Catastrophic Evaporation of Rocky Planets
Short-period exoplanets can have dayside surface temperatures surpassing 2000 K, hot enough to vaporize rock and drive a thermal wind. I will present a radiative-hydrodynamic model of atmospheric escape from strongly irradiated, low-mass rocky planets. We find that rocky planets with masses ≤ 0.1 M_Earth and surface temperatures ≥ 2000 K disintegrate entirely in ≤ 10 Gyr. When we apply our model to Kepler planet candidate KIC 12557548b—believed to be a rocky body evaporating at a rate of dM/dt ≥ 0.1 M_Earth/Gyr—we find its present-day mass to be ≤ 0.02 M_Earth (less than twice the mass of the Moon).
Surface Layer Accretion in Protoplanetary Disks Driven by X-ray and FUV Ionization
How do protoplanetary disks accrete? Whether accretion of the disk surface layers by the magnetorotational instability (MRI) occurs at observationally significant rates, depends on how well ionized they are. Surface layers ionized by stellar X-rays are susceptible to charge neutralization by small condensates. Ion densities in X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI. I will show that ionization by stellar far-ultraviolet (FUV) radiation produces a plasma so dense that it is immune to ion recombination on condensates. MRI-turbulence in the FUV-ionized layer behaves in the ideal magnetohydrodynamic (MHD) limit and can accrete at observationally significant rates at radii ≥ 1–10 AU. At smaller radii, surface layers ionized by both X-rays and FUV radiation cannot sustain the accretion rates generated at larger distance and an additional means of transport is needed.