Radiative Thrusters on Close-in Extrasolar Planets

The atmospheres of close-in extrasolar planets absorb most of the incident stellar radiation, advect this energy, then reradiate photons in preferential directions. Those photons carry away momentum, applying a force on the planet. Here we evaluate the resulting secular changes to the orbit, known as the Yarkovsky effect. For known transiting planets, typical fractional changes in semi-major axis are about 1% over their lifetime, but could be up to ~5% for close-in planets like OGLE-TR-56b or inflated planets like TrES-4. We discuss the origin of the correlation between semi-major axis and surface gravity of transiting planets in terms of various physical processes, finding that radiative thrusters are too weak by about a factor of 10 to establish the lower boundary that causes the correlation.