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