Today’s factismal: The Solar System officially had eleven planets 406 years ago today.
Names are a funny thing. Sometimes, they are just meaningless symbols. But many times in science, they are actually carefully crafted to help sort data into information. For example, the word planet has little meaning from a scientific point of view. It has been used variously to mean “anything moving through the sky in an unexpected manner” (the Greeks), “anything orbiting our Sun or orbiting something that orbits our Sun” (during the Renaissance), “anything big that orbits a star” (during the Industrial Revolution) , and “anything that orbits our Sun and cleans up after itself” (the IAU). Thanks to the number of times it has changed, the word planet really means very little in science. But the way that the various things in the Solar System are named has a lot of meaning.
Planets are typically named after a god or goddess. Because the biggest planets were discovered back when the scientists mostly spoke Greek and Latin, the planets were given the names of Greek and Roman gods and goddesses. Our Moon took its official name from Luna, the goddess of the Moon. Saturn was named for the eldest of the Roman gods. Thanks to the speed with which it whipped about, Mercury was named for the fastest Roman god. And so on. And for thousands of years, there were just the seven planets (Luna, Sol, Mercury, Venus, Mars, Jupiter, and Saturn).
But then Galileo came along and he started breaking the rules. Using the telescope that he invented, Galileo found four more planets. He called them Medici planets I, II, III, and IV in a successful attempt to get more funding from his patron. And everyone accepted that those planets were indeed planets, including Medici planet IV which was discovered 406 years ago today. They called them planets even though they orbited another planet (Jupiter).
And while everyone called Galileo’s discovery “planets”, they weren’t too fond of the names Galileo had given them. And so the names were changed to be lovers of Jupiter. After just a few years, nobody talked about Medici planets I, II, III, and IV; instead, they bragged about seeing Io, Europa, Ganymede, and Callisto. Interestingly, the names then fell out of favor for nearly 300 years. It wasn’t until the early 1900’s that the old new names were used.
Now just finding Callisto was amazing. But even more astonishing are the facts about Callisto. Though fiery Io and frozen Europa get all of the press, Callisto is well worth knowing. It is about as large as Mercury but has just 1/3 of Mercury’s mass. That’s because Callisto is made up of a small rocky core wrapped in a thick slushy ocean and covered with a thin, icy crust. Europa, which has a similar structure, is just 1/4 the volume and 2/3 the mass of Callisto. And Ganymede is even larger than Callisto!
If you’d like to try finding Galileo’s planets yourself, all you need is a good pair of binoculars and a dark night when Jupiter is visible. Look at Jupiter through the binoculars and you’ll get a better view of Io, Europa, Ganymede, and Callisto than Galileo ever did. And if you’d like to try finding a new planet yourself, why not head to Planet Hunters? They are using Kepler data to identify new worlds around other stars.