Neil deGrasse Tyson Rejects Pluto Planethood Proposal on 'Colbert'

Scientists from the New Horizons mission to Pluto are making their case that the dwarf planet should be considered a planet again — and science popularizer and "Cosmos" host Neil deGrasse Tyson came out vehemently against them on late-night television.

While speaking on "The Late Show with Stephen Colbert" Wednesday (March 15) Tyson pointed out that Pluto's orbit crosses Neptune's from time to time.

"That's no kind of behavior for a planet. No!" deGrasse Tyson exclaimed. ['Land' on Pluto's Icy Plains in This Amazing New NASA Video]

"You've got to stay in your lane," Colbert responded.

"Stay in your lane!" deGrasse Tyson shouted, repeating, "Stay in your lane!"

You can watch the clip on YouTube.

In 2006, the International Astronomical Union (IAU) agreed that to be considered a planet, an object would have to orbit the sun, be round and be large enough to clear the local neighborhood of debris — a definition that excluded Pluto. This definition came under renewed scrutiny after New Horizons' 2015 flyby of Pluto revealed a world with vast mountains and other surprising terrain for such a small world.

The IAU definition will be contested in a poster session at the Lunar and Planetary Science Conference in The Woodlands, Texas, on March 21. Johns Hopkins University Ph.D. student Kirby Runyon, who worked on the New Horizons mission, will lead the session. His proposal, which he drafted in collaboration with several other members of the New Horizons team, offers a new definition of planethood.

"In a short paragraph, they define a planet as 'a sub-stellar mass body that has never undergone nuclear fusion' and that has enough gravitational heft to maintain a roughly round shape, even if it bulges at the equator because of a three-way squeeze of forces created by its gravity and the influence of both the sun and a nearby larger planet," Johns Hopkins representatives said in a statement.

Unlike the IAU definition, the new proposal makes no mention of the planet's surroundings being cleared. Previously, New Horizons principal investigator Alan Stern said the IAU's definition of "planet," if applied strictly, wouldn't even apply to Earth, Mars, Jupiter and Neptune, which share their orbits with asteroids.

Under Runyon's definition, there would be roughly 110 planets in our solar system instead of the current eight. In the statement, he said that would be an excellent way to engage the public in exploration, as objects defined as planets would naturally carry more weight and draw additional interest.

In the early 2000s, Tyson, who directs the Hayden Planetarium at the American Museum of Natural History in New York City, endorsed a new exhibit describing Pluto as an object more akin to icy bodies in the outer solar system, together known as Kuiper Belt objects. 

However, Pluto's official status change only came after a flurry of discoveries of similarly sized bodies, most notably by California Institute of Technology astronomer Mike Brown. 

"People like Mike Brown, who discovered the objects in the outer solar system that forced that vote — he's guilty. His Twitter handle is called @plutokiller. He admits it!" deGrasse Tyson said to Colbert.

"Who's this Mike Brown you're throwing under the bus right now?" Colbert asked.

"He put himself under the bus," deGrasse Tyson quipped. "I'm pointing it out."

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