Hypothetical solid celestial body that accumulated during the last stages of accretion. These bodies, from ~1-100 km in size, formed in the early solar system by accretion of dust (rock) and ice (if present) in the central plane of the solar nebula. Most planetesimals accreted to planets, but many – such as the asteroids– never combined to form large bodies. The very largest asteroids can be considered protoplanets.

Bodies ranging in size from a few meters to a few hundred kilometers that accreted early in solar-system history.