Our solar system has at least twelve planets, not nine as asserted in school books all over the world. That is the implication of a draft resolution for the definition of a planet, presented today by the International Astronomical Union (IAU). If the resolution is accepted, the solar system will gain three planets overnight: Ceres, formerly known as the biggest asteroid; Charon, formerly known as the largest satellite of Pluto; and 2003 UB313 (nicknamed Xena), a Pluto-like object that was discovered last year.
After the discovery of Xena, which is slightly larger than Pluto, the IAU asked a special committee to come up with a clear-cut definition of a planet. The committee, chaired by astronomer and historian Owen Gingerich of Harvard University, now proposes to define a planet as a nonstellar spherical body orbiting a star. The object must be large and massive enough to have become spherical under its own gravity. Pluto certainly fits the bill. So do the others.
"It's a highly scientific approach," says Pluto expert Alan Stern of the Southwest Research Institute in Boulder, Colorado. "I am quite happy with it." Daphne Stam, a planetary scientist at SRON Netherlands Institute for Space Research in Utrecht, adds that "it's a nice definition, although we will end up with a very large number of planets." Indeed, twelve objects, including three asteroids and nine objects beyond Neptune, are currently on the IAU's list of candidate planets.
The committee also proposes to officially classify Pluto and its large moon Charon as a double planet, because their common center of gravity lies somewhere in the empty space between the two bodies. Consequently, Charon itself must also be a planet, even though it is considerably smaller than our own moon. Jim Christy, who discovered Charon in 1978, says, "Thinking of myself as the discoverer of a planet is a new concept. It would be an honor."
The committee also suggests labels for different types of planets, recommending that those smaller than Mercury could be called "dwarf planets," while the eight big ones (from Mercury through Neptune) might be called "classical planets." Moreover, the small, frozen planets beyond Neptune, which take more than 200 years to orbit the sun, would officially be labeled as "plutons," to set them apart from the big ones.
Not everyone is happy with the new proposal. "I think it's too complicated," says outer solar system expert Scott Sheppard of the Carnegie Institution of Washington in Washington, D.C. Some of the very massive planets found around distant brown dwarf stars would now technically be classified as plutons because of their long orbital periods. "That would be confusing," he says, because the plutons in our own solar system are low-mass objects.
Over the next few days, the draft resolution will be discussed and possibly altered at the 26th General Assembly of the IAU in Prague, Czech Republic. On 24 August, some 2500 astronomers from all over the world will be asked to vote "yes" or "no." Says Sheppard: "I won't be there, but if I were to vote, I would vote it down." So don't throw away those old school books just yet.