A planet is a large, round object that orbits a star. In our solar system, eight planets circle the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Each planet travels in its own path called an orbit, taking anywhere from 88 days (Mercury) to 165 years (Neptune) to complete one trip around the Sun.

What makes something a planet? Scientists say it must be big enough that gravity pulls it into a sphere, it must orbit a star, and it must be the dominant object in its neighborhood of space. That last rule is why Pluto lost its planet status in 2006: it shares its orbital zone with many other icy objects.

The word comes from ancient Greek meaning “wanderer,” because planets seemed to wander across the sky while stars stayed in fixed patterns. Ancient astronomers noticed these bright points of light moving against the background of stars night after night.

Beyond our solar system, astronomers have discovered thousands of exoplanets orbiting distant stars. Some might have conditions similar to Earth's. When someone talks about saving the planet, they mean protecting Earth's environment and ecosystems, since it's the only planet we know that supports life.