In grade school, we memorized a mnemonic sentence that helped us remember the order of the planets. My Very Energetic Mother Just Served Us Nine Pies. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto — nine planets that have been orbiting the sun peacefully for eons. But a new cacophony is intruding on the harmony of the spheres. Some astronomers are ready to demote Pluto from planetary status — they want to take away pies. Pluto’s story begins shortly after the 1846 discovery of Neptune. Astronomers determined that the new planet was just not behaving itself, refusing to conform to Newtonian laws of motion. The only conceivable explanation at the time was that a massive trans-Neptunian planet existed whose gravitational pull was disrupting Neptune’s orbit. Eventually dubbed Planet X, the search for this planet was a dismal failure until the 20th century.
On Feb. 18, 1930, a 24-year-old Kansas farm boy, who had been working at Arizona’s Lowell observatory for a mere three months, stumbled across the most distant of the nine planets. While diligently analyzing thousands of photos of the night sky, Clyde W. Tombaugh discovered the mysterious Planet X, soon called Pluto at the suggestion of an 11-year-old school girl from Oxford, England. And that was supposed to be the end of it. It did not take long for problems to become evident. To explain Neptune’s perturbations, Percival Lowell had predicted that Planet X had to be 6.6 times as massive as Earth. But by the end of 1930, astronomers believed that Pluto was only 0.11 Earth masses. In 1978, when Pluto’s moon Charon was discovered, Pluto’s mass shrank to 0.002 that of Earth, a value that has remained roughly intact to the present. Clearly, Pluto is not the source of Neptune’s deviations since it is a lightweight as far as planets go.
Pluto’s radius of 1,137 kilometers, just about big enough to lay across the United States, is smaller than seven moons: four Jovian (Io, Europa, Ganymede and Callisto), one Saturnalian (Titan), Neptunian (Triton) and our own creatively named Moon. Only Mercury is likewise lacking in size, being smaller than two moons, but it is at least more massive than both, a claim that Pluto cannot make. Perhaps the world was a bit hasty in calling this small ball of rocks and ice a planet.
When you start to really investigate Earth’s distant cousin, things get downright strange. Consider Pluto’s orbit. All planetary orbits around the Sun are elliptical. Pluto takes this idea to an extreme. Its orbit is so eccentric that for 20 of the 248 years it takes for Pluto to circle the Sun, it is not the ninth planet, but the eighth, moving in closer than Neptune. This has been the case since 1979. Moreover, Pluto’s orbit does not sit on the ecliptic, the cosmic dinner plate on which all well-behaved planets reside. Pluto rises above the ecliptic and then crashes below it at an angle of 17 degrees.
Then there is Pluto’s rotation. It rotates around poles with an equator just like any other planet, but Pluto’s equator is inclined to the ecliptic by 122 degrees. This means that Pluto is effectively rotating perpendicular to the rest of the planets, except Neptune which also has an odd tilt.
And let’s not forget Pluto’s moon, Charon, named after the boatman who ferried souls across the River Styx to Pluto’s underworld realm. Charon orbits Pluto in exactly one Plutonian day. If you were standing on the surface of Pluto and looking up at Charon directly overhead, it wouldn’t move. It would just hang there in the same place in the sky looking ready to come crashing down on your head, which is no small threat since Charon’s relative size to Pluto is really big, the largest in the solar system. This ratio has led some astronomers to refer to Pluto and Charon as a double planet.The latest assault on Pluto’s planetary distinction comes from the discovery of a bunch of Pluto-looking objects beyond Neptune. Similar to Pluto, the icy balls that compose the Kuiper Belt maintain similar eccentric orbits. Although none is quite as large as Pluto, it is being argued by some astronomers that Pluto is the grand-daddy of all Kuiper objects and not, in fact, a planet.The problem, the deep dark secret that astronomers don’t want you to know, is that they have no idea what a planet is. Sort of. We know which objects are planets and which are not. Any junior high student (at least the ones that can find the United States on a world map) can tell you the names of the nine planets. What neither they nor the astronomers can tell you are the precise conditions that qualify one object as a planet and another as not. Most people propose two criteria to differentiate the planets from all the other solar bodies like moons, asteroids and comets. First, the object must orbit the Sun, as opposed to a moon which orbits a planet. Second, the object must be pulled together in a roughly spherical shape by its own gravity.While this may sound good, it forces us to admit more objects to the exclusive family of planets than we want. For example, the asteroid Ceres meets both conditions. A third proposed qualifier for planets to resolve this difficulty is the presence of an atmosphere. While this eliminates Ceres, we think, from contention, it causes problems for both Pluto and Mercury. Mercury’s atmosphere is dubious, consisting of particles blasted off the surface by the solar wind which are quickly lost into space. Pluto, on the other hand, has an atmosphere, but when it begins to move farther from the Sun this atmosphere will freeze and settle on the icy surface for more than 100 years. And don’t forget that several moons have atmospheres far more robust than these. Maybe it’s enough that we just know which are the planets, admitting the arbitrariness of such a definition. These nine things are planets, and everything else is not. Unfortunately, that’s not the way science works. Science finds its validation in precise definition. As we discover objects orbiting other stars, a suitable definition of “planet” will be indispensable. Realistically, Pluto is not a planet nor a twin planet. It is the black sheep of the family that just does not fit in with everyone else. Pluto is not big enough to be a planet, and its eccentric behavior rules it out. On the other hand, it is bigger than comets, asteroids and Kuiper objects and has an atmosphere and satellite. It exists in the gray area that our definitions do not cover. It will be up to the International Astronomical Union to finalize any definition. Should they exclude Pluto? As Americans, we hate to see it lost as it is the only planet discovered by one of our countrymen, and our inherent love of the underdog gives Pluto a special place in many of our hearts. As citizens of the world, it has been customary to call it a planet no matter how strange its behavior. Besides, if we change our minds, the plaques on Voyager and Pioneer missions that depict nine planets will be wrong. But it is probably better for the IAU to err conservatively. When they do formally adopt a definition, in order to keep the planetary neighborhood somewhat exclusive, Pluto will probably be eliminated by not meeting certain criteria. Perhaps, though, there is a solution, even if an arbitrary one. When the official definition is written, include all the properties that make a planet a planet. After all that, we can add an addendum: Everything that meets all these properties is a planet and so is Pluto. The scientists will have a good working definition, and we will have tradition. Hopefully everyone will be happy, and we will still have nine delicious pies to eat.

Chris Trejbal’s column appears every Tuesday. He welcomes comments to [email protected]