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Eighty-four years ago today, on February 18, 1930, the celestial body known as Pluto was discovered. In contrast to the dramatic discoveries of other things made by humans in the past–Antarctica, the moons of Jupiter, penicillin–the discovery of Pluto was pretty dull. Clyde Tombaugh, a 23-year-old astronomer fresh on the job at Lowell Observatory in Flagstaff, Arizona, was staring at some photographic plates of the stars side-by-side when he noticed a tiny speck was out of place in one of the plates. All the other stars were in the same place. Hence, something out there was moving. From mathematical predictions made by the founder of his employer institution, Perceval Lowell, Tombaugh and his colleagues already suspected there was a planet beyond Neptune. Now they had the proof.
When the discovery of the new planet hit the front pages, the big question was what they were going to name it. Venetia Burney, the 11-year-old daughter of an Oxford University professor, wrote to Lowell Observatory suggesting the name Pluto, lord of the underworld in classical mythology. The directors of the observatory loved the idea. Since 1846 the solar system officially only had eight planets, after William Herschel discovered Neptune. Now the solar family grew to nine. Walt Disney named Mickey Mouse’s dog after the new planet.
1930, however, was probably Pluto’s heyday. Its fortunes began to decline almost immediately. In 1931 the new planet was thought to be about the size of the Earth, but new astronomical work done in the late 1940s determined that was too generous and Pluto was in fact about the size of Mars. In the 1970s the official estimates dropped again–now researchers thought Pluto was 1/100 the size of Earth. In 1978, improved optics revealed there was something else out there: Pluto had a moon, quickly named Charon, but instead of orbiting Pluto the two seemed to orbit each other like drunken co-dependent lovers. The discovery of the moon made it possible to be more precise about how big Pluto was. Final estimate: a measly 0.0218 the size of Earth. By the turn of this century it was clear that Pluto, never very big to begin with, was a really dinky place.
Why should this matter? Well, the size of Pluto affects how much gravitational pull it has, and hence how it can affect other celestial bodies. Perceval Lowell had predicted his “Planet X,” which in 1930 everybody thought Pluto was, based on gravitational anomalies in the orbit of Neptune, which were believed to have been caused by another planet. (We’ve seen before on this blog how conclusions like that can turn out to be erroneous). But at roughly the size of a large Winnebago, Pluto was far too small to have that kind of pull. (Okay, it’s larger than a Winnebago–I just made that up. But still). The answer? It turned out that astronomers had been miscalculating the mass of Neptune all along. There never was any “Planet X.”
The next blow to Pluto’s reputation came in 1979. At the time of its discovery it was the most distant planet, so far that it takes 248 years to go around the Sun once. Its orbit is on a totally different plane than all the other planets, however, and in 1979 its screwy orbit carried it, for a brief time, slightly closer to the sun than Neptune. This remained true until 1999, when again Pluto became more distant than Neptune. By this time, however, especially aided by the Hubble Space Telescope, astronomers were discovering all sorts of stuff out there, mostly icy rocks hurtling around at the far reaches of the solar system. Pluto didn’t really seem that much different than them.
Finally, in 2006, Pluto suffered the ultimate indignity. An astronomer named Mike Brown discovered another icy rock out beyond Neptune that was (wait for it…) even bigger than Pluto. The thing he discovered is called Eris, and it caused an immediate problem: if Eris is bigger than Pluto, and Pluto is “officially” the ninth planet of the solar system, does that make Eris the tenth planet? Worse still, if there are other chunks of ice and rock still out there that turn out to be that same size or bigger–which is likely–are we going to call those things planets too? How many planets are we going to end up with? How many is too many? The International Astronomical Union, which in a 2003 document tried to define what a “planet” actually was, didn’t want to go there. It declared that neither Pluto nor Eris nor any of the other icy rocks that Mike Brown has discovered–including two, I am not making this up, named after characters from Xena: The Warrior Princess–are really planets. They’re now “TNOs,” Trans-Neptunian Objects. Pluto’s reign as the ninth planet of the solar system (1930-1979, and 1999-2006), or even as the eighth planet of the solar system (1979-1999), was over.
Clyde Tombaugh, who was very young when he discovered Pluto and died very old, did not live to see “his” planet demoted. He passed away in 1997. Venetia Burney, however, did. She died in 2009, saying that she didn’t really care that much but tended to think Pluto should still be a planet.
All is not lost, however. In 2006, the year of Pluto’s demotion, NASA launched a probe called New Horizons, which is scheduled to reach Pluto next year. Clyde Tombaugh’s ashes are on board. Assuming New Horizons completes the trip, he will truly have gone where no man has gone before.