The effects of a resisting medium on protoplanetary orbits

The equation of motion of a body in orbit about a central star emitting a radially moving stream of gas is obtained on the assumption that all the gas hitting the body is accreted by it. Exact analytical solutions are found for the case when the body has a constant radius. The variations in the orbital parameters are discussed and the results applied to protoplanets. It is shown that protoplanets originally close to the Asteroidal Belt, and hence outside the Roche limit are affected in such a way that they move through this limit and have their gaseous envelopes tidally disrupted. In this way an explanation for the composition of the terrestrial planets is offered. (author)