the surface of the white dwarf, possibly through streamers of gas that follow the white dwarf's magnetic field lines. As in Saturn's rings, there might be streaks in the dust due to material that is darker than the surrounding dust, or perhaps caused by shadows of rocky bodies with sizes larger than the scale height of the disk.
Planetary Systems around White Dwarfs
White dwarfs are the corpses of sun-like stars. By themselves, they are rather simple objects, C/O hulks about half the mass of the Sun crammed into volumes about the size of the Earth. Often, they have a thin atmosphere of helium or hydrogen. Any element heavier than helium often sinks below this atmosphere on a scale of days or years. These objects seem pretty barren, and yet a growing fraction of white dwarfs might show evidence for holding systems of planets and asteroids--some white dwarfs show extra light at infrared wavelengths due to small disks of dust, and others show evidence for accreting dust and gas from asteroids. Studying the planetary systems around white dwarfs can tell us the ultimate fate of our Solar System, and might just tell us a little bit about how planets form in general.
A Menagerie of Disks
The first dusty white dwarf, G29-38, was discovered in 1987 by Ben Zuckerman and Eric Becklin in a search for brown dwarfs areound white dwarfs. Since the launch of Spitzer, almost 20 WD disks are known, shown below ordered by year of discovery (click on the image to zoom in to full detail):
One thing you will notice is that most of these disks are not much bigger than the extent of Saturn's outermost rings, and that to date there is actually a fair bit of variability in terms of the location of the inner and outer edges of these disks. It is not precisely known how these disks form and how long they last. All of these disks, however, are directly accreting onto the surface of the white dwarfs, which shows up in the spectra of the WDs themselves as narrow absorption lines caused by elements such as Ca, Mg, and Fe. If enough elements are detected, you can learn the atomic abundance of the dust accreting onto the WD, giving you an unparalleled view of the composition of the dust in these disks. Most of the abundances done to date show that the dust is very similar to the bulk composition of the Earth and asteroids.
A Chaotic Rebirth in the Stellar Graveyard?
My graduate thesis focused on explaining what might happen to planetary systems after post-main sequence evolution. What I found was that the mass loss from a central star acts to destabilize planetary systems, so much so that some of them might perturb large groups of asteroids. These asteroids could swoop so close to the white dwarf that they get shredded from tidal forces, ending up as a bright dust disk to be observed in the Infrared. This paper describes my results--it is my most cited paper. If dusty white dwarfs originate from the tidal disruption of asteroids, at least one planet is required to send the asteroid close enough to the white dwarf. Post-main sequence evolution scours the inner regions of a planetary system clean, and planets must perturb asteroids from further away after the central star turns into a white dwarf. Therefore, dusty white dwarfs are easy signposts to planetary systems that have survived the death of their parent star.