The debate over Pluto’s classification as a planet has been fought by astronomers and the public ever since the International Astronomical Union, a global assembly of astronomy experts, voted to establish a definition of a planet that excluded Pluto in 2006.

Pluto was deemed not to be a planet because it had not “cleared the neighborhood,” around its orbit, according to the IAU definition.

Michael Allen, senior instructor of physics and astronomy at WSU, said clearing an orbit is about being the most dominant mass within the orbit.

For example, Allen said the Earth makes up to 99 percent of all the mass within its orbital width. Pluto is the largest object within its orbit, but it makes up less than half of the mass of the objects in the same orbit.

What exactly the IAU means is a little fuzzy, said Guy Worthey, associate professor of astronomy at WSU.

By IAU’s definition this makes Pluto a dwarf-planet, a classification of celestial objects separated from planets solely because they have not cleared the neighborhood around their respective orbits.

Pluto and a few other dwarf planets are located within the Kuiper belt, a large region of frozen gases extending around Neptune’s orbit to the outer edges of the solar system.

“They should have come up with quantitative evidence for how planets are classified,” Worthey said. “It could be simpler.”

Worthey suggested comparing a planet’s distance from its sun to determine whether it should be a planet or not. It is worth noting Pluto would likely not be considered a planet by this method either.

Worthey said it is not necessarily significant to planetary science whether Pluto is classified as planet or not.

“You have to understand there is no overarching theory in planetary science like there is in other science disciplines,” Allen said. “We can only appeal to data and observation.”

Allen said the debate surrounding Pluto’s classification is really about processes of gaining knowledge and the way scientists come to understand the universe.

Worthey said the debate is a “people thing,” and not a “science thing.”

When Clyde Tombaugh discovered Pluto in 1930, it was immediately called a planet, Worthey said. Its status as a planet wasn’t publicly questioned until after he died in 1997.

A lot of what is known about Pluto has changed since its discovery.

“When I was a kid, Pluto was thought to be twice the size of the Earth based on its brightness,” Worthey said. “Turns out, they guessed wrong.”

It wasn’t until Pluto’s moon Charon was discovered in 1978 that scientists were able to measure Pluto’s mass, he said. This is when astronomers discovered Pluto’s mass was one two-hundredth of Earth’s mass, a big difference from what was originally believed.

Worthey said what is important about Pluto is not its classification. He said Pluto’s glaciers are made of nitrogen, with complex geological processes that create nitrogen rivers and unique geological features seen from advanced telescopic imaging.

“Let’s talk about what it is,” he said. “It’s a really cool place.”