The inner planets are Mercury, Venus, Earth, and Mars. They are composed mostly of rock, are smaller and denser than the outer gas giants, and have few to no moons and no rings. Mercury is the closest planet to the sun, has no moons, and experiences extreme temperature ranges. Venus has a thick, toxic atmosphere that creates a runaway greenhouse effect. Earth has a protective atmosphere and one moon, and is the only known planet capable of supporting life. Mars is also called the Red Planet for its rust-colored surface and has a thin atmosphere and two small moons.