OK, dumb question: When astronomers describe the new planets that have been discovered outside the solar system, they use the term "gas giant." This term is especially vexing because it implies these are Jupiter-like planets: huge bodies composed of lighter elements that are "blown" into the farther reaches of the solar system by the parent star. Yet some of these planets orbit their suns very closely, which has astronomers scrambling to alter their theories about solar system formation. My question is this: Why must these planets be "gas giants"? Why can't they simply be very large terrestrial-style planets, or very dense planets? I mean, gravity doesn't care if it's a big planet with low density or a little planet with high density, right?