Astrophysicist Chronicles The Battle Over Pluto

Published February 2, 2009 at 1:39 PM EST

When Pluto was officially demoted from planet to "dwarf planet" status in 2006, astrophysicist Neil deGrasse Tyson caught a lot of flak.

The director of the Hayden Planetarium at the American Museum of Natural History in New York was widely blamed for what many saw as shabby treatment of America's beloved planet.

"I had to think long and hard about why [Pluto had] such a grip on the American body and soul," Tyson tells NPR's Melissa Block. "And after sifting through all kinds of possible arguments, I landed on one very simple one: There's a dog that shares the name. I'm blaming Disney completely — Mickey's dog, Pluto."

Tyson chronicles America's love affair with Pluto and how its status has evolved in a new book, The Pluto Files: The Rise and Fall of America's Favorite Planet.

Tyson started noticing that Pluto was different from the other planets in the 1990s. At the time, other ice bodies were discovered in the outer solar system that looked similar to Pluto. Those bodies were also like Pluto in another way: They crossed orbits. Tyson says Pluto was the only planet whose orbit crossed the orbit of another planet.

"It's kind of misbehaving, if you think of it in those terms," Tyson says.

When Tyson put together an exhibit showing the relative size of planets at the Hayden Planetarium in 2000, he decided not to include Pluto.

"We wanted to be sure that the exhibits had high shelf life," he says, adding that the exhibit was a $250,000 investment. "So we had the need to look carefully at what was going on in the solar system. That's what led us to organize the solar system as like properties rather than as an enumeration of cosmic objects, as it's so commonly taught in school."

Other astrophysicists said Tyson was off base. Museum visitors noticed Pluto was missing and sent in vitriolic letters. And when it was reported in The New York Times in January 2001 that the exhibit did not include Pluto, Tyson says he received a "new wave of hate mail."

Although critics viewed the omission as a demotion for Pluto, Tyson says, he viewed it as "a new swath of real estate in the outer solar system called the Kuiper belt, and comets. Maybe Pluto was, in fact, the first discovered object of the Kuiper belt rather than the ninth planet."

Finally, in 2006, the International Astronomical Union ruled that Pluto is a dwarf planet.

"In the end, we were kind of vindicated," he says.

Tyson says there is no stigma attached to being labeled a "dwarf planet." In fact, Tyson says he thinks Pluto is a comet because it's mostly ice by volume.

"If you slid Pluto to where Earth is right now, heat from the sun would evaporate that ice, and it would grow a tail," Tyson says. "Now, that's no kind of behavior for a planet. What is that about? It's one of the odd things about Pluto that no other planet shares.

"I think Pluto is happier" in its current classification, Tyson says. "We didn't lose a planet, we gained the Kuiper belt. And now Pluto is one of the kings of the Kuiper belt. Pluto is just fine."