Here's how the IAU defines a planet: "A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit."
Pluto ticks the first two boxes, but fails on the third because it lives in the Kuiper Belt, an area of the solar system packed with icy bodies. But Metzger argues that third requirement isn't valid.
Metzger says there's no support in the research literature for requiring a planet to clear its orbit. He looked back at over 200 years of publications and says he found only one -- from 1802 -- that used that requirement to classify a planet, and that the sole publication was based on now-disproven reasoning.
Metzger refers to Pluto as "the second-most complex, interesting planet in our solar system" and calls the IAU definition "sloppy." He suggests instead that planets should be classified based on being large enough that their gravity allows them to be spherical.
Metzger points to Pluto's moons and its complex geology and atmosphere, saying "It's more dynamic and alive than Mars."
This isn't the first time scientists have questioned the IAU's definition of a planet.
New Horizons team, which guided the spacecraft in a close-up study of Pluto, proposed a radical new definition for planets in 2017. That proposal would return Pluto to planet status, but also qualify some moons and other solar system objects as planets.
IAU press officer Lars Lindberg Christensen tells CNET there have been no resolutions proposed to revisit Pluto's classification. "It is nevertheless good and healthy to debate such topics," he says.
The Metzger-led study, which focuses on how asteroids came to be classified differently than planets in scientific literature, was published online this week in the journal Icarus.