The remains of the pre-WD-phase solar systems are revealed in the form of heavy element ‘pollution’ in WD atmospheres, excess emission from dust discs, and—only recently—in transits of planetary debris. In principle, WDs can host not only debris, but also whole planetary systems.
The first transiting planetesimal orbiting a WD was detected in K2 data of WD 1145+017 by Vanderburg et al. 2015 and has been followed up intensively. The multiple, long and variable transits suggest the transiting objects are dust clouds, probably produced by a disintegrating asteroid...
Recent Kepler photometry has revealed that about half of WDs have periodic, low-level (∼10−4−10−3), optical variations. Hubble Space Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of WDs are actively accreting rocky planetary debris...