Update BelowRecently, a group of astronomers met to begin deciding a weighty question - should the planet Pluto really be called a planet? (Background
here.) At the end of the day it was clear that nobody was ready to demote Pluto. Instead, a new definition of "planet" was proposed, a definition specifically designed to include Pluto (without including too many other planetoids orbiting the Sun). So much for scientists being rational and unsentimental. I don't care about Pluto's scientific status, but I am fascinated by the sentimentality. We appear to be haunted by the ghost of Planet X.
Percival Lowell was a wealthy Bostonian who occupied himself as a "gentleman scientist" in the late 1800's. He had enough wealth to build and run an astronomical observatory in Arizona. He also had enough money to widely publicize his thoughts and observations, and thus it was that he planted his first ghost in the American psyche - Martians.
Lowell was aware of the observations of Mars by Giovanni Schiaparelli, who had published drawings showing surface details, including what he labeled "canali." Lowell took the word literally to mean canals, something deliberately built, and convinced himself that there must be canal-building Martians.
Now, the idea of Martians is so imbedded in our culture that it can seem odd that 1) we are unique in this regard, and 2) one person started the whole idea of Martians. Of course, in the 21st century, no one really believes that there are Martians, but just to be sure, we have been sending spacecraft to Mars for some time now, looking for them. As I write this, there are two rovers ranging about the surface of Mars looking for signs of water. Why water? Water supports life, and life means Martians. Maybe the life is just microbial, or maybe we'll just say that Mars could have supported life in the past, but we're still looking for Martians.
During Lowell's life the planet Neptune was discovered. Neptune is invisible to the naked eye, and its existence had to be inferred. Locations were calculated, and eventually the planet was spotted by telescope. There also seemed to be evidence that maybe there was one more planet out there, and Lowell took the bait. The great American astronomer would attempt to find Planet X.
Lowell did not find Planet X in his lifetime. The task was passed on to Clyde Tombaugh, who never once questioned the existence of Planet X in spite of nobody ever finding it where it was supposed to be. To be fair, no one knew then what we know now - the evidence was flawed, and there is no Planet X.
But what about Pluto? When Lowell envisioned Planet X, he pictured a giant gas planet like Uranus or Neptune. He had calculated where it should be, and it wasn't there. Tombaugh gave up this particular search, and just systematically started looking everywhere remotely possible. It was almost miraculous that he was able to find a tiny ball of ice smaller than our Moon, and of course he assumed it was Planet X.
So the idea, the ghost, of Planet X, a great planet out in the unknown, just beyond our seeing and our calculating, infiltrated our imaginations. When it became clear, over time, that Pluto was not the Planet X Lowell had hoped for, the idea of Planet X still remained. If Pluto, a small planet, was all we had for now, well then all right. When Michael Brown discovered the planetoid he calls Xena, and it turned out to be bigger than Pluto (by a hair), wasn't all the tenth-planet
excitement really about the unfulfilled promise that there must be a Planet X?
To demote poor Pluto further would be almost admitting there is no Planet X, and that would be like denying Martians. How many other ghosts comfort us like this in the night?
BC
Update: 24 Aug 06
A larger international body of astronomers have
decided that Pluto is not a planet after all. However, in deference to Pluto's former status, they have created a category called "dwarf planet" to which Pluto now belongs.
Here's Mike Brown's
homepage where you can read his reaction to all this, plus his op-ed for the NYT.
And here's Wikipedia on
Planet X.
(My friend Mike has alerted me to the possibility that Rhode Island might now be classified as a dwarf state.)