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Space Topics: Pluto Top TenIs Pluto a Planet?by Bill Nye
In 1999, the International Astronomical Union (IAU) issued a statement declaring that Pluto is a planet akin to ours (Earth...). The IAU did this to address an Earth-wide argument raging among astronomers and other Earthlings about whether or not the distant relatively small object we call Pluto should be thought of as a “planet.” Or, should it be something else, something less significant than, say, our own planet Earth? One might easily dismiss this debate as trivial when compared with global (Earth-borne) terrorism, deadly pandemics, and catastrophic climate change. But, this sort of thinking affects our world or worlds view, if you will. This argument came about, because Pluto is different. For one thing, its orbit is different. It’s tilted or inclined relative to the orbits of Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. That suggests an origin quite different from, say, Mars or Earth. Pluto is small, less than half the size of Mercury. Although Pluto has Charon, a moon almost as big as Pluto itself, they’ve both recently been found to be part of a complex system of remote icy bodies gravitationally tugging on each other, blurring the conventional definitions of moons and solar orbits. Because of the discoveries of other icy bodies out there, like Sedna, Quaoar, and 2003 UB313, all similar in size to Pluto and Charon, it’s likely that if Pluto were discovered today, it would not be called a “planet.” Either we stop referring to Pluto as a planet, or we reckon this 75 year-old definition with the new discoveries. When Pluto was discovered, “planet” was pretty much the only noun around. But after all, Pluto and Charon do orbit the Sun, and they do have enough gravity to become spherical— cold worlds suspended in the darkness. The eight traditional planets orbit the Sun in very nearly the same plane, as though they were all sliding on a Solar System-sized invisible tabletop, what I like to call the “Main Plane.” So, we can describe our Earth and the seven others as “the Main Plane” planets. Then, Pluto and the other icy objects out there would be described with a different modifier. They could be the “Icy Dwarf Planets,” for example. Or how about this, “the Plutonian Planets.” That would be the worlds past Neptune and at least, say, 2000 kilometers in diameter. The IAU and the rest of us can keep calling Pluto a planet (diameter about 2274 km), but we would all just have to get in the habit of referring to these objects as Plutonian bodies to distinguish them from Main Plane bodies. In astronomy and in science education, we would find ourselves being consistent. We have always added descriptors as we go along. When we refer to planets orbiting other stars, for example, we just add “extra-solar” (separate from the Sun). This month we’re launching the New Horizons spacecraft to go have a close look at Pluto. Whatever we discover there, you can bet it will be spectacular. And my fellow Main Planers, I’d bet we wouldn’t be sending this mission, committing all the required resources, if we hadn’t thought of Pluto as a planet since its discovery in 1930. The mission is called New Horizons, because although we know what we’re looking for, we’ve never had cameras up close that far out. We’re not sure what we’re going to find. On to Pluto, the first of the non-Main Plane, icy dwarf, Plutonian planets.
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