Pluto agonistes
Oct. 17th, 2010 09:09 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
calimacI've been interested in Pluto since long before the controversy over its demotion from planet status came up, because I'm always interested in obscure things at the ends of lists or the edges of categories. In a high-school astronomy class I chose Pluto as the planet to write my report on, a challenge because virtually nothing was known about it at the time. By good fortune, just before then, I went on a family vacation that took us to the Lowell Observatory in Flagstaff, where Pluto was discovered, so I was able to insert a bit of "I was there" local color into my report. What particularly struck me about the place was - this was February - the large quantities of this cold white stuff sitting around on the ground, the likes of which we never saw at home. I'd always been under the impression that snow was fluffy, but this seemed more like crushed ice, and I wasn't sure if it was actually snow or something else. (If snow is fluffy, I've still never seen any yet.)
But in all my reading on Pluto, then and since, I've never seen any straightforward answers to some basic questions I had about its discovery. Until now. A new book called The Case for Pluto by Alan Boyle (Wiley, 2010), which I found in the public library, answered them simply.
Astronomers suspected the existence of a ninth planet in the first place because of some discrepancies in Uranus's orbit which could be caused by gravitational influence from another large planet outside its orbit. Indeed, such discrepancies had already successfully produced Neptune. Someone calculated where a planet that would produce these effects should be located, someone else looked at that spot in the sky, and sure enough, there it was. But it turned out there were some discrepancies left over, and the hunt was on for the next gas giant. Instead, they found Pluto, which was too small to account for the equations' needs, and turned out to be smaller still every time astronomers measured it more accurately. So my first question was,
Q. If Pluto was too small to account for the remaining discrepancies in Uranus's orbit, what did account for them?
A. There weren't any. They were the result of miscalculations of Neptune's mass and orbit. This was suspected after Pluto's size was estimated, and proved when the results of the Voyager flyby came in.
Q. If Pluto didn't have anything to do with Uranus's orbit, what was it doing in just the right spot to be responsible for it?
A. It wasn't there. Neptune had indeed been discovered this way, but looking for Planet 9 in the calculated (or repeatedly re-calculated, after it repeatedly didn't turn up) spot is why Percival Lowell never found it. When Clyde Tombaugh resumed the search some years after Lowell's death, he wasn't using any calculations. He was just scouring any old place along the ecliptic where there might be a planet, and it was sheer luck that he found one. It was coincidence that it did match one of Lowell's search spots, and it turned out that Lowell had actually photographed it, but hadn't noticed it because it was far too dim to be the gas giant he thought he was looking for.
Q. If Tombaugh found Pluto by happenstance, how do we know there weren't other bodies in similar orbits findable at the time?
A. Partly because Tombaugh kept looking. He kept up his systematic sky survey for another dozen years and found plenty of asteroids, but no planets. Also partly because Pluto's orbit is shaped by orbital mechanics involving Neptune, and there's a limited number of places where a stable orbit of a Pluto-like object at such a distance could exist. In the last 20 years, we've discovered a large number of objects outside Neptune's orbit, and they are collectively known as the Kuiper Belt. But Pluto is by far the brightest, and the others are too faint to have been findable by Tombaugh's telescope.
Q. Is Pluto really named after Percival Lowell?
A. Sort of. As is well known, the name was suggested by an 11-year-old girl from England, but one of the reasons the Lowell Observatory's astronomers chose it over other proposals was that they liked the P-L initial letters.
But in all my reading on Pluto, then and since, I've never seen any straightforward answers to some basic questions I had about its discovery. Until now. A new book called The Case for Pluto by Alan Boyle (Wiley, 2010), which I found in the public library, answered them simply.
Astronomers suspected the existence of a ninth planet in the first place because of some discrepancies in Uranus's orbit which could be caused by gravitational influence from another large planet outside its orbit. Indeed, such discrepancies had already successfully produced Neptune. Someone calculated where a planet that would produce these effects should be located, someone else looked at that spot in the sky, and sure enough, there it was. But it turned out there were some discrepancies left over, and the hunt was on for the next gas giant. Instead, they found Pluto, which was too small to account for the equations' needs, and turned out to be smaller still every time astronomers measured it more accurately. So my first question was,
Q. If Pluto was too small to account for the remaining discrepancies in Uranus's orbit, what did account for them?
A. There weren't any. They were the result of miscalculations of Neptune's mass and orbit. This was suspected after Pluto's size was estimated, and proved when the results of the Voyager flyby came in.
Q. If Pluto didn't have anything to do with Uranus's orbit, what was it doing in just the right spot to be responsible for it?
A. It wasn't there. Neptune had indeed been discovered this way, but looking for Planet 9 in the calculated (or repeatedly re-calculated, after it repeatedly didn't turn up) spot is why Percival Lowell never found it. When Clyde Tombaugh resumed the search some years after Lowell's death, he wasn't using any calculations. He was just scouring any old place along the ecliptic where there might be a planet, and it was sheer luck that he found one. It was coincidence that it did match one of Lowell's search spots, and it turned out that Lowell had actually photographed it, but hadn't noticed it because it was far too dim to be the gas giant he thought he was looking for.
Q. If Tombaugh found Pluto by happenstance, how do we know there weren't other bodies in similar orbits findable at the time?
A. Partly because Tombaugh kept looking. He kept up his systematic sky survey for another dozen years and found plenty of asteroids, but no planets. Also partly because Pluto's orbit is shaped by orbital mechanics involving Neptune, and there's a limited number of places where a stable orbit of a Pluto-like object at such a distance could exist. In the last 20 years, we've discovered a large number of objects outside Neptune's orbit, and they are collectively known as the Kuiper Belt. But Pluto is by far the brightest, and the others are too faint to have been findable by Tombaugh's telescope.
Q. Is Pluto really named after Percival Lowell?
A. Sort of. As is well known, the name was suggested by an 11-year-old girl from England, but one of the reasons the Lowell Observatory's astronomers chose it over other proposals was that they liked the P-L initial letters.
