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 system, out in a region of icy objects called the Kuiper Belt. Part of the reason scientists decided to change Pluto’s classification is because it looks and behaves like other icy objects that aren’t considered planets.

Until now, scientists haven’t really been able to get a good look at Pluto. But after a nine-year journey, NASA’s New Horizons spacecraft will pass Pluto this week.

The spacecraft has already taken a few pictures of the surface. We now know the surface is reddish-brown and has some strange dark spots, Jones said.

My friend Katie Cooper is a Washington State University professor. She’s an expert on the geology of Earth and objects out in space. She said New Horizons’ flyby of Pluto once again highlights the question of what makes a planet. It has also made your question a very popular one again.

“And this is a good question,” Cooper said. “It may seem like scientists arguing over words, but good classifications help us build good models for understanding our solar system and how it was formed.”

She told me the International Astronomical Union has three rules for planets. First, it needs to orbit around the Sun.

“Pluto has got this down,” Cooper said.

Second, it needs to be massive enough for its gravity to pull it into a spherical shape. This is the rule sparking a new debate as we’ve seen new pictures of Pluto, Cooper explained.

“We’ve known it’s round for ages,” Cooper added, “but the new images just make it seem so planet-like.”

Even if it orbits the sun and it is round, planets need to follow one last rule. They have to “clear the neighborhood.” They must be big enough to knock other bodies out of their orbit. This is where little Pluto fails. It is hanging out in Neptune’s chilly orbit. Some other icy objects from the Kuiper Belt are in the orbit, too.

“Some scientists like to point out that other established planets also don’t meet all three criteria, including the Earth, because these planets share their orbits with asteroids,” Cooper explained.

Still, Cooper said because Pluto doesn’t meet this last rule, it means the astronomical union probably won’t put Pluto back into the “good graces of planet-hood” anytime soon.

Dr. Universe

WSU, Pullman

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