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White and brown dwarfs are astrophysical objects that are bright enough tosupport an insolation habitable zone (IHZ). Unlike hydrogen-burning stars, theycool and become less luminous with time, and hence their IHZ moves in withtime. The inner edge of the IHZ is defined as the orbital radius at which aplanet may enter a moist or runaway greenhouse, phenomena that can remove aplanet's surface water forever. Thus, as the IHZ moves in, planets that enterit may no longer have any water, and are still uninhabitable. Additionally, theclose proximity of the IHZ to the primary leads to concern that tidal heatingmay also be strong enough to trigger a runaway greenhouse, even for orbitaleccentricities as small as 10^-6. Water loss occurs due to photolyzation by UVphotons in the planetary stratosphere, followed by hydrogen escape. Young whitedwarfs emit a large amount of these photons as their surface temperatures areover 10^4 K. The situation is less clear for brown dwarfs, as observationaldata do not constrain their early activity and UV emission very well.Nonetheless, both types of planets are at risk of never achieving habitableconditions, but planets orbiting white dwarfs may be less likely to sustainlife than those orbiting brown dwarfs. We consider the future habitability ofthe planet candidates KOI 55.01 and 55.02 in these terms and find they areunlikely to become habitable.

Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary