If planets could re-form or migrate inwards to just outside the Roche limit of white dwarfs stars, they would be warmed to Earth-like temperature for billions of years. These planets would be easy to detect in edge-on orbit via a large depth transit lasting a couple of minutes and repeating every 12 hours; thus, a ground-based transit survey of cool white dwarfs would be sensitive to Earth-sized planets in their habitable zones, should they exist in sufficient abundance. I will discuss the prospects for detection and characterization of Earth-like planets in the habitable zones of white dwarfs, as well as scenarios for planet formation and potential constraints on habitability.