Pluto aficionados, rejoice! Pluto is a planet. So are the giant asteroid Ceres, Pluto’s moon Charon, and a large outer-solar system object called 2003 UB313. The solar system has 12 planets instead of the familiar 9, according to a proposal that the General Assembly of the International Astronomical Union (IAU) will vote on next week in Prague, Czech Republic.
The IAU had asked a panel of seven astronomers, writers, and historians to better define what constitutes a planet. According to that panel’s proposal, announced this week in Prague, a planet is any body that orbits a star, is neither a star nor a satellite of a planet, and has gravity strong enough to pull it into a rounded shape.
“We finally have a definition of a planet after 2,500 years, and I applaud any definition that gives us an unambiguous answer,” says Neil deGrasse Tyson, director of the Hayden Planetarium in New York City.
Planethood has become increasingly controversial since 1992, when astronomers began discovering objects beyond Neptune in a region known as the Kuiper belt. Astronomers consider Pluto to be in that belt. Pluto has a small size relative to the other planets, an oddly shaped orbit, and other features shared by many of the nearly 1,000 objects now known to reside in the belt. Furthermore, last year astronomers found that 2003 UB313, a belt object, is larger than Pluto (SN: 8/6/05, p. 83: Bigger than Pluto: Tenth planet or icy leftover?).
The simplest solution would be for astronomers to admit that they erred in originally calling Pluto a planet, but “it takes guts to demote a planet that many people claim to love,” says Mike Brown of the California Institute of Technology in Pasadena, a codiscoverer of 2003 UB313.
The IAU panel not only sees Pluto as a planet but also promotes Charon to planethood. Because Charon is about 15 percent as massive as Pluto, the panel didn’t consider it to be a satellite like the moons of other planets. In fact, the group calls Pluto and Charon “double planets.”
According to the panel’s proposal, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune would make up the classical planets. Ceres would be the lone member of a group termed dwarf planets.
Pluto, Charon, and 2003 UB313 would form a new class, the icy plutons. This class would eventually include many more members—at least 41 already identified objects in the outer solar system, according to Brown.
He says that he objects to another part of the proposal, which would call for a committee to evaluate planethood if there’s disagreement about candidates. “That is just crazy,” he says, noting that a new discovery should stand on its scientific merits.
Tyson says that he worries that the proposal, which he calls a “scientifically informed, cultural decision,” could mislead the public. Scientists don’t have any new understanding of these bodies or how they’re grouped in nature, he says.
Panel member and planetary scientist Rick Binzel of the Massachusetts Institute of Technology says that a diverse group of IAU astronomers has already embraced the proposal and that it will sail through the Aug. 24 vote.
Brown says that he’s not surprised. “Most astronomers are so sick of this issue, they’d pass anything.”
The solar system has eight planets, and Pluto has been relegated to the status of dwarf planet. That’s the official word from the General Assembly of the International Astronomical Union (IAU) in Prague. The asteroid Ceres and the outer solar system object 2003 UB313, informally dubbed Xena, are also dwarf planets. On Aug. 24, astronomers at the meeting voted overwhelmingly in favor of the first-ever formal definition of a planet, after an IAU-appointed committee revised their original proposal.
Under that original proposal, submitted by the committee in mid August, the number of planets in the solar system would have expanded from 9 to 12 planets. These would have included the eight classical planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, the largest asteroid, Ceres, and a trio of outer solar system objects termed plutons, including Pluto, its moon Charon, and Xena. Geologists soon pointed out, however, that they already use the word plutons to refer to chunks of molten rock that solidify before they reach Earth’s surface.
Two elements of the originally proposed definition called any object a planet if it is not a satellite and is massive enough to pull itself into a rounded shape. But under pressure by astronomers at the Prague meeting, the proposal was revised a few days ago to include an additional criterion—a planet must be heavy enough to clear other objects from its path. That qualifier took Pluto out of the running, as well as Ceres, Xena, and Charon.
Reached in Prague, Rick Binzel, a planetary scientist at the Massachusetts Institute of Technology and a member of the IAU definition committee, says that he was “surprised that this [revised definition of a planet] passed by such a big margin.”—R. Cowen