  Short-period exoplanets can have dayside surface temperatures surpassing 2000
K, hot enough to vaporize rock and drive a thermal wind. Small enough planets
evaporate completely. We construct a radiative-hydrodynamic model of
atmospheric escape from strongly irradiated, low-mass rocky planets, accounting
for dust-gas energy exchange in the wind. Rocky planets with masses < 0.1
M_Earth (less than twice the mass of Mercury) and surface temperatures > 2000 K
are found to disintegrate entirely in < 10 Gyr. When our model is applied to
Kepler planet candidate KIC 12557548b --- which is believed to be a rocky body
evaporating at a rate of dM/dt > 0.1 M_Earth/Gyr --- our model yields a
present-day planet mass of < 0.02 M_Earth or less than about twice the mass of
the Moon. Mass loss rates depend so strongly on planet mass that bodies can
reside on close-in orbits for Gyrs with initial masses comparable to or less
than that of Mercury, before entering a final short-lived phase of catastrophic
mass loss (which KIC 12557548b has entered). Because this catastrophic stage
lasts only up to a few percent of the planet's life, we estimate that for every
object like KIC 12557548b, there should be 10--100 close-in quiescent
progenitors with sub-day periods whose hard-surface transits may be detectable
by Kepler --- if the progenitors are as large as their maximal, Mercury-like
sizes (alternatively, the progenitors could be smaller and more numerous).
According to our calculations, KIC 12557548b may have lost ~70% of its
formation mass; today we may be observing its naked iron core.
