Sure, but Jupiters and Saturns only form because of the heavier elements. You can't make a planet just out of hydrogen and helium (actually some models suggest

Sure, but Jupiters and Saturns only form because of the heavier elements. You can't make a planet just out of hydrogen and helium (actually some models suggest you can, via some weird dynamical instability, but it's not widely believed that this is what actually happens in most cases). Instead, you form a planetary core first, out of rock and ice, and once it gets to a sufficient size (something like 20 M_earth) it's big enough to be able to hold onto hydrogen and helium, and it quickly hoovers up all the gas in its path, growing from 14 to hundreds of Earth masses in a relatively short time (less than a million years). First-generation stars are thought to be very unlikely to have planets, unless they're formed by the weird disk instability thing.