Sorry Pluto, You Still Aren’t a Planet

A new test for planetary status leaves the diminutive world and its dwarf planet kin out of the family portrait

This composite image features Pluto and its largest moon Charon in enhanced color. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

When Pluto was voted out of the planetary family in 2006, textbooks were rewritten, solar system models were remade—and the public was outraged.

Plenty of astronomers also disputed the decision, and the icy world still generates heated debate. Now, one astronomer has come up with a simple test to determine planetary status based on quantifiable factors. While a recent NASA flyby has shown Pluto to be a dynamic, complicated place, the new test agrees with the 2006 ruling: Pluto is still not a planet.

There wasn’t really an official definition of a planet until scientists in 2005 discovered a rocky body that at the time appeared to be larger than Pluto, now called Eris, zipping around the Kuiper belt, a reservoir of icy bodies past the orbit of Neptune. Questions flew about Eris’ status, especially since a growing body of evidence suggested that granting it the coveted classification could mean dozens of potential future additions to the planetary lineup.

“There are over 100 objects like Pluto [in the solar system], so we’re not going to have the schoolchildren of the world memorize over 100 planets,” says Jay Pasachoff, director of Hopkins Observatory at Williams College.

In a controversial vote, the International Astronomical Union (IAU) established criteria for planets that kicked our beloved Pluto (and the newly discovered Eris) out of the family. Instead the union named them dwarf planets, along with Ceres, Makemake and Haumea. 

According to the IAU, a planet must orbit a star, be fairly round and must clear the orbital path around its star by ejecting or accumulating most debris. Though the first rule is easy to observe, the last two are harder to measure definitively.

“How round is round?” planetary scientist Jean-Luc Margot, of the University of California, Los Angeles, emphasized during a press conference yesterday at the American Astronomical Society's Division of Planetary Sciences meeting.

This ambiguity led Margot to devise a test to clearly separate planets from other orbital objects. His test requires estimating the star’s mass, the planet’s mass and its orbital period—values all easily measured from ground or space-based telescopes.

Plugging these numbers into his equation, Margot can determine whether an orbiting body is capable of clearing its path of debris without having to go looking for the clutter around other worlds. That's especially useful for objects in the far-off Kuiper belt, which are relatively small and dim.

He then put to the test the eight recognized planets of our solar system plus Eris, Pluto and its moon Charon, and he found a definitive split. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune fall in the “planet” category, while the others do not.

“Make no mistake about it, we have two very clear types of bodies in our solar system,” says Margot, whose paper on the subject was recently accepted in the Astronomical Journal.

This equation even works for the flood of exoplanets—worlds that orbit stars other than the sun—discovered in recent years. The test can rapidly classify 99 percent of the thousands of known exoplanets, says Margot. Bodies lacking good estimates for their mass or orbital period make up the remaining one percent. 

Despite the results, some astronomers remain staunchly pro-planet status for Pluto, which, for the record, currently holds the crown as the largest body in the Kuiper belt. Angela Zalucha, an atmospheric scientist at the SETI Institute, points to Alan Stern, leader of NASA's New Horizons mission to study Pluto. Stern regularly stands in team photos flashing the "Pluto salute": nine fingers to show the nine planets.

But perhaps Pluto’s dwarf planet status isn’t such a bad thing, says Pasachoff: “I call its new categorization a promotion for it, because it is the biggest and the best” of its kind.

Editor's Note 11/13/15: This story has been updated to reflect the latest data on sizes for Pluto and Eris.

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