I knew this strange, grey world was not my own,
But Yuggoth, past the starry voids
— Fungi from Yuggoth, H. P. Lovecraft
The perturbations in Saturn's orbit led to the discovery of Uranus. The perturbations in Uranus's orbit led to the discovery of Neptune. And then it seemed that Neptune's orbit also had perturbations, so astronomers followed the trail of gravitational clues still further out, looking for yet another planet, "Planet X," the so-far-unknown.
The planet was found in 1930 by Clyde Tombaugh, at the Lowell Observatory in Flagstaff, Arizona. It was a heroic effort, taking hundreds of hours comparing before and after shots of the suspect part of the sky, to see if any of the dots had moved – completely without computers.
Eventually, Tombaugh found the dot that moved. It was christened "Pluto." The discovery made world-wide news and Disney Studios may have named Mickey's new pet dog after it (though the Disney folk say they can't find any definite records on the subject). Planet X had been found.
Or had it? Pluto is very far away, and it's very dark out there, so far from the Sun. It was hard to say how big it was. Every time the measurements got better, Pluto got smaller. When I was a kid, Mercury was known as the smallest planet in the Solar System, but after a few more years, Pluto was smallest. Clearly, it was too small to be perturbing the orbit of Neptune. Was there another Planet X out there, X for Roman numeral ten as well as unknown, perturbing the orbit?
Well, Neptune is hard to observe, too, though much easier than Pluto. As measurements of its mass and movements refined, the perturbations simply went away. Nothing was perturbing its orbit. There was no need to suppose a Planet X. Pluto had been discovered by lucky accident.
Meanwhile, Pluto had been having an active career in science fiction. As the furthest, coldest, darkest planet, it had a fascination similar to that of Antarctica in earthbound adventure tales, a great example of the Back of Beyond. E. E. Smith put a colony of four-dimensional Palanians there, in his Lensman series. Robert Heinlein used it as a staging area for the invading "wormface" aliens, in Have Spacesuit, Will Travel. Most famously, H. P. Lovecraft made it the home of intelligent space-flying fungi, who called it "Yuggoth."
Pluto is so far away, we had only the blurriest images of it, even with our best telescopes. Finally, in 2015, the New Horizons probe gave us a clear view of the last known planet.
But was it the last planet? Was it really a planet at all?
After decades of speculation, telescopes got good enough and astronomers finally began discovering more bodies, in Pluto's general part of the Solar System:
...and so on.
All these bodies, including Pluto, lie in what is call the Kuiper belt – pronounced "KY-per belt," and named after astronomer Gerard Kuiper. Comets come in two types: those that come back every few decades or centuries, like Halley's Comet, and those that show up once and, so far as we know, never again. Kuiper predicted that repeating comets come from a belt beyond Pluto.
The new bodies, Quaoar et al., confirmed the existence of the Kuiper belt. This also made Pluto look less like a planet, with its own orbit and all, and more like a large member of an asteroid belt, like Ceres. (And Ceres used to be called a planet, too.)
Then they discovered Eris, distinctly larger than Pluto and named after the goddess of discord. Very appropriately. Instead of just being the tenth planet, Eris started people arguing over whether Pluto is a planet.
We know of many object, now, out there in the same orbital neighborhood as Pluto, smaller than it (except for Eris), but still big enough for their own gravity to pull them into a spherical shape. Were they planets because they were big enough to be round? But then, there were round asteroids too, and these Kuiper belt objects look rather like icy asteroids more than planets.
Turns out, there had never been a formal, official definition of "planet." The International Astronomical Society decided they had to have one, and they wanted to exclude Pluto, since any definition that let Pluto be a planet would let a large number of other objects be planets too. Who wants a planet list longer than the alphabet?
So a "planet" is now officially something that orbits the Sun (or other star), is big enough to make itself spherical by gravity, and has been able to clear its own orbital neighborhood. This makes planethood depend on situation. But then so does moonhood; an object larger than Earth would still be a moon if it were orbiting a still larger planet, say a gas giant like Jupiter.
The IAS went on to create the category "dwarf planet" and awarded that status to Pluto as a sort of consolation prize. A dwarf planet is big enough to be spherical, but has not cleared out its orbit. The list of known dwarf planets is: Ceres, Pluto, Haumea, Makemake, and Eris. Four other objects may also qualify: Orcus, Quaoar, Sedna, and the still-unnamed "2007 OR10."
Many people resent this "demotion" of Pluto. I've even seen bumper stickers about it. They'd probably be happy to let Eris be the tenth planet and let it go at that. Let planethood be a matter of tradition. None of this affect Pluto, of course; it's just a matter of the labels we make. A geologist, watching the furor, waspishly remarked that geology never found it necessary to define "continent," after all.
The Kuiper belt still isn't the end of the Solar System. But the next frontier is a long way out – the outer edge of the Oort cloud, named after Jan Hendrik Oort, a Dutch astronomer who theorized its existence. In fact, it is still theoretical. It is a great, thinly scattered spherical swarm of comets left over from the formation of the Solar System. It is the theoretical source of "long-period" comets. They circle for long, quiet ages, then get jostled out of orbit by some gravitational flutter and fall into the inner system, appearing unexpectedly, then leaving for more ages.
The Kuiper belt stretches from roughly the orbit of Pluto to about twice that distance. The Oort cloud is thought to be about a thousand times as wide. That's about a light-year. And out there, at the edge of the Oort cloud, the Solar System finally ends.
But I'm not quite done...
On to the Legendary Solar System
Back to Neptune
Return to Gallery of Planets
Return to Introduction to Essays
Return to Wind Off the Hilltop
Copyright © Earl Wajenberg, 2012