Survival of planetary systems

Gravitational interactions (i.e., disk tides) between a newly formed protoplanet and its precursor disk give rise to a net torque that drains angular momentum from the protoplanet's orbit. As a result, protoplanetary objects suffer orbital decay as the disk attempts to destroy the very system it spawns. Survival of a planetary system may be a rather uncertain outcome, and the fraction of circumstellar disks that produce an extant system could be significantly less than unity. Newly discovered close stellar companions may be circumstantial evidence of such large-scale orbit migrations. A scheme for in situ accretion of such objects is outlined that is consonant with a strong tidal influence.