  Proxima b is a planet with a minimum mass of 1.3 MEarth orbiting within the
habitable zone (HZ) of Proxima Centauri, a very low-mass, active star and the
Sun's closest neighbor. Here we investigate a number of factors related to the
potential habitability of Proxima b and its ability to maintain liquid water on
its surface. We set the stage by estimating the current high-energy irradiance
of the planet and show that the planet currently receives 30 times more EUV
radiation than Earth and 250 times more X-rays. We compute the time evolution
of the star's spectrum, which is essential for modeling the flux received over
Proxima b's lifetime. We also show that Proxima b's obliquity is likely null
and its spin is either synchronous or in a 3:2 spin-orbit resonance, depending
on the planet's eccentricity and level of triaxiality. Next we consider the
evolution of Proxima b's water inventory. We use our spectral energy
distribution to compute the hydrogen loss from the planet with an improved
energy-limited escape formalism. Despite the high level of stellar activity we
find that Proxima b is likely to have lost less than an Earth ocean's worth of
hydrogen before it reached the HZ 100-200 Myr after its formation. The largest
uncertainty in our work is the initial water budget, which is not constrained
by planet formation models. We conclude that Proxima b is a viable candidate
habitable planet.
