text:  portmanteau of a planet plus infinitesimal so a very small part of a planet so asteroids and comets they are these so-called planetesimals or as i like to call them the building blocks of planets right and then once you build it that size gravity comes into play and then they can keep growing to the size of the earth or bigger and at some point it gets so massive that they attract gas from the disk and then form a gas planet like jupiter