You may have heard that Pluto is no longer a planet. Here's the skinny on what was once known as our Sun's smallest planet.
In 2006, astronomers reclassified Pluto from an official planet to a "dwarf planet," but it is still considered to be one of the largest bodies in the Kuiper Belt, a band of icy objects at the edge of our solar system.
What defines a planet?
Rules have now been created:
- It must orbit the Sun
- It must be round (so big enough for gravity to squash it into a round ball)
- It must have cleared other objects from its orbit.
Pluto did not meet the third rule. It orbits among other floating icy bodies in the Kuiper Belt.
A "dwarf planet" has a definition too. It must meet the criteria of a planet as well as a fourth rule: it must not be a satellite.
No spacecraft have ever visited Pluto. We may know more in 2015, when a NASA probe named New Horizons is scheduled to reach Pluto. It launched on January 19, 2006.