oligarch

Oligarch is a term used in planet formation theory, to signify planetesimals that have grown larger than the majority, in some sense dominating the subsequent development into the resulting planets. Such theories necessarily have evolved, based on observation and computer simulations, and any "complete" theory must include means of creating both rocky planets and giant planets, the differing results implying some difference in the growth process.

A general theory is that at some early point, numerous planetesimals undergo a period of rapid growth (runaway growth) during which any larger ones have the advantage on adding mass, i.e., the largest grow the most. Eventually the larger ones (termed the oligarchs) reach a point where their growth (oligarchic growth aka oligarchic accretion) slows, when they have basically accreted the material near enough for accretion, after which their further growth generally consists of smaller planetesimals merging with them on impact. Interactions without such an impact sometimes fling smaller planetesimals out of the system. This slower growth phase is presumed to last far longer than the earlier accretion. The end result is that after growth ceases, just a few planets remain, such as the solar system's eight planets. The oligarch theory was developed when very little detail was known of any planetary system other than the solar system.