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 are as likely to add as much mass as the smaller ones, if not more. Generally some end up significantly larger than average (termed the oligarchs) and can reach a point where their growth slows (oligarchic growth aka oligarchic accretion), once they have accreted the nearby the material that enlarges planetesimals, and much of their further growth consists of smaller planetesimals merging with them on impact. Also at this time, their interactions with planetesimals can fling the planetesimals out of the system. Given the slower growth, this is presumed to be a far longer growth phase. The end result is that 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.