ROCKS AND GAS
You can sort planets into two categories—two ways. The first pair of categories refers to the planets' physical nature. Earth is a terrestrial planet, so too are Mars, Venus, and Mercury. Jupiter, on the other hand, is a gas giant, along with Saturn, Uranus, and Neptune.
The terrestrial planets are small and rocky with relatively thin atmospheres. Gas giants are at least a dozen times more massive and consist of deep, heavy atmospheres that surround small, rocky cores. Pluto is an oddball that does not fit either category well. Scientists are beginning to think it most resembles an enormous comet nucleus. But its nature remains in question, partly because it has never been visited by spacecraft.