THIS IS NOT A PLANET?
How to save Pluto and rescue planetary science from endless ridicule.
In his 1929 painting “The Treachery of Images”, Belgian surrealist Rene Magritte wrote, “This is not a pipe” beneath what is obviously a pipe. What at first appears as nonsense becomes, on reflection, a thoughtful statement about the differences between words, human concepts, and actual things. In 2007 the International Astronomical Union (IAU) declared “A dwarf planet is not a planet.” This unintentional surrealism intensified a dispute about planetary definition and the status of poor little pluto.
Passions were unleashed among astronomers and the general public. Flame wars erupted. Leno joked and pundits pontificated. As a planetary scientist and educator, I’ve been amazed, amused and annoyed by this dust up. I’ve had to reassure kids that Pluto is OK. I’ve tried to explain that there is actually a wonderful truth beneath the foolishness: revolutionary new discoveries about planets orbiting other stars and the variety of objects orbiting our sun.
The hastily written IAU resolution seemed to define planets as objects that orbit only the sun. Hello! Worse still, it said an object can only be a planet if it has cleared all small objects out of its neighborhood – something impossible to determine for planets around other stars. It also seemed to imply that the same object could be considered a planet in one location but not if you moved it farther from its star.
Many planetary scientists rushed to criticize the decision. Others, eager to put the whole thing to rest, urged acceptance of the decree. This devolved into a wrestling match between those who favor the dynamical criterion the IAU used (a planet must be big enough to have gravitationally kicked out any smaller bodies that come near it) or physical criteria (a planet must be big enough to have gravitationally pulled itself into a round shape).
Some mainstream media, comfortable quoting scientific “authorities”, assumed the “official” IAU process was the last word. But the IAU has no such authority. They name objects and surface features, but defining categories and words, especially those with widespread cultural and historical resonance, is quite different. If they adopt a clearly flawed definition, nobody is under any obligation to accept it.
But I’m getting sick of this. Do planetary scientists really want to be known as the community that can’t stop fighting about what a planet is, during a decade when we are actually finding more planets every year than in all of human history, and launching spacecraft to solve mysteries of planetary climate, landscapes and habitability?
Its time for a compromise definition of “planet” that includes both the physical and the dynamical perspectives. How about: A planet is a round object orbiting a star. If we learn that it has not gravitationally dominated its surroundings, then it goes in a sub-class called dwarfs. Dwarf planets join terrestrial planets like Earth and jovian planets like Jupiter or (extrasolar gas giant) as full fledged citizens of their planetary systems with all the rights and priveledges thus implied.
The point of definitions is clear communication, so we don’t need one that is perfect. Good enough is good enough. The universe is a delightfully messy place and much as we like to tidy it up with our concepts and categories, it will (let us hope) continue to surprise us. So lets accept this provisional definition, knowing that – reveling in the fact that – new discoveries will surely render it inadequate. Lets embrace a certain amount of ambiguity and contradiction and an awareness – that would satisfy Magritte – of the limits of words and categories. Exceptions should not trouble us unduly. Moons are moons because they orbit planets, even though some resemble other planets more than they resemble other moons. Some asteroids are actually planets, if they are big enough to be round, and are orbiting the sun. Why not? Mostly, lets just accept that a dwarf planet is a planet. Fix this, tidy up the sloppy definition, and lets be done with it for now.
Put that in your pipe and smoke it.