You might have heard that Pluto was demoted a couple of years ago. (The object in outer space, not Mickey Mouse’s dog.)
Pluto had been discovered in 1930 by Clyde Tombaugh, a young employee of Lowell Observatory in Texas, as a result of a search for a “missing” planet that was disturbing the orbits of the other outer planets. It actually turned out that there was no missing planet: the astronomers’ calculations had been wrong. Pluto had merely been in the right place at the right time.
The name of the new planet was suggested by a scholar with interests in both astronomy and classical myths: Venetia Burney, aged 11. Pluto, of course was the Greek God of the Underworld.
In 1930, Pluto was just a speck on a long-exposure photographic plate. Although it was now realised that Pluto could not be big and heavy enough to affect the orbits of the other planets, astronomers still were not able to make a true estimate of its size, and as late as 1971 guessed that it was probably as big as Mars.
Boy, were they wrong. Improved telescopes spotted that Pluto has a moon (named Charon, the ferryman of the dead) and observation of its orbit allowed Pluto’s and Charon’s masses and sizes to be worked out accurately from about 1978. Pluto is only 2300 km in diameter, much smaller than Earth or Mars, and even smaller than our Moon (about 3400 km).
That was why astronomers eventually decided that it was silly to keep calling Pluto a planet, and invented the term “dwarf planet”. Pluto was officially a dwarf. But more humiliation was to come. Pluto probably isn’t even the biggest dwarf planet. Out in the darkness, in Pluto’s own domain, Eris (Greek Goddess of Discord) was discovered in 2003: over 25% heavier. Thus Eris is probably physically larger than Pluto, unless it turns out to be composed of much denser material.
The astronomers who discovered Eris had actually been using a working name of Xena [Warrior Princess] for her, but that wouldn’t have been allowed as an offical title. They did get to name Eris’s small moon Dysnomia, though: Lawless-ness.
The rule for naming outer dwarfs is that they should be deities associated with a creation myth. For example, the two next largest are Makemake from Easter Island mythology and Haumea from Hawaii. That still leaves many for subsequent discoveries. Jehova, maybe? Flying Spaghetti Monster?
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