Is Pluto a Planet After All? A New Argument Emerges

It’s been a tumultuous few years for Pluto. The dwarf planet, first discovered in 1930 by astronomer Clyde Tombaugh, was stripped of its more esteemed planet status in 2006 by the International Astronomical Union (IAU) because its orbit overlapped that of Neptune. A new set of IAU criteria mandated that a planet must orbit the sun, be spherical as a result of gravity, and "clear" the "neighborhood" around its orbit, asserting itself as the dominant presence. Pluto met the first and second edicts but not the third, relegating it to the lesser dwarf planet designation.

That declaration led to an ongoing debate over whether Pluto really earned its demotion. The newest and potentially most compelling argument comes courtesy of a paper from researchers at the University of Central Florida's Space Institute and published in the planetary science journal Icarus. In it, first author Philip Metzger asserts that no one since 1802 has used the cleared-space argument to define a planet. Referring to the IAU's definition as "sloppy," Metzger and his co-authors point out that no one else has separated asteroids from planets by using the clearing mandate. Planets, the paper argues, should not be held to dynamic descriptions of bodies that may change over time.

"We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it's functionally useful," Metzger said in a statement. "It's a sloppy definition. They didn't say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit."

Metzger is advocating instead for a planet obtaining its status due to being large enough to achieve a gravity-influenced spherical shape that activates geological changes.

Speaking with CNN, IAU spokesperson Lars Lindberg Christensen indicated a motion could be put forward to have the group reevaluate the classification but that no one had yet done so.

Whatever Pluto is or may one day become, it was a planet to Tombaugh, who wasn't around long enough to experience the reclassification. He died in 1997. In 2015, his ashes, attached to the New Horizons space probe, entered Pluto's orbit after nine years of travel.

[h/t Science Alert]