  The precise astrometric measurements of the Gaia Data Release 2 have opened
the door to detailed tests of the predictions of white dwarf cooling models.
Significant discrepancies between theory and observations have been identified,
the most striking affecting ultramassive white dwarfs. Cheng et al. (2019)
found that a small fraction of white dwarfs on the so-called Q branch must
experience an extra cooling delay of $\sim 8\,$Gyr not predicted by current
models. $^{22}$Ne phase separation in a crystallizing C/O white dwarf can lead
to a distillation process that efficiently transports $^{22}$Ne toward its
center, thereby releasing a considerable amount of gravitational energy. Using
state-of-the-art Monte Carlo simulations, we show that this mechanism can
largely resolve the ultramassive cooling anomaly if the delayed population
consists of white dwarfs with moderately above-average $^{22}$Ne abundances. We
also argue that $^{22}$Ne phase separation can account for the smaller cooling
delay currently missing for models of white dwarfs with more standard
compositions.
