Accretion is the process of matter gravitationally collapsing from a cloud of dust or gas or (usually) both. For example, our Sun makes up 99.86% of the matter in the solar system. By the way, there’s still a lot of “unaffiliated” junk in our solar system that’s still waiting to “join” a planet. Our solar system formed as a disk with all of the planets forming within that disk in the “plane of the ecliptic”. In fact, Jupiter’s moons are laid out so much like the rest of the solar system (all orbiting in the same plane) that they helped early astronomers to first understand the entire solar system.