Perplexing Pluto

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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:

  1. It must orbit the Sun
  2. It must be round (so big enough for gravity to squash it into a round ball)
  3. 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.

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