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Astronomy 101: Planets

In this episode, explore the variety of worlds both within our stellar neighborhood and throughout the galaxy and learn about the controversy around the definition of a planet.
RELATED TOPICS: SOLAR SYSTEM
Solar-system-planets
The planets of our solar system
NASA/JPL

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.

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