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.