Scientists generally agree today that comets were formed at the same time that our solar system was formed – about 4.6 billion years ago. At that time, the solar system was an enormous spinning disk of dust and gas called the Solar Nebula. Over time, gravitational attraction caused the dust and gas in the center to form our Sun and other areas consolidated into clumps of matter called planetesimals. Planetesimals were about one kilometer across.
Comet Origins: Blame the Solar Nebula!
