The 180

Pluto is not a planet!

A decade after it was kicked off the list of planets, a movement is afoot among some NASA scientists to win planetary status back for Pluto. But astronomy professor Jaymie Matthews warns that if Pluto gets its planetary status back, we are heading down a dangerous path.
New Horizons’ views of the informally named Sputnik Planum on Pluto (top) and the informally named Vulcan Planum on Pluto's moon, Charon (bottom). (NASA/JHUAPL/SwRI)

Pluto is beautiful, interesting and worthy of more exploration, but it's not a planet.

That's the opinion of UBC astrophysics professor, Jaymie Matthews.

And he warns that if the movement to make Pluto a planet again succeeds, it takes us down a dangerous path.

"The story of Pluto is the story of discovery," he says.

In Matthews's view, what might seem like a debate between scientists around the classification of Pluto is, in fact, a test of how scientists and the broader public adapt to an ever-increasing body of knowledge.

This is just a side-effect of our expanding knowledge and in some sense our recognition of additional areas of ignorance. The more we learn, the more questions we have to pose- Jaymie Matthews

Discovered in 1930, Matthews says Pluto was thought to be the size and mass of the Earth and so was designated a planet.

But by the 1970s, Matthews says, astronomers were "shocked" to find its mass was 0.2% of the Earth's mass. In the decades following, as larger telescopes and more sensitive detectors became available, the ability to search the solar system outside the orbit of Neptune improved.

In 1992, the second TransNeptunian Object after Pluto was found, and in the next 7 years, Matthews says 200 TNS were discovered. Today, he adds, 100,000 objects are thought to exist.

Jaymie Matthews, UBC astronomy professor, argues that while Pluto is a world worthy of exploration, it is not a planet. ( Jaymie Matthews/UBC/CSA)

"This presented for the first time, how do we define a planet?" he says, adding: "People are upset that it has been reclassified, but the reason it's been reclassified is because we know a lot more now about our solar system than we did when it was first found, and even just a few decades ago."

To Matthews, the purpose of science is to help people understand the world around them, and he says if the language is poorly defined, then the ideas are poorly defined.

He agrees the definition of a planet probably does need revision, but Matthews doesn't think classifying Pluto and dozens of other objects as planets satisfies either the needs of astrophysics or planetary geology.