That each of the giant planets formed around rocky-metal cores but turned out so differently is an important clue to their origins. Why do Jupiter and Saturn have so much hydrogen and helium compared to Uranus and Neptune? Why is hydrogen-rich Jupiter so much more massive than hydrogen-rich Saturn? The answers may lie both in the time that it took for these planets to form and in the distribution of material from which they formed. It is probable that all of the hydrogen and helium in the giant planets was captured from the protoplanetary disk by the strong gravitational attraction of their massive cores. The much lower hydrogenhelium content of Uranus and Neptune suggests that their cores were smaller and formed much later than those of Jupiter and Saturn, at a time when most of the gas in the protoplanetary disk had been blown away by the emerging Sun.