What is a Planet?

Thousands of years before the invention of the Internet or Super Mario Kart, people entertained themselves by peering into the night sky and connecting the stars to create pictures—called constellations—of animals, objects, and legendary heroes. But these ancient stargazers noticed that some stars didn’t cooperate with the constellations. These oddball balls of light followed their own paths across the night sky, so the ancient Greeks called them “planetes,” meaning wanderers. By 1930, nine planets had been discovered: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

But what exactly is a planet? It seems like an easy question. After all, you’re standing on one right now! But the scientific definition was fairly loose until recently. The discovery of other solar-system “wanderers” rivaling Pluto in size suddenly had scientists asking what wasn’t a planet. They put their heads together in 2006 and came up with three conditions for planethood: A planet must orbit the sun, be large enough so that its own gravity molds it into a spherical shape, and it must have an orbit free of other small objects. Unfortunately for Pluto, our one-time “ninth planet” failed to meet the third condition. It was downgraded to a “dwarf planet,” a classification of solar-system objects you can read about here.

Far-Out Facts

• Our solar system’s eight planets come in two flavors: smaller rocky planets with solid ground (Mercury, Venus, Earth and Mars) and larger gas giants (Jupiter, Saturn, Uranus, and Neptune).
• You can spot six of the solar system’s planets with the naked eye from Earth—and all eight if you have binoculars or a telescope.
• Astronomers have discovered hundreds of worlds beyond our solar system and are spotting more every day. Scientists believe some of these “exoplanets” may support alien life. Who knows. Maybe an alien kid elsewhere in the galaxy is wondering if you exist.

Visit the NASA Kids' Club to play games, read about the current space station crew, see cool pictures of NASA missions, and more for students in grades K-4.



STEMonstrations

Watch as astronauts on the space station bring STEM concepts to life.

This school year, take advantage of all the student challenges and opportunities NASA has to offer. Find the challenge that's right for you, gather your team, and become part of the mission!

ACTIVITIES

Explore Glow-in-the-Dark Water!

Have you ever been fascinated by things that glow in the dark? It can be a lot of fun to play with bracelets, wands, and other toys that are glow-in-the-dark, like some stickers and creepy, crawly, plastic insects! Have you ever wanted to make something at home that glows? It turns out that it is not that hard to do — all you need is tonic water and a black light! Some common household chemicals can also affect this beverage's glow. In this science activity, you will make tonic water glow… Read more

Measure Up with a Homemade Thermometer

Are you eager to understand how everyday items work, or interested in making useful objects and instruments yourself? Have you ever imagined you could build your own thermometer? In this activity, you will make a liquid thermometer to track how temperatures vary with location, indoors or outdoors. What will turn out to be the hottest spot in your home? What about the coolest? Your very own homemade thermometer will be able to tell you! Read more