The four inner planets — Mercury, Venus, Earth, and Mars — are all small, rocky worlds, very different from the gas giants further out.
Mercury is the closest planet to the Sun and has almost no atmosphere to trap heat, so its temperature swings wildly: scorching hot during the day, freezing cold at night.
Venus is the hottest planet in the Solar System, even hotter than Mercury, because its thick atmosphere traps heat in a runaway greenhouse effect. Its surface pressure is so high it would crush an unprotected spacecraft.
Earth is the only known planet with liquid water on its surface and life as we know it, thanks to its distance from the Sun and its protective atmosphere.
Mars, the Red Planet, gets its color from iron oxide (rust) covering its surface. It has the largest volcano in the Solar System, Olympus Mons, and evidence suggests it once had liquid water flowing across its surface.