Pluto could return to planet status, and another planet may be added this year

Ceres is the largest object in the asteroid belt, and NASA’s Dawn spacecraft will arrive there on March 6.

Pluto is the largest object in the Kuiper belt, and NASA’s New Horizons spacecraft will arrive there on July 15.

These two events will make 2015 an exciting year for solar system exploration and discovery. But there is much more to this story than mere science. I expect 2015 will be the year when general consensus, built upon our new knowledge of these two objects, will return Pluto and add Ceres to our family of solar system planets.

The efforts of a very small clique of Pluto-haters within the International Astronomical Union (IAU) plutoed Pluto in 2006. Of the approximately 10,000 internationally registered members of the IAU in 2006, only 237 voted in favor of the resolution redefining Pluto as a “dwarf planet” while 157 voted against; the other 9,500 members were not present at the closing session of the IAU General Assembly in Prague at which the vote to demote Pluto was taken. Yet Pluto’s official planetary status was snatched away.

Ceres and Pluto are both spheroidal objects, like Mercury, Earth, Jupiter and Saturn. That’s part of the agreed upon definition of a planet. They both orbit a star, the Sun, like Venus, Mars, Uranus and Neptune. That’s also part of the widely accepted definition of a planet.


Planet Problems

Unlike the larger planets, however, Ceres, like Pluto, according to the IAU definition, “has not cleared the neighborhood around its orbit.” The asteroid belt is, apparently, Ceres’ neighborhood while the Kuiper Belt is Pluto’s neighborhood – though no definition of a planet’s neighborhood exists, and no agreed upon understanding of what “clearing the neighborhood” yet exists. Furthermore, no broad-based agreement exists as to why “clearing the neighborhood” need be a requirement in order for an object to be considered a planet.

Some planetary astronomers would argue that were the Earth placed in the Kuiper Belt, it would not be able to clear its neighborhood and thus would not be considered, by the IAU definition, a planet; apparently location matters. Here a planet, there not a planet. I’d argue that location shouldn’t matter; instead, the intrinsic properties of the objects themselves should matter more. And so we are led back to Ceres and Pluto.