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Some NASA scientists have joined together to propose a new definition for a planet that would let Pluto back into the fold.

It's been more than a decade since Pluto got demoted from planet to dwarf planet, thanks to a controversial decision by the International Astronomical Union (IAU).

As far as planetary geologist Kirby Runyon is concerned, letting astronomers who "mainly study stars, galaxies and black holes" define planets is ridiculous.

"It would be like a Portuguese speaker telling an Italian what the grammatical rules of Italian should be," he told CBC Radio's As It Happens guest host Helen Mann.

I consider Pluto a dwarf planet because I'm fine with adjectives modifying nouns. - Kirby Runyon, NASA's New Horizons

Runyon was one of the scientists behind NASA's New Horizons mission, which embarked on a flyby of Pluto in 2015.

Now, members of that team are hoping to bring the distant, icy object back to its former glory by proposing a new definition for a planet, which they will introduce at the Lunar and Planetary Science Conference in March.

A close-up image from NASA's New Horizons spacecraft shows a region near Pluto's equator where a range of mountains rises as high as 3,500 metres. (NASA/APL/SwRI via Getty Images)

Despite Runyon's opinions about astronomers defining planets, the IAU will make the final call about whether to adopt the new definition.

Dwarf planets are planets

The current definition of a planet, according to the IAU, is "a celestial body that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, and has cleared the neighbourhood around its orbit."

Pluto's atmosphere is seen in infrared wavelengths in this image from NASA's New Horizons spacecraft captured on July 14, 2015. (NASA via Reuters)

The proposed definition? "A sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."

Or, in other words, says Runyon, "Anything round in space that's smaller than a star is a planet."

'Anyone with an esthetic eye for nature would hopefully think Pluto's a beautiful place. - Kirby Runyon, planetary geologist

The idea, he says, is to define planets by their intrinsic properties, like their mass, rather than their extrinsic properties, like their orbit.

Pluto was booted from the list of planets in 2006 because its orbit is inclined relative to the rest of the solar system and crosses over the orbit of Neptune.

"I consider Pluto a dwarf planet because I'm fine with adjectives modifying nouns. Earth, for instance, is a terrestrial planet. Jupiter is a giant planet," he said.

"The problem is that the IAU then goes out of their way to say, 'Oh by the way, dwarf planets are not planets, which is grammatical nonsense."

Under the proposed definition, our solar system would go from having eight planets to 110, including Earth's moon and dwarf planets like Pluto.

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Runyon admits part of the reason Pluto has such vehement defenders is emotional rather than scientific.

"Casually glancing at the incredible images from the New Horizon spacecraft passage by Pluto, you see Pluto in all of its complex geologic glory — and it's not just for science nerds. Anyone with an esthetic eye for nature would hopefully think Pluto's a beautiful place," he said.

"People have this non-scientific emotional attachment to the planet Pluto and that's legitimate."