Sorry Pluto, You Still Aren’t a Planet
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.