After nearly a decade of debate, astronomers have finally determined Pluto’s fate: it’s not a planet. The decision has generated controversy throughout the scientific community, and upset everyone from schoolkids to stargazers. So why, 76 years after Pluto’s discovery, has it lost its planetary status?

The move stems from the newly established definition of a planet, voted on by the International Astronomical Union (IAU), a society of astronomers from 75 countries. This new definition states “a planet is a celestial body that is in orbit around the sun, has sufficient mass for self-gravity to overcome rigid body forces so that it assumes a almost round shape, and has cleared the neighborhood around its orbit." Under this definition, Pluto is excluded. For one, it has what is known as a highly eccentric orbit, which at times overlaps Neptune’s path, and also veers away from the solar system’s main plane. The other eight planets, on the other hand, have large, clearly defined orbits. In addition, Pluto is one of a vast group of celestial bodies located in the far reaches of the solar system, and has not “cleared its orbit.” Because Pluto is not as large as it was believed to be when discovered, and because it is surrounded by so many other small bodies, many people believe it was a mistake to classify it as a planet.

Because of the IAU’s Aug. 24 decision, Pluto is now considered a “minor planet” or “dwarf planet,” and is known as minor planet 134340 Pluto. The IAU plans to create a catalogue of dwarf planets, which will include two others: the asteroid Ceres, and a distant object officially named 2003 UB313, but nicknamed “Xena.”

Discovered in 1930, Pluto was at first thought to be the answer to an astronomical mystery. Astronomers had noticed irregularities in the orbits of some of the outer planets, and theorized that a large, unidentified planet must be disrupting their movements. Astronomer Percival Lowell calculated where this mystery planet might be, and though he didn’t discover it, his work did lead to the discovery of Pluto by Clyde Tombaugh, 16 years after Lowell’s death. However, Pluto is smaller than initially thought--too small to have an effect on the large planets that triggered the search. Because of this, some scientists argued that Pluto was nothing more than a large asteroid.

Despite the IAU’s vote, dispute over Pluto’s status continues. The group had considered another proposal, one that preserved Pluto’s standing as a planet, while also classifying three other bodies as planets. There are also rumors of a petition to overturn the IAU’s decision, and members of California‘s state assembly introduced a resolution condemning the IAU‘s move. One thing is certain: we shouldn’t get too attached to any one definition of our solar system, because we never know when it might change.







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