I picked up a compendium of H.P. Lovecraft’s short stories a couple of weeks ago – mainly so that I could amuse myself by playing Lovecraft Bingo (counting how many times he uses words like ‘accursed’, ‘pnakotic’ and ‘squamous’ ) – and ended up being rather amused when I came across the story “The Whisperer in Darkness”. This consists of about sixty pages of the usual confused rambling while the reader waits for him to get to the damn point already, but ultimately turns out to be about advanced aliens from Pluto who have set up a mining colony in the furthest reaches of the American hills that they’re trying to keep secret. This brought a wry smile to my lips for two reasons:
- Lovecraft’s shameless attempt to piggyback off of something that was very much part of the public zeitgeist at the time, what with him starting this story the same month that Pluto was discovered. It’s the literary equivalent of somebody sticking dark matter into their awful sci-fi novel because they think it sounds exotic and dangerous.
- That there was ever a time when people thought Pluto was an important place that might support life of any kind.
The whole Pluto “controversy” a few years ago stems from a variety of roots: a bureaucracy which ran a sloppy vote, scientists with vested interests and pet theories on both sides of the argument, and – bizarrely – a great deal of public sentiment for what is essentially a lump of ice and rock out in the middle of nowhere. If you want to trace it back to the beginning, though, you have to start with the discovery of Pluto in 1930. Are you sitting comfortably? Time to Listen with Mother.
Once upon a time there was a man called Alexis Bouvard. Bouvard was engaged in the tedious task of compiling astronomical tables predicting and describing the orbits of the outer gas giants according to Newtonian mechanics, but he hit a snag when he came to Uranus: it wasn’t orbiting the way Newtonian mechanics said it should. There were only two possible explanations for this: either Newtonian mechanics were wrong (unthinkable at the time, although this eventually turned out to be the case) or else there was some unknown eighth planet orbiting beyond Uranus exerting a gravitational force that was interfering with its orbit. The matter then passed to Urbain le Verrier, a specialist in celestial mechanics, who cranked through some feverish calulations based on nothing more than the minor perturbations in Uranus’s orbit and came up with a predicted position for the hypothetical eighth planet. He mailed his prediction to the Berlin observatory, who duly pointed their telescopes at the appropriate piece of sky and wound up discovering Neptune in 1846.
This was an astonishing achievement for le Verrier, but all was not hunky dory in astronomy land. After some examination of Neptune it was determined that the new planet was, on its own, not enough to account for the irregularities in the orbit of Uranus. This led astronomers to posit the existence of a ninth planet, which they eventually dubbed Planet X.
Fifty years went by. Percival Lowell spent a decade looking for Planet X, a search which ended with his death in 1916. The search stalled for another thirteen years while a legal battle over Lowell’s bequest to his observatory was wrung out, before being handed off to the 23-year old Clyde Tombaugh in 1929. Tombaugh spent the next year examining pairs of photographic plates; each pair of plates was a picture of the same portion of the night sky taken several days apart, and Tombaugh’s job was to pretty much to see if he could spot the difference. If he saw something on the second plate which had moved from its position on the first plate, he had a candidate for Planet X. This was tedious work involving the painstaking examination of hundreds and hundreds of plates, but it eventually paid off and he discovered his candidate body in January 1930.
Everything up till now has been Good Science, but this is where the Bad Science starts. Telescope technology in 1930 wasn’t that great. The new “planet” — named Pluto by an English schoolgirl – appeared as a point of light to astronomers rather than a distinct disc like the other planets. This could mean any or all of a number of things:
a) It wasn’t very big.
b) It was a very very long way away.
c) It wasn’t reflecting much sunlight (it had a low surface albedo).
However, because astronomers thought Pluto must be responsible for the additional perturbations in the orbit of Uranus it followed that it had to have enough mass to exert the level of gravitational force required to do so. This led them to discount point a) and estimate the size of Pluto as being about 8,000 km in diameter – or just a bit larger than Mars. In order for Pluto to be this big it would need a very, very low albedo to account for such a large object reflecting such a tiny amount of sunlight, and so astronomers decided it was about as reflective as a lump of road asphalt. Despite a vocal minority proclaiming that these astronomers were full of shit and that Pluto’s orbital eccentricity and brightness meant it had far more in common with comets than it did with planets, the size estimate and the planet title stuck.
Time continued to pass. Pluto’s size was revised further and further downwards as telescope technology improved and more accurate observations were made. Eventually Pluto’s “moon”, Charon, was discovered in 1978. This allowed astronomers to measure the total mass of the Pluto-Charon system based on their mutual gravitational effect, which turned out to be about five hundred times smaller than that of the Earth. After Charon serendipitously made a series of planar transits across Pluto from 1985-1990 their respective sizes were worked out as well*. Pluto has a volume less than one two-hundredth that of the Earth. It is tiny.
So Pluto was now known to be unlike any other planet in the Solar System. Fortunately (for Pluto) nothing else had yet been found that remotely resembled Pluto, so Pluto remained a planet simply through dint of having been called one so long that nobody could think of a good reason not to. Nothing lasts forever, though, and Pluto should have started looking over its shoulder in 1992 when the catchily-named (15760) 1992 QB1 was discovered. This was the first confirmed occupant of the long-posited region of space called the Kuiper Belt, a population of icy bodies outside the orbit of Neptune which are left over from the formation of the Solar System. I’ll be posting more on that later, but after the Kuiper Belt was discovered and more people started combing it for interesting objects it was only a matter of time before somebody found something in it that was decidedly Pluto-like. Quaoar, Haumea, Ixion and Varuna were all discovered in the period between 2000 and 2005, and all of them had sizes comparable to – but not exceeding – that of Pluto (between 500 km and 1,200 km diameter; Pluto has a diameter of 2,000 km). The straw that broke the camel’s back was the discovery of Eris in 2005; this had a diameter of 2,400 km, making it some 25% larger than Pluto.
Eris really brought things to a head. After the size downgrades certain segments of the astronomical community had been complaining for years that Pluto wasn’t a planet at all, but Eris presented the IAU with an outright logical conundrum. Scientists love consistency. To call Pluto a planet would also mean calling Eris a planet. But if they called Eris a planet there’d be no good reason not to call the other four planets as well – they were all orbiting in the same area of the solar system and they were all of comparable size, so why not? And – even worse – it was almost certain that astronomers would go on finding these big Pluto-like objects as more people looked with better instruments. Do they call all of them planets? If they did, it’d be entirely possible that in a hundred years time we’d end up with a list of planets that was 100-200 items long.
And so the IAU conference in 2006 to decide what exactly constituted a planet and what didn’t. Exactly what happened at that conference I do not know and there are some scientists (notably Alan Stern, the principal investigator behind the New Horizons probe that’ll be arriving at Pluto in 2015) who are still very bitter over its result, which was that the largest of the Pluto-like bodies (the ones with the most accurate size measurements) were to be placed in a new category of objects called “dwarf planets”. Further, Pluto itself was to be demoted from planet status and put in the category of dwarf planet.
Now, you can argue all you want about how good the new definitions for dwarf planets and planets are is. There’s definitely an argument to be had since the two defining criteria of a planet are now that it is “is in hydrostatic equilibrium” and “has cleared the immediate area around its orbit”, but nobody really knows what the hell the second one means given that a strict reading of it leads to us discounting Earth as a planet given the existence of 3753 Cruithne and other similar bodies, not to mention the Jupiter Trojans. However, what is clear to me after studying the Kuiper belt for most of the last five years is that Pluto is rather obviously a member of this discrete population of objects and not a planet in its own right. It formed with the belt, it evolved with the belt and it’ll die with the belt. Pluto might or might not be a dwarf planet, but what it definitely isn’t is a planet.
Which is why the outrage over Pluto’s demotion eluded — and continues to elude – me. Even I get a little bit unsettled when I run into schoolkids who have been taught from day one that Pluto is a dwarf planet and nothing else, but the scientific argument for it no longer being a planet is irrefutable. When respected scientists like Alan Stern come out with something like
“Firstly, it is impossible and contrived to put a dividing line between dwarf planets and planets. It’s as if we declared people not people for some arbitrary reason, like ‘they tend to live in groups’.”
My response is an entirely predictable collection of four-letter words. It is not in the least bit contrived to draw a distinction between planets and dwarf planets, just as it is not contrived to draw a distinction between adults and children, or between gas giants and terrestrial planets, We called Ceres an asteroid for two hundred years simply because we had nothing else to call it, even though it was demonstrably different from everything else in the asteroid belt, and we didn’t start calling it a planet when we figured out it was round. Pluto is demonstrably different from a comet, or a small Kuiper Belt Object (they’re pretty much the same thing), but that doesn’t mean it automatically qualifies as a planet since it’s demonstrably different from those as well. So if you ask me, the classification of dwarf planet works quite well even if the definition does not.
But then the media and the public came up with far more spurious reasons as to why Pluto shouldn’t be demoted and should instead be “grandfathered” in as a planet, as if science worked that way. It’s been a planet for seventy-five years! It’ll upset schoolkids who will have to find a new mnemonic to remember the planet names! We’ll have to rewrite all the textbooks! Cry me a river, guys; nobody discounted tectonic plate theory just because it would have been a pain in the ass to change our scientific paradigm of the interior of the earth. There’s always cultural resistance to a new idea but it still surprised me in this case because they were kicking up a fuss over basically nothing. Pluto is the same object discovered by Clyde Tombaugh eighty-two years ago. We just call it something different now, that’s all.
Oh, and I nearly forgot to mention: those peturbations in the orbit of Uranus that started this whole mess? When the Voyager 2 probe flew by Neptune in 1989 astronomers were able to use its orbital trajectory to get a more accurate measurement of Neptune’s mass, which turned out to be 0.5% less than previously thought. This meant that Neptune on its own was now enough to account for the variations in the orbit of Uranus, and that ultimately the entire Pluto = planet debacle has been the result of science running down this massive blind alley.
(Of course nobody ever thinks about the real victims of the IAU’s decision: the astrologers.)
P.S – I’ll post later on in the month in more detail about some of the science behind why Pluto is so different and the general mechanics & formation of the Kuiper belt/scattered disc. I’m by no means done on this subject.
*In detail: sometimes a planet or a satellite will move in front of some light-emitting or –reflecting object, blocking some of the light we see from that object. If the planet/satellite is close enough to us to block out all the light from the object, this is called occultation**. Solar eclipses are the most obvious example of occultation, but stars are small enough to be occluded all the time by various Solar System objects. Charon occluded a star in 1980, allowing an estimate of its diameter to be made – astronomers knew how fast Charon was moving and they knew how long it took to move from one side of the star to the other, which gave them the size.
Now, if the blocking object appears smaller to us than the light-emitting object, all it will do is block a portion of the object’s light and so reduce the intensity of the light we receive from that object. This is called a transit, and this is what Charon did to Pluto a number of times between 1985 and 1990 – it passed between Pluto and the Earth, which was remarkably lucky given the precession of Charon’s orbit means that happens only once every hundred years or so. Astronomers measured the decrease in light intensity as Charon moved across Pluto, and since they already knew how big Charon was from the 1980 occultation they were able to use the transit time to work out the size of Pluto.
**Not to be confused with occlusion, which is a word dentists use to describe how the top and bottom teeth fit together when your mouth is shut.