Is Pluto a Planet? Astronomers Vote, JHU Takes Straw Poll

Generations of schoolchildren have memorized “My Very Energetic Mother Just Served Us Nine Pizzas” (or a variation thereof) in order to remember the order of the nine planets in our solar system. For 70 years – ever since it was discovered – the “p” in that oft-repeated mnemonic device has stood for Pluto, the ninth and smallest planet.

But is Pluto actually a planet? Or is it something else? Astronomers have been debating this issue – often, hotly — since Pluto’s discovery seven decades ago.

That debate may finally end next week, when members of the International Astronomical Union meeting in Prague vote on a formal definition of the word “planet.” According to the proposed definition, a planet is any star-orbiting object with enough mass for its own gravitational force to pull it into the shape of a ball. Furthermore, the object must orbit a star, without being a star itself.

If this definition is adopted, our current nine-planet solar system would be joined by Ceres (the largest of the asteroids), Charon (Pluto’s largest moon), and 2003 UB313 (the provisional name for a recently discovered object larger and farther away from the sun than is Pluto). In addition, the IAU resolution also would establish a new “junior” class of planets called “plutons” to describe objects such as Pluto, which have less-than-circular orbits and take more than 200 years to orbit the sun.

An informal poll of astronomers at the Henry A. Rowland Department of Physics and Astronomy at the Johns Hopkins University’s Krieger School of Arts and Sciences and the Johns Hopkins University’s Applied Physics Lab reveals that though most favor Pluto’s continued existence as a planet and establishing a new definition of the word, the notion of a “pluton” is less well received. Here is a sampling:

* Karl Glazebrook, professor in the Department of Physics and Astronomy: “My take is that a physical definition like the one proposed makes sense: something large enough to be spherical and which is not a satellite. But if I were in charge, I would insist on a diameter of greater than 1,000 kilometers to define a ‘planet’ in order to remove Ceres from the list, but that would be an arbitrary cut to preserve the order of the main nine and to save the hassle of rewriting textbooks. * Definitions and naming really matter little physically, anyway.”

* William P. Blair, research professor in the Department of Physics and Astronomy and chief of observatory operations for NASA’s Far Ultraviolet Spectroscopic Explorer Satellite, operated by JHU: “This is really not a burning issue for most astronomers. It really is a ‘classification’ problem more than an issue of science. From what I can tell, they have tried to come up with a consistent definition: an object roughly spherical under its own gravity, orbiting a star, and not orbiting something else. OK, so far. Then they are apparently willing to immediately bend this relatively simple definition and allow Charon to itself be called a planet, with Pluto and Charon being a ‘double planet’ system. This goes too far and seems inconsistent to me.”

* Richard Conn Henry, professor in the Department of Physics and Astronomy: “I think the notion that Pluto is a planet is absurd. When it was initially discovered, it was thought to be vastly more massive than it turned out to be. Its orbit is radically different from that of all the other planets. Down with Pluto, is what I say!”

* Harold A. Weaver Jr., project scientist, Johns Hopkins University Applied Physics Laboratory: “Personally, I’m in favor of the resolution, mainly because it’s about time we had a formal definition of the word ‘planet,’ and the proposed definition is relatively straightforward. However, we must not forget that any attempts by us to pigeonhole objects in the universe are bound to have shortcomings. The classification schemes that we invent help us to place diverse objects in context, but we must also recognize that nature often doesn’t adhere to our attempts to categorize things. Although a ‘planet,’ Pluto has more in common with the horde of ‘Kuiper belt objects’ on the outskirts of our solar system than it does with, say, Jupiter. On the other hand, to those who complain that Pluto doesn’t deserve to be in the company of planets like Jupiter, I point out that there are also huge differences between Jupiter and Earth, for example. In any case, I doubt that the IAU could come up with a resolution that would please everyone.”

* Andy Cheng, Johns Hopkins University Applied Physics Laboratory: “Yes, keeping Pluto as a planet is the correct decision. However, the new definition of planet does not work for me, because “hydrostatic equilibrium” is an idealization — it is approximately correct for planets like Earth but is not exact. There is still no criterion for deciding how far from hydrostatic equilibrium an object can be and still qualify as a planet. Much of the science of geophysics deals with the different ways, and the reasons why, planets are not quite in hydrostatic equilibrium. Also the suggested term ‘pluton’ is a bad idea, in my opinion.”