To some people, Pluto's demotion from planet to dwarf planet status in 2006 was one of the most devastating astronomical losses since the death of Spock. But a new study suggests Pluto should (also like Spock) be resurrected as a planet, since the definition the object failed to meet was not based on precedents set in scientific literature.

Pluto enjoyed 75 years in the planet club, but its membership was first threatened in 2005 when astronomers discovered a distant object now known as Eris. Although it was believed to be bigger than Pluto, scientists were reluctant to label it a "planet," worried that doing so would open the floodgates for hundreds of trans-Neptunian objects (TNOs) to suddenly swamp the exclusive clubhouse of planets.

So the International Astronomical Union (IAU) voted on a new definition in 2006. According to the official statement, a planet is "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 neighborhood around its orbit."

It was the third point that forced Pluto to hand in its gun and badge. It orbits in the Kuiper Belt, a messy neighborhood chock-full of rocks and debris. As a result Pluto, along with fellow TNOs like Eris, Haumea, Makemake and Sedna, were relegated to a new class of object called "dwarf planet," which essentially meet the first two criteria for a planet but not the third.

Pushback

Not everyone was onboard with the change though. Planetary scientists Philip Metzger and Alan Stern – key figures on the New Horizons mission, which gave us our closest look at Pluto in 2015 – have long argued that the definition doesn't fit.

"It's a sloppy definition," says Metzger, lead author of the new study. "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."

Not only that, the researchers argue the IAU definition is based on a concept that nobody uses in the scientific community. They conducted a literature review of work from the past 200 years and found only one publication that classifies planets based on them clearing their orbits. And this paper, published in 1802, was based on ideas that have long been disproven.

"The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research," says Metzger. "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."

The researchers say that the IAU justified its decision by saying that an object clearing its orbit was a standard used to distinguish planets from asteroids. But the lit review found that this so-called standard wasn't in use anymore – asteroids and planets are distinguished by their geophysical characteristics.

New definition

So how should a planet be defined? According to the authors of the new study, the answer is simply (b) – the object is large enough that its gravity pulls it into a ball shape.

"And that's not just an arbitrary definition," says Metzger. "It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body."

Interestingly, Metzger and co imply that their new, broader definition of "planet" might include moons as well. Saturn's moon Titan and Jupiter's Europa, for example, are spherical and seem to be geologically active. To keep moons and planets separate, the IAC's definition also includes the caveat that a planet or dwarf planet is not orbiting a body other than a star, but whether the new proposal includes such a distinction remains to be seen.

We're sure we haven't heard the last of this argument, which has been raging for more than 12 years now.

The paper was published in the journal Icarus.

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