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Cover Story

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Why Pluto Isn't a Planet

As the director of the Hayden Planetarium, in New York City, Neil deGrasse Tyson is paid to gaze at the sky. He studies how stars form and explode. He examines distant galaxies and our own galaxy, the Milky Way. Tyson has written a new book for adults called The Pluto Files. In it, he explains "the rise and fall" of Pluto's status as a planet. It was reclassified as a "dwarf planet" in 2006. Tyson spoke to TIME's Gilbert Cruz.

What's so special about Pluto?
Disney's dog Pluto was sketched the same year the object was discovered. Pluto was discovered by an American. Americans fell in love with a planet that really is just a tiny ice ball.

When did we figure out that Pluto is actually pretty small?
People started getting suspicious in the 1970s. There are seven moons in the solar system bigger than Pluto.

What exactly is a planet?
We abandoned the word planet. We organize by what objects look like. The terrestrials Mercury, Venus, Earth and Marsoare small, all rocky, all dense. Jupiter, Saturn, Uranus and Neptune are all big, gaseous.

You played a part in getting Pluto demoted. How did that happen?
We rebuilt the planetarium. New science needed to go in. I called in a panel of experts. It was clear that Pluto's day was over.

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