Difference between planets and satellites
Answers
The definition of "planet" (which is actually a newish thing) is an object orbiting a star that has cleared the area of its orbit of debris. Any object orbiting a star that has not cleared the area of its orbit of debris is called an asteroid, a comet, or a dwarf planet depending on its size, its location, and what it's made of.
Rocky planets like Earth, Mars, Venus, and Mercury are rocky because they aren't big enough to hold much gas. Farther out, Jupiter, Saturn, Uranus, and Neptune are made mostly of gas. They're huge compared to the rocky planets, but still pretty small compared to the Sun.
There are some objects called "brown dwarfs". These can be seen as huge planets or tiny stars. They are large enough to fuse some hydrogen, but not large enough to have sustained stable fusion.
Red dwarfs are a little bigger than brown dwarfs, and can sustain hydrogen fusion. These are tiny stars. The closest star to us other than the Sun is a red dwarf, but you can't see it without a really good telescope because it's so dim and puny (puny by star standards, huge by planet standards).