  The last several years have brought about a dynamic shift in the view of
exoplanetary systems in the post-main sequence, perhaps epitomized by the
evidence for surviving rocky planetary bodies at white dwarfs. Coinciding with
the launch of the Spitzer Space Telescope, both space- and ground-based data
have supported a picture whereby asteroid analogs persist at a significant
fraction of cool white dwarfs, and are prone to tidal disruption when passing
close to the compact stellar remnant. The ensuing debris can produce a
detectable infrared excess, and the material gradually falls onto the star,
polluting the atmosphere with heavy elements that can be used to determine the
bulk composition of the destroyed planetary body. Based on the observations to
date, the parent bodies inferred at white dwarfs are best described as
asteroids, and have a distinctly rocky composition similar to material found in
the inner Solar System. Their minimum masses are typical of large asteroids,
and can approach or exceed the mass of Vesta and Ceres, the two largest
asteroids in the Solar System. From the number of stars surveyed in various
studies, the fraction of white dwarfs that host terrestrial planetary system
remnants is at least a few percent, but likely to be in the range 20% to 30%.
Therefore, A- and F-type stars form terrestrial planets efficiently, with a
frequency at least as high as the remnants detected at their white dwarf
descendants.
