The vast majority on literature on solar sailing has thus far considered solar radiation pressure (SRP) as the only contribution to the sail force. However, when a sail orbiting a planet is considered, a new contribution can be added. A planet, being a body in thermodynamic equilibrium with its environment, emits the so called Black-Body Radiation. This radiation is emitted uniformly in all direction, with most of the energy emitted in the infrared range. The specific spectrum and intensity of this radiation depends only on the body temperature. Moreover, the radiation coming from the Sun will hit the planet and it will be reflected diffusely from the planet surface into the outer space as a function of the position of the Sun.