When the Sun formed over 4.6 billion years ago, dust and gas that was not incorporated into the Sun formed a disk around it called the protoplanetary disk. Planets, like the Earth, formed in the protoplanetary disk by accretion of the dust and gas and by collisions of planetesimals (planetary bodies that are about 1 km in diameter). The planet-forming process heats up the rock so that it melts, completely changing its original form and composition. However, not all of the material in the disk was used for planet-making. The leftover material made up smaller bodies that experienced less heating than the planets. This material includes asteroids, where most meteorites come from. Visit the links below to learn more!