What is the story behind Pluto?

Answer 1

Pluto is a little chunk of ice and rock out beyond Neptune.

Uranus was found by accident, more or less. William Herschel, who discovered it, was doing some parallax studies on stars, and when he went to observe the parallax of a star he had observed before, he discovered that it had moved significantly ... more than could be accounted for by parallax. He therefore supposed it to be a comet, but eventually it became clear to him that he had discovered a seventh planet in the Solar System. He called it by the rather ungainly name of Georgium Sidus (George's Star), after King George III ... Hershel was British.

Nobody outside of Great Britain liked this name much, and several other names were proposed, among them Herschel and Neptune. Johann Bode argued that since Saturn was both Jupiter's father in mythology and the name of the next planet out from the planet Jupiter, the next planet out from Saturn should be named after Saturn's father, Uranus. This name eventually stuck.

Studies were done of its orbit, and it was discovered that it wasn't moving quite the way it should, even after accounting for the gravitational effects of Jupiter and Saturn. It was therefore supposed that there must be an eighth planet out beyond Uranus (or, as the British increasingly desperately insisted on calling it, Georgium Sidus). The location such a hypothetical planet would have to have was calculated by a Frenchman, Urbain Le Verrier, and when the German astronomer Johann Galle looked, he found another planet within a degree of Le Verrier's prediction.

Galle suggested Janus for the name of the eighth planet. Le Verrier proposed Neptune, arguing that since he was the one who had calculated its position, he should get to name it. He then changed his mind and tried to name it Le Verrier, after himself, but this proposal was not well liked outside of France. French astronomers attempted to revive Herschel as the name of the seventh planet to give legitimacy to Le Verrier's claim, but eventually Neptune became the name generally used for the eighth planet.

Oh, right, Pluto. We're getting there.

See, Neptune's orbit didn't quite match up with theory, either. The success in predicting Neptune's existence led some people to attempt to repeat this feat by calculating the position of the hypothetical "planet X" that was perturbing Neptune's orbit.

It was calculated that planet X must have about the same mass as the Earth and be located in a particular region of the sky. Several astronomers looked in vain until Clyde Tombaugh of the Lowell Observatory discovered, by comparing photographic images taken several nights apart, a moving "star" that he concluded must be Planet X. He suggested it be given a better name than Planet X, but didn't propose any; Percival Lowell's widow Constance Lowell suggested Zeus, then Percival, and finally Constance but none of these were taken seriously (Zeus is Greek, while all the other planets have Latin names; not only that, but Zeus was the Greek name for Jupiter... "Percival" and "Constance" were disregarded for reasons we hope are obvious). The name was finally chosen in a contest; the winning suggestion... Pluto, the Roman god of the underworld... seemed appropriate for such a dark, dismal planet, and the fact that the name started with Percival Lowell's monogram probably didn't hurt.

This is a great little story of the triumph of mathematical prediction... except for one thing. The more we learned about Pluto, the less likely it seemed that it could possibly affect Neptune's orbit. Its size was revised downward from about the size of Earth, to about the size of Mars, to, eventually, significantly smaller than the Moon ... way too small to have any noticeable affect on the much more massive Neptune.

So what was wrong with Neptune's orbit? As it turns out, nothing. The initial observations weren't very good, and later observations and more precise calculations showed that Neptune was moving precisely as it should be, with no need for Pluto or any other hypothetical Planet X. The tiny little ball of ice and rock that we call Pluto just happened to be in about the right place at the right time for Tombaugh, searching diligently for anything moving in that region, to spot it. We now know that there are lots of bodies out there only slightly smaller than, or in one case even slightly bigger than, Pluto, and it was pure luck that one of them ... Pluto ... happened to appear on Tombaugh's plates.

Pluto is no longer considered a planet; it's officially designated a "dwarf planet", which means it's big enough for gravity to pull it into a ball but not so large that it "clears its orbit" ... in fact, its orbit crosses Neptune's, and Neptune so gravitationally dominates the region that Pluto and a group of other bodies collectively called "plutinos" are locked in a 3:2 orbital resonance with Neptune.