And Then There Were Twelve
For going on two centuries, astronomers have debated what exactly a planet is and which celestial bodies should qualify. The question was finally forced a year ago by the discovery of 2003 UB313, tentatively named Xena, a body larger than Pluto residing in an even more distant orbit. If Pluto is a planet, then Xena must be one, too -- along with lots of other bodies in the outer reaches of the solar system. But if you don't want to open the floodgates to so many new planets, you have to relegate Pluto to some lesser status. In either case, tradition must give way.
The debate is nearing its resolution. This morning, a committee of the International Astronomical Union announced a new definition of planet at the triennial IAU General Assembly being held in Prague. Astronomers will vote on it a week from tomorrow. The committee proposes to define a planet as a body that orbits around a star and is large enough that gravity (as opposed to material strength) controls its shape. Formally, the definition reads:
A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.
The new solar system
Part (a) would typically translate into a mass larger than 5 x 10^20 kilograms or a diameter greater than about 800 kilometers. Smaller bodies tend to have a craggy shape, but it is possible for them to qualify as planets if, for want of structural integrity, they have settled into a round shape.
Regarding part (b), the committee allows for multiple planets sharing an orbit, such as double planets -- two planets orbiting each other and also moving as a pair around the star. A pair counts as a double planet, rather than a planet and satellite, if they are roughly comparable in size -- more specifically, if their center of gravity lies outside the surface of the larger body. Thus Earth's moon remains a satellite, but Pluto's companion Charon is now a planet.
By these criteria, the sun has 12 planets: the nine usual ones plus Charon, Xena, and the largest asteroid, Ceres. More may soon join them. Astronomers could find that other asteroids have a round shape, and new discoveries of Xena-like bodies are almost guaranteed.
So it is a reasonable definition that confirms most people's intuition about planets. I'd been worried that they might try to undefine Pluto as a planet, which would violate well-established precedent, or lock in the classical designations, which would deny scientific progress. The definition also sticks to observable criteria. Lying in the back of everyone's mind is that the real definition of a planet is an object that forms by accretion in a circumstellar disk of gas and dust, rather than direct collapse from an interstellar gas cloud. But origins are often murky.
That said, the system strikes me as odd in several respects. For one, it sweeps the problem of what a star is under the rug, possibly leading to inconsistencies and disagreements; it might have been better to develop a full, coherent system of definitions. Also, the proposal explicitly rejects planetary status for bodies that have been ejected from orbit and consigned to wander through intergalactic space; yet it offers no new nomenclature for these bodies, leaving them orphaned twice over. The committee adds a category known as "plutons" for transneptunian, or Kuiper Belt, objects; yet this coinage is not in widespread usage among astronomers. Other people will surely be unhappy that the committee does away with the term "minor planet" and instead introduces the clunky, acronym-resistant "small solar system objects".
