Almost everyone can rattle off at least some of the planets in our solar system. But the formal definition of the word was only recently set, and even professional astronomers sometimes disagree. So, what do we mean by planet?
The word planet comes from the Greek term asteres planetai, which means “wandering star.” Greek skygazers were intimately familiar with the night sky — no light pollution back then! — so they noticed that while most of the stars remained motionless relative to each other, some objects seemed to wander among them. Now known as the classical planets, the Greeks named these objects after their gods: swift Mercury, beautiful Venus, warlike Mars, powerful Jupiter, and bountiful Saturn.