PRAGUE, CZECH REPUBLIC--Our solar system lost a planet today, but it gained a whole new class of objects in the process. That's the surprising outcome of a historic vote on the definition of a planet, held earlier here today at the 26th General Assembly of the International Astronomical Union (IAU).
Far-flung Pluto, which is smaller than Earth's moon, was classified as a planet when it was discovered in 1930. Scientists have searched for a tenth planet ever since. But while numerous rocky and icy bodies have been found on the outskirts of our solar system, none of them were ever classified as planets.
The trouble started last year when Michael Brown, an astronomer at the California Institute of Technology in Pasadena, announced that he had spotted a new planet beyond Pluto. Or was it a planet? If size is all that matters, then 2003 UB313, as it is officially known, certainly should be a planet: It is larger than Pluto. But Pluto and objects like 2003 UB313 (nicknamed Xena, after a popular television character) are just the largest members of the Kuiper Belt--a huge population of icy bodies beyond Neptune's orbit. To settle the issue, the IAU decided it needed a formal definition of the term "planet."
The organization threw together a special committee of seven experts (including a historian and a science writer) to come up with a definition. But the proposal they put forward on 16 August didn't get far among skeptical astronomers who angrily complained that its chief criterion for planethood--roundness--could open the door for hundreds of still-undiscovered future planets in orbits beyond Pluto (ScienceNOW, 16 August). So a revised version, first put forward on Tuesday, states that, in addition to being round and in orbit around a star, a planet must be massive enough to have "cleared the neighborhood around its orbit."
Pluto would not qualify as a planet by this definition because it shares its part of the solar system with numerous other objects in the Kuiper Belt. Xena and Ceres--the largest rocky body orbiting the sun between Mars and Jupiter--were also denied planethood and will join Pluto as dwarf planets. A third class of smaller objects that orbit the sun, such as asteroids and comets, will now be known as "small solar system bodies."
Although a vast majority of astronomers at the meeting accepted the new definitions (about 400 participated in the vote), not everyone is happy with how the debate played out. "[It] should never have become this emotional," says astronomer George Miley of Leiden Observatory in the Netherlands. Even IAU President Ron Ekers says, "There is no correct answer" to the question of what is a planet and what is not. "There had to be a compromise."
In the near future, many more dwarf planets are expected to be found, says Keith Noll of the Space Telescope Science Institute in Baltimore, Maryland. But the number of "real" planets in the solar system will be fixed at eight. "In a sense, I hope this definition won't last long," says Rick Binzel of the Massachusetts Institute of Technology in Cambridge, who was a member of the IAU planet definition committee. "If we would discover something new in the solar system that would make it necessary to completely revise our current views, that would be tremendously exciting."