Why Is Pluto Not A Planet? The Answer For Kids

The Long Answer

Clyde Tombaugh discovered Pluto in 1930 at Lowell Observatory in Arizona, and for 76 years it was the ninth planet. Then astronomers started finding other round, icy worlds out past Neptune. In 2005 they found Eris, about the same size as Pluto and a little heavier. Either the solar system had a lot more planets than anyone had noticed, or the word planet needed a definition.

In 2006 the International Astronomical Union wrote one. A planet has to pass three tests:

1. It goes around the Sun.
2. It is heavy enough that its own gravity has pulled it into a ball.
3. It has cleared the neighborhood around its orbit, so it is the biggest thing on its path by far.

Pluto passes the first two and fails the third: it travels with millions of other icy bodies in the Kuiper Belt. So it became a dwarf planet, one of five the IAU recognizes: Ceres, Pluto, Haumea, Makemake and Eris.

Pluto is small, about 2,377 kilometers across, smaller than our own Moon. NASA's New Horizons spacecraft flew past it on 14 July 2015 and found mountains of ice and a heart-shaped plain of frozen nitrogen.