  Several tens of white dwarfs are known to host circumstellar discs of dusty
debris, thought to arise from the tidal disruption of rocky bodies originating
in the star's remnant planetary system. This paper investigates the evolution
of such discs if they are very massive, as may be the case if their progenitor
was a terrestrial planet, moon, or dwarf planet. Assuming the discs are
physically thin and flat, like Saturn's rings, their evolution is governed by
Poynting-Robertson drag or viscous spreading, where the disc's effective
viscosity is due to self-gravity wakes. For discs with masses >10^26 g, located
in the outer parts of the tidal disruption zone, viscous spreading dominates
the evolution, and mass is transported both in- and outwards. When
outwards-spreading material flows beyond the Roche limit, it coagulates into
new (minor) planets in a process analogous to the ongoing formation of moonlets
at the outer edge of Saturn's rings. The newly formed bodies migrate outwards
by exchanging angular momentum with the disc and coalesce into larger objects
through mutual collisions. Eventually, the disc's Roche-limit overflow recycles
tens of percent of the original disc mass; most ends up in a single large body
near 2:1 mean-motion resonance with the disc's outer edge. Hence, the recycling
of a tidally disrupted super-Earth, for example, could yield an Earth-mass
planet on a ~10-h orbit, located in the habitable zone for 2-to-10-Gyr-old
white dwarfs. The recycling process also creates a population of smaller bodies
just outside the Roche limit, which may explain the minor planets recently
postulated to orbit WD 1145+017.
