noun. one of many small solid celestial bodies thought to have existed at an early stage in the development of the solar system.
- celestial body
Example sentences of the word planetesimal
1. Noun, singular or mass
Some of these scientists began referring to Pluto as a planetesimal.
One theory of how the moon was formed is that a planetesimal about the size of Mars hit the Earth and spun off a chunk of material that later became the moon.