Why isn't Pluto a planet anymore?

When I was a kid, Pluto was a planet, the smallest planet in our solar system.  But in 2006, the International Astronomical Union (IAU) determined that Pluto is not really a planet, but a dwarf planet.  A dwarf planet is a celestial body that 1) is in orbit around the sun, 2) keeps a nearly round shape, 3) hasn’t cleared the neighborhood around its orbit and 4) is not a satellite.  Pluto was determined to be a dwarf planet because it “hasn’t cleared the neighborhood” around its orbit meaning that there are other celestial objects in its orbital path around the sun. 

Pluto is actually part of the Kuiper Belt.  The Kuiper Belt is in the solar system extending beyond Neptune and it is made up of frozen objects made up of methane, ammonia and water.  Pluto is the largest dwarf planet in the Kuiper Belt.  The two other dwarf planets in the Kuiper Belt are Haumea and Makemake.

NASA has sent a spacecraft to visit Pluto and the Kuiper Belt called New Horizons.  New Horizons left Earth in January 2006 and is not expected to reach Pluto until 2015.

Click here to see NASA’s homepage for Pluto.