When the International Astronomical Union came up with an official definition of a planet in 2006, they booted Pluto out of the club and reclassified it as a dwarf planet. But some say the discovery of exoplanets requires that we revisit this definition and give Pluto a second chance.

The current, official definition says that a planet is a celestial body that:

  1. is in orbit around the Sun
  2. is round or nearly round, and
  3. has "cleared the neighborhood" around its orbit.

But this definition only applied to planets in our solar system. What about all those exoplanets orbiting other stars? Are they planets?


A few days ago, the Harvard-Smithsonian Center for Astrophysics decided to once again tackle the question of "what is a planet?" It hosted a debate among three leading experts in planetary science, each of whom presented their case as to what a planet is or isn't. The goal: to find a definition that the eager public audience could agree on.

As Astrobiology Magazine reports:


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.

Gingerich argued that "a planet is a culturally defined word that changes over time," and that Pluto is a planet. Williams defended the IAU definition, which declares that Pluto is not a planet. And Sasselov defined a planet as "the smallest spherical lump of matter that formed around stars or stellar remnants," which means Pluto is a planet.

After these experts made their best case, the audience got to vote on what a planet is or isn't and whether Pluto is in or out. The results are in, with no hanging chads in sight.

According to the audience, Sasselov's definition won the day, and Pluto IS a planet.

The video of the debate is now available online: