As most everybody knows, there are eight planets. This is according to the International Astronomical Union, which famously demoted beloved Pluto back in 2006 to a “dwarf planet”. But the definition of planet changes so often, that there is really no one way to define a planet.
To the ancient Greeks and Romans, a planet meant “any object that moved in the sky that wasn’t a bird”. This meant there were seven, the seven planets famously depicted in the Ptolmatic Model (by Ptolmey): Luna (moon), Mercury, Venus, Sol (sun), Mars, Jupiter, and Saturn. These all orbited the Earth, because they all appeared to move in the sky, and were thus planets. This model made the most sense and stuck around until a fellow named Copernicus noticed that Ptolmey’s model had some fancy math error that could be rectified by a heliocentric model of the universe. This is why the Sun and Moon move differently in the sky from all the other planets, since they orbit the Sun, and not the Earth. From this model there were only 5 planets, and the definition expanded to mean “a moving object in the sky that orbits the Sun”.
Then in the 19th century, as telescopes improved, a whole bunch of itty bitty planets were discovered. At the time, Uranus was newly discovered by William Herschel (who wanted to call it George’s Star, after the king). So when Giuseppi Piazzi found a dinky little planet between Mars and Jupiter in 1801, the scientific community were eager to add it to the official list, to be the 9th planet. They called it Ceres, in the tradition of naming planets after Roman gods. Then the next year another was found, but it was a lot smaller. And then another. And another. By this point astronomers were scratching their heads at these little oblong (except for Ceres) lumps that were all clustered together, and decided to call them “asteroids” (meaning star-like). Soon after, in 1846, a new planet (big this time) was discovered beyond Uranus, and they named it Neptune.
For years afterwards, astronomers studied the orbit of Neptune and realized that the orbit was slightly affected. They assumed that this was because of the influence of a large body’s gravity on it. They called this theoretical planet Planet X. The idea was Percival Lowell’s, and he poured a lot of money into finding it. And then, who should find the planet, but a new guy at the observatory, who was comparing photographs. The world was soon in delight to have found a new planet, despite it not being big enough to be Lowell’s Planet X. They called it Pluto.
From 1930 until 2006, Pluto has enjoyed status as a planet. We all grew up learning mnemotic devices in school to remember the nine planets. However, astronomers always were unsure about Pluto, especially once other objects in the area were found. They were of course much smaller than Pluto, in the same way that Ceres contains an entire third of the mass of the Asteroid Belt. So the Kuiper Belt was discovered, a mass of comets, and icy rocks beyond Neptune. The objects within are normally called Trans-Neptunian Objects (TNOs), and yes, Pluto is included. So why isn’t Pluto a planet anymore? Well, astronomers figured that despite being tiny (for a planet), and not having a “clear neighborhood” (the official term), it could still be a planet unless something larger than Pluto was found, demoting its special status. While many objects were found that rivaled Pluto in size, and even being given names, none came close to being bigger.
I’ll give you one guess as to what was found in 2005.
Ladies and gentlemen, Eris is the reason Pluto was demoted. It’s not actually that much larger than Pluto, but is twice as far. It even has a moon, appropriately named Dysomnia (moons are usually given names from Greek mythology). As we all know from the “PLUTO IS NOT A PLANET ANYMORE” headlines of 2006, the IAU decided that neither Pluto nor Eris was to be a planet, and a new classification was to be made. Planets had to be 1) in orbit around the Sun, 2) have enough gravity to become round (or close enough), 3) has cleared the neighborhood (meaning that it can’t be in an orbit among a ton of space junk. For those objects that fulfilled 1 and 2, but not 3, the classification of “dwarf planet” was given.
The current definition of Dwarf planet means that there are five official dwarf planets (and several more objects that fit the definition but aren’t on the list) The official ones are Pluto, Eris, Ceres (the only round member of the Asteroid Belt), Haumea, and Makemake. There are four others which are most likely dwarf planets, and they are Orcus, Quaoar, Sedna, and 2007 OR10. In case you were wondering what that last one was, it doesn’t actually have a name yet. The designation is the astronomical designation, since no name has yet been given to 2007 OR10. Interestingly enough, all nine of these bodies have their own Wikipedia pages.
Here is where I make my point: These objects are certainly deserving to be called “planets”. They are big, they are round, and some of them are really really cool. Every planet has its own interesting facts: Venus’s runaway greenhouse effect, Earth’s uniqueness, Uranus’s being sideways and having all its moons named after Shakesperean characters. Pluto and Co. have their own traits of Awesomeness.
First there’s Ceres. It’s a big asteroid that’s big enough to have obtained hydrostatic equilibrium (which is science talk for “it’s round”), and it might even have a water ocean! All the other asteroids look like potatoes (use Mars’s two “moons” as an example). Next there’s Pluto. Oh esteemed Pluto, how it has sat in the hearts of humans for eighty years. What can we say about Pluto? It has five moons, one of which is so big, Charon, that it’s half the mass of Pluto itself, and so the two are a binary system, rotating around each other. It’s orbit is an ellipse, rather than a circle, so for a period of 20 years (recently too, from 1979-1999), it is actually inside Neptune’s orbit! A quarter of its surface is in permanent sun, and another quarter in permanent darkness, due to its rotational axis. And despite being really cold, it even has an atmosphere. Needless to say, Pluto is awesome.
Eris is really far out (although not the furthest on this list), beyond the Kuiper Belt. It orbits at a huge angle compared to the rest of the Solar System (all the planets are coplanar for some reason). Makemake is one of the few dwarf planets with no moons, and is really bright (considering its great distance from the Sun). Haumea looks like an egg, has two moons, and water ice about 80% of the surface (mixed with cyanide). Orcus and Quaoar just have really cool names (not much is known about them, except for Quaoar being bright red). And then there’s Sedna. Sedna tops them all in coolness. Sedna’s orbit is so long, that it takes approximately 11,400 years to complete one. Just look at this diagram for comparison. At the moment, it is at its nearest point (perihelion) to the Sun, which is how it was able to be discovered. It’s so far out, it’s in the darn Oort Cloud at aphelion! The Oort Cloud marks the outer edge of the Solar System. It’s so far, the only reason it’s known to exist is by mathematics, and the evidence of the Voyager probes.
As we can see, all of these minor Solar System bodies are highly underappreciated by the IAU. They ought to be planets. How cool would it be to have 17 planets in the Solar System?