The original definition of planet is wanderer, from the Greeks who watched these bright lights wander through the firmament of fixed stars. Observers discerned nine of these travelers over the course of human history, the last being Pluto in 1930. But recent discoveries of more objects orbiting the sun, both bigger than Pluto and similarly rounded in shape, called into question the arbitrary limit of nine, with some proposing that Pluto did not merit its planetary status. Now the International Astronomical Union (IAU) has crafted a new definition for what constitutes a planet that would expand the solar system to Pluto and beyond, encompassing 12 bodies in all.
Earlier this year, a special team convened by the IAU struggled to establish the criteria that defines a planet. Various proposals included size (mass) and orbit. "On the second morning several members admitted that they had not slept well, worrying that we would not be able to reach a consensus," writes Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics and chair of the Planet Definition Committee. "But by the end of a long day, the miracle had happened: we had reached a unanimous agreement."
The new proposed definition of a planet is: a celestial body with sufficient mass to assume a nearly spherical shape that orbits a star without being another star or a satellite of another planet. By this definition, the list of planets in order from the sun now reads: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto-Charon (considered a double-planet system) and the newly discovered and officially unnamed 2003 UB313, otherwise known as Xena. The committee also proposed a new category of planets, called plutons, be applied to those bodies that, like Pluto, both take longer than 200 Earth years to revolve around the sun and have eccentric orbits outside the typical orbital plane.
The solar system thus gains its first double planet, the Pluto-Charon pairing, as well as several so-called "dwarf planets," such as Ceres, which, while only 952 kilometers in diameter, still fulfills the new planet criteria. In fact, there are at least 12 more planet candidates, including Sedna and Quaoar that the IAU will be called upon to include or dismiss during future deliberations, along with giving 2003 UB313 a proper name. For the moment, attendees will simply debate the proposed definition and vote on whether to accept it or not on August 24.
No matter the outcome of that vote, this new definition does not neatly wrap up all the confusion engendered by the multiplicity of bodies in our solar system. Pluto's two newly discovered satellites elude precise classification because they orbit the gravitational center between it and Charon and the proposal does nothing to distinguish between large gaseous planets and brown dwarfs. "Did our committee think of everything, including extrasolar planet systems? Definitely not!" Gingerich notes. "Science is an active enterprise, constantly bringing new surprises." Twelve planets may just be the starting point of a growing system.