Pity poor Pluto. Introduced as a new member of the planetary pantheon upon its discovery in 1930, it suffered a harsh dismissal in 2006 when the International Astronomical Union (IAU) downgraded the little guy to “dwarf planet” status.

This hasn’t gone over very well. Millions of Pluto proponents — from schoolchildren to amateur stargazers and astronomers — have refused to let the planet go gentle into that good night, saying that the demotion was arbitrary.

As the Harvard-Smithsonian Center for Astrophysics put it: “Is a dwarf planet a small planet? Not according to the IAU. Even though a dwarf fruit tree is still a small fruit tree, and a dwarf hamster is still a small hamster.”

This particular celestial conundrum was borne during the 2006 IAU meeting when it was determined that given the discovery of a whole slew of other planet-sized objects orbiting about, a new definition of a “planet” was required. Not an easy task, but in the end a vote was held and the criteria were set. From thereon, a planet was a celestial body that:

  • Is in orbit around the sun.
  • Is round or nearly round.
  • Has "cleared the neighborhood" around its orbit (be gravitationally dominant in its area).
And it was then that Pluto was kicked out of the elite club for its lack of being able to “clear the neighborhood.” Its small size, which has made it so endearing to many, was its undoing.

Fast forward eight years and the Harvard-Smithsonian Center has just not been able to let it go. Last month they hosted a debate to discuss the finer points of planetary definition. They invited three leading experts in planetary science, who each presented their argument as to what makes a planet. The aim was to find a definition that the public could agree on.

Science historian Dr. Owen Gingerich, who chaired the IAU planet definition committee, presented the historical viewpoint. Dr. Gareth Williams, associate director of the Minor Planet Center, presented the IAU's viewpoint. And Dr. Dimitar Sasselov, director of the Harvard Origins of Life Initiative, presented the exoplanet scientist's viewpoint, according to a news release for the event.

Gingerich’s case: "A planet is a culturally defined word that changes over time," and that Pluto is a planet. (Score one for Pluto.)

Williams stuck with the IAU party line: Pluto is not a planet. (Bah humbug.)

Sasselov defined a planet as: "The smallest spherical lump of matter that formed around stars or stellar remnants," which means Pluto is a planet. (Go, Pluto!)

The audience voted and voila: Pluto, they determined, is indeed a planet.

Of course the planet’s fate is ultimately in the hands of the IAU, but the public’s vote of confidence is heartening. Maybe with enough support, the IAU will consider granting a grandfather clause to Pluto, because it sure doesn’t look like Pluto’s defenders are going away anytime soon. Long live Pluto!

You can watch the debate and audience vote in the video below:

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