Pluto: 9 + 1 = 10, not 8

I grew up in Hershey, Pennsylvania, just a few miles downwind from Three Mile Island. When I was studying math in the 1970s, Hershey was not known for the quality of its public schools. The local school board seemed to think that the kids needed to know just enough to work in the chocolate factory. But when I learned that 9 + 1 = 10, I was apparently taught correctly. I confirmed this fact when I went to college.

Yet amazingly, some of the most learned professional astronomers, and even some of my esteemed S&T colleagues, think that 9 + 1 = 8. They argue that because of last summer's announcement of a new Kuiper Belt object (KBO) larger than Pluto (2003 UB313), the solar system now has 8 major planets. In other words, astronomers have been saying since 1930 that the solar system has 9 planets, then they find another planet, then all of a sudden we have 8 planets. Am I missing something here?

The campaign to drum Pluto out of the category of "major planets" has actually been gathering steam since the early 1990s, when David Jewitt and Jane Luu discovered the first KBO besides Pluto and its moon Charon. The argument goes something like this: With a diameter of just 2,300 kilometers (1,400 miles), Pluto is just too much of a pipsqueak to deserve membership in the exclusive club of major planets. It's merely the largest known member of the Kuiper Belt, several of whose members are half to two-thirds Pluto's diameter. If we count Pluto as a major planet, we should also count all of these other KBOs, and pretty soon we’ll have more planets than Elvis impersonators.

Admittedly, the critics' contentions have considerable merit. Until last year, Pluto really was just the largest known member of the Kuiper Belt. I also have to concede their point that if Pluto had been discovered in 2006 instead of 1930, few astronomers would call it a major planet.

But that's not the whole story.

For every argument against Pluto being a planet, Pluto's defenders can summon an equally legitimate counterargument. When critics say Pluto is small, defenders can point out that it's easily big enough so that gravity can pull it into a sphere, and that astronomers from Jupiter might consider Earth to be an overgrown asteroid. When critics say Pluto has a highly elongated orbit, defenders can point out that many of the 200 known extrasolar planets have even more eccentric orbits. When critics say Pluto shares its region of space with zillions of other KBOs, defenders can point out that Earth shares its orbit with thousands of asteroids (and even mighty Jupiter and Neptune share their orbits with swarms of Trojan asteroids).

The bottom line is that Pluto is big enough to be round, it has an atmosphere for at least part of its orbit, and it has at least three satellites. It might even have rings. While asteroids and moons share some of these characteristics, we don't know of any that have the whole kit and kaboodle. And since these are all traits one normally associates with "planets," it's not at all obvious that Pluto should be booted from the realm of planethood.

What is clear is that because Mother Nature makes objects in a continuum of sizes, wherever you draw the line that distinguishes a major planet from a minor planet, your boundary will be arbitrary. Astronomers have been calling Pluto a major planet since 1930, and they haven't been saying that just to kids, they've been saying it to each other in their books and research papers. The simplest thing to do is to simply grandfather Pluto into the planet club, and set its 2,300-km diameter as the minimum size for a major planet.

The debate has festered because there is no official definition of what constitutes a "planet." For years, the organization that resolves these nomenclature matters, the International Astronomical Union (IAU), has sidestepped the controversy. But Mike Brown's announcement last summer of 2003 UB313 forced the issue. The IAU has to give this object an official name, and that name will depend on whether it's a major planet, or just another KBO. And since 2003 UB313 is only slightly larger than Pluto, once the IAU decides its official status, it will resolve Pluto's as well.

The IAU has appointed a committee, chaired by the eminent astronomy historian Owen Gingerich, to come up with a definition of "planet." Gingerich's committee, which includes members on all sides of the Pluto debate, has reached a consensus after a period of intense discussion, and National Public Radio reported today that the definition will include Pluto. However, the report was based on what 5 committee members thought before the committee even met, and S&T has learned that some of the "facts" that NPR reported are wrong — but we don't know which ones! According to the NPR report, the definition will establish several classes of planets, such as terrestrial planets (Mercury, Venus, Earth, and Mars), giant planets (Jupiter, Saturn, Uranus, and Neptune), and dwarf planets (Pluto and the larger KBOs, and perhaps several main-belt asteroids). In other words, if the IAU adopts this classification scheme, we might have dozens of objects that fall under the term "planets."

This scheme makes sense scientifically in terms of our solar system, but I predict that as we learn more about planets around other stars, we'll find plenty that don't fit neatly into these categories. The history of science tells us time and time again that Mother Nature refuses to conform to human expectations and nomenclature systems. I also predict that a lot of people in the general public would find such a classification scheme confusing and dissatisfying.

The committee will present its definition at the IAU's General Assembly in Prague next week, and a vote by the full membership is expected around August 25th. S&T editor in chief Rick Fienberg will be at the conference to cover this unfolding story, and will keep you posted with accurate and authoritative coverage. I have great confidence that whatever it decides, the IAU will eventually come to a sensible conclusion that will lay the controversy to rest. While I enjoy discussing the Pluto debate, ultimately the science is more important than the semantics, and it's time to move on. When it comes to the solar system, the world needs to know whether 9 + 1 = 10, or whether we need to learn New Math.