How I Killed Pluto and Why It Had It Coming - Mike Brown [11]
The eighth planet? Ceres? Most people today have never heard of “planet Ceres,” but there was little question at the time that Ceres was indeed a planet. Within a few years it could be found in all astronomy textbooks, alongside Uranus and the others. In keeping with tradition, the element cerium, discovered two years later, was named for the new planet. Most people today have never heard of the element cerium, either, but it is used in the walls of most self-cleaning ovens.
Planet Ceres’s problems began just a year after its discovery, when German astronomer Heinrich Olbers, investigating the new planet with his telescope, accidentally stumbled upon yet another unknown object wandering through the sky: the ninth planet, Pallas! Again, there was little question that Pallas was the ninth planet, and the element palladium was named for it in 1803.
Ceres and Pallas, though considered full-fledged planets, had a few puzzling properties. While all of the other planets were well spaced in their orbits around the sun, Ceres and Pallas were, in the cosmic scheme of things, right on top of each other between Mars and Jupiter. They were different from the other planets in other ways, too. The recently discovered Uranus was too faint to be seen without a telescope simply because it was so far past Saturn. With the aid of a telescope, though, the green outline of the disk of Uranus was apparent. But Ceres and Pallas were closer to us than Jupiter, closer than Saturn. They could not be seen without the aid of a telescope not because they were far away, but simply because they were so small compared to all of the other planets. They were so small, in fact, that even with the best telescopes of the day they just looked like little points of light. Herschel, the discoverer of Uranus—wanting to preserve the uniqueness of his own discovery, I suspect—coined the term asteroid (“aster” is Greek for “star,” as in astronomy, while “oid” means “like”) to describe these new objects. To Herschel, Ceres and Pallas weren’t like real planets with their visible disks; they appeared “starlike” instead.
Astronomers quickly found two more planets in this same region between Mars and Jupiter—the tenth planet, Juno, in 1804 and the eleventh planet, Vesta, in 1807—and then, for almost forty years, nothing new came along. This was too many new planets for some people, chemists in particular. There are no elements named after Juno or Vesta. But still, forty years was long enough for the eleven-planet solar system to firmly emplace itself into the teachings of the day. In a secondary school text from 1837, the chapter between “The fourth planet, Mars” and “The ninth planet, Jupiter” is simply called “The fifth, sixth, seventh, and eighth planets.” The schoolkids who had learned about the eleven planets were probably unhappy with what was about to come.
I have never seen these fifth, sixth, seventh, or eighth planets, even though they are just as easy to see in my binoculars as the satellites of Jupiter, and I look at the satellites of Jupiter in my binoculars all the time. In fact, I love a solar system tour with good binoculars. The rings of Saturn pop out, as does the redness of Mars, and sometimes even the little crescent-moon-shaped sliver of Venus that proved to Galileo that Venus orbits the sun. I can explore the craters and mountains and shadows on the moon for hours. I’ve carefully tracked down the position of Uranus and stared at it several nights in a row just to experience how Herschel might have felt about his discovery. But I’ve never even thought to look for any of these objects that were the most exciting astronomical discoveries of the early nineteenth century.
The reason I’ve never looked for these four individuals, I think, is that just as the four new small planets were becoming accepted as part of our understanding of the universe, a deluge of new objects started to be discovered. By 1851, fifteen more of the new asteroid planets were found, as well as one more large planet—Neptune. Neptune was even