n.(cosmology) the theory that the solar system was formed by the gravitational accumulation of planetesimals
英汉双解大词典
planetesimal hypothesis/ˌplænɪˈtɛsɪməl/
1.
N（名词）
the discredited theory that the close passage of a star to the sun caused many small bodies (planetesimals) to be drawn from the sun, eventually coalescing to form the planets 星子假说