 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. 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 they get so massive that they attract gas from the disk and then form a gas planet like Jupiter.