This drawing shows how forming planets drew gas to themselves. Click on image for full size version (317K GIF)

As shown in this picture, while they were forming in the solar nebula, the nucleii of the planets-to-be (called protoplanets) drew material to themselves from the cloud of gas and dust around them. The bigger protoplanets were able to attract even more gaseous material unto themselves. Because of it's position in the solar nebula, the proto-Jupiter, was able to draw an enormous amount of gas unto itself, and become the biggest of the planets.