  To date the search for habitable Earth-like planets has primarily focused on
nuclear burning stars. I propose that this search should be expanded to cool
white dwarf stars that have expended their nuclear fuel. I define the
continuously habitable zone of white dwarfs, and show that it extends from
~0.005 to 0.02 AU for white dwarfs with masses from 0.4 to 0.9 solar masses,
temperatures less than 10,000 K, and habitable durations of at least 3 Gyr. As
they are similar in size to Earth, white dwarfs may be deeply eclipsed by
terrestrial planets that orbit edge-on, which can easily be detected with
ground-based telescopes. If planets can migrate inward or reform near white
dwarfs, I show that a global robotic telescope network could carry out a
transit survey of nearby white dwarfs placing interesting constraints on the
presence of habitable Earths. If planets were detected, I show that the survey
would favor detection of planets similar to Earth: similar size, temperature,
rotation period, and host star temperatures similar to the Sun. The Large
Synoptic Survey Telescope could place even tighter constraints on the frequency
of habitable Earths around white dwarfs. The confirmation and characterization
of these planets might be carried out with large ground and space telescopes.
