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. NASA’s 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.