Ask Dr. Universe

Asked by Heidi, Cincinnati, OH

Dear Heidi

It’s a big week for Pluto as NASA’s New Horizons spacecraft gets a close-up look at the distant, icy world. But first, the answer to your question: Pluto is not a planet.

Scientists called it a planet when it was discovered in 1930. They needed a name for it and an 11-year-old girl living in London at the time came up with “Pluto.” Things changed in 2006.

“Pluto is now classified as minor planet 134340,” said my friend Jessica Jones at the Washington State University Planetarium. “It was a sad day for Pluto-lovers.”

Pluto lies on the edge of our solar...