Astronomers Propose New Criteria to Classify Planets, but Pluto Still Doesn’t Make the Cut

Nearly two decades after Pluto got kicked out of the planet club, astronomers are proposing an updated way to define “planet” based on more measurable criteria. The current definition is “problematic” and “vague,” they write in a paper published Wednesday in The Planetary Science Journal.

Unfortunately for fans of the dwarf planet, however, Pluto would remain excluded, even if the proposal is approved.

In 2006, the International Astronomical Union updated its definition for a planet, ruling that for a celestial body to officially make the cut, it must orbit around the sun, contain enough mass to become “nearly round” due to its own force of gravity, and clear away any debris (excluding moons or other satellites) in its orbital path. The last point is what ultimately declassified Pluto, since it is not large enough to influence other objects in the neighborhood around its orbit.

Now, the three planetary scientists involved in the new proposal have raised several issues with that definition. For starters, the fact that a planet must orbit the sun—as in our sun—narrows down the number of “planets” to exactly the eight in our own solar system, excluding the rest of the universe. These criteria also don’t involve any numbers.

“The problem in the past was, you had a word ‘planet’ but you didn’t have a quantitative definition of it,” Brett Gladman, an astronomer at the University of British Columbia and one of the authors of the paper, tells Live Science’s Kristel Tjandra.

In other words, how round is “nearly round”? And exactly how clear should the celestial body’s orbit be of debris?

“Jupiter’s orbit is crossed by comets and asteroids, as is Earth’s,” Gladman points out in a university statement. “Have those planets not cleared their orbit and thus, aren’t actually planets?”

In a bid to correct for this ambiguity, Gladman and his two colleagues propose a more measurable definition. According to their model, a celestial body is a planet if it: orbits one or more stars, brown dwarfs or stellar remnants; is more massive than 10²³ kilograms (a size big enough to clear its orbit of debris); and is less massive than 2.5 x 10²⁸ kilograms (equivalent to 13 Jupiter masses).

Pluto’s mass is 1.31 x 10²² kilograms, so it would remain excluded—but our current eight planets would retain their classification.

To confirm the legitimacy of this new definition, the team took a mathematical algorithm that groups together similar objects and applied it to the bodies in our solar system. The algorithm grouped our eight planets in one cluster and the dwarf planets in another, confirming the existence of shared qualities among them and validating the logic of their new categorization criteria.

The proposed definition has another benefit—it removes the need to measure an object’s roundness. Especially when it comes to distant exoplanets, it is difficult to make an exact observance of just how spherical a celestial body is. But, at least in our solar system, objects that fall within the researchers’ proposed range of masses tend to be round. These new guidelines, then, would make it easier to consider far-off worlds to be planets.

In the words of Gizmodo’s Isaac Schultz, “the move would take us away from an isolationist classification of our own existence; instead of planets just being our cosmic neighbors, planets could refer to any one of the countless worlds in our universe.”

Ultimately, the astronomers hope to inspire more discussions on what exactly makes a planet a planet, even though any formal change to the definition is probably a few years away. Not many astronomical objects have a very clear definition. But harnessing the meaning of “planet” gets at a human curiosity.

“There is no definition of what a star is, what a galaxy is, what a nebula is,” says Mike Brown, an astronomer at the California Institute of Technology who was not involved with the new paper, to New Scientist’s Jonathan O’Callaghan. “Planets could have been the same way, except that the public want to know what the planets in the solar system are.”

Margherita Bassi | READ MORE

Margherita Bassi is a freelance journalist and trilingual storyteller. Her work has appeared in publications including BBC Travel, Discover magazine, Live Science, Atlas Obscura and Hidden Compass.