New Solar System? Twelve Planets And Counting

[freeon]

The vote is apparently today.
Here's my question: will "the twelfth planet" ever rear it's head?



Ron Cowen

"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.



ASTRONOMER'S DOZEN. In a newly proposed scheme, Pluto would retain planethood and there would be three new planets—Ceres, Charon, and 2003 UB313. (These and Pluto enlarged above.)
M. Kornmesser, IAU

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: http://www.sciencenews.org/articles/20050806/fob2.asp).

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." "