An Astronomer Proposes a New Mathematical Definition for What Counts As a Planet

Pluto still doesn’t count as a planet, but our moon does.

Nov 14 2015, 7:00pm

Image: NASA

It was difficult to not feel a twinge of existential angst when the International Astronomical Union (IAU) demoted Pluto from its planetary status to a mere Kuiper Belt object back in 2006. The IAU has been responsible for naming planets since the early 20th century and although it was this organization that had upheld Pluto's status as a planet since 1930, the first formal definition of a planet proposed and accepted by the IAU General Assembly in 2006 meant that Pluto would have to go – the IAU giveth, the IAU taketh away.

Yet if Pluto wasn't immune from losing its lofty rank after 76 years as a faithful member of planetary canon, what was to stop the solar system's tyrannical overlords at the IAU from demoting Neptune, Mercury or – dare I say it – Earth to lowly Kuiper Belt objects with the stroke of a pen?

As it turns out, not a whole lot.

The IAU's famously vague definition, where 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," has since come under fire from a number of astronomers who feel that such a loose definition hardly works for classification in our own solar system, to say nothing ofthe thousands of exoplanets that have been discovered since Pluto's demotion.

In an attempt to solve this definitional crisis, UCLA planetary scientist Jean-Luc Margot proposed a new method for defining planets at a meeting of the American Astronomical Society on Thursday, the full outline of which is forthcoming in the Astronomical Journal. Elegant in its simplicity, Margot's mathematical schemata allows for a quantification of the third element of the IAU's definition, which would allow astronomers to utilize the definition to classify all exoplanets.

According to Margot, the problem with the final clause of the IAU's definition is that a planet can never completely clear its orbital zone – either accumulating nearby objects or flinging them away – because gravity and radiative forces continually send comets and asteroids into planet-crossing orbits.

So, Margot devised a relatively simple mathematical formula which takes into account the mass of an exoplanet's star and the size of its orbit around the star in order to precisely quantify whether or not the object is capable of meeting the IAU's orbit clearing criterion. As it turns out, when an object reaches this critical mass to qualify it as a planet, it also tends to satisfy the IAU's second criterion.

"When a body has sufficient mass to clear its orbital neighborhood, it also has sufficient mass to overcome material strength and pull itself into a nearly round shape," said Margot in astatement.

According to Margot, his test is very easy to implement and is capable of classifying 99 percent of the over 5000 exoplanets that have been discovered to date. This is important because in many cases, astronomers can't actually get high resolution images of a planet to determine its shape, yet in almost all cases they have the data necessary to complete Margot's formula. Ultimately, Margot hopes this will allow for more robust inquiries into potentially habitable exoplanets.

"One should not need a teleportation device to decide whether a newly discovered object is a planet," he said.

There is still no word on whether the IAU will consider Margot's criteria for mathematically defining planets, however there is one interesting result of his formula that might make it a tough sell at the next IAU generally assembly in 2018. According to his schema, Earth's moon is above the critical mass necessary to qualify as a planet, technically making the Earth and moon binary planetary system. This might not be a fatal blow to his proposal however, given another definitional conundrum faced by the IAU.

"We should be careful here [because] the IAU has not defined the term 'satellite'. When they do, that will affect what they might decide about double planets versus satellites," Margot told New Scientist. "Of course it's just a proposal. I don't know whether it will stick, whether people will love it, hate it or be indifferent."