Planet Formation on the Fast Track

It's textbook astronomy. Planets form little by little, as material slowly congeals within the disk of gas, dust, and ice known to swaddle young stars. First, gravity gathers together bits of dust, which merge to form boulder-size bodies, which themselves coalesce into bigger and bigger objects. In about a million years, these form rocky planets, like Earth and Mars. Over the next few million years, gas from the disk settles around some of these solid bodies, and they grow far bigger, becoming giants like gaseous Saturn and Jupiter.

http://www.sciencenews.org/20030125/bob9.asp