Hey. I’m Eris. No, not the goddess of discord and strife- although a knowledge of Greek mythology is always appreciated. No, I’m the Eris that was called the tenth planet back when I was discovered in 2005 by the astronomer, Mike Brown. But then you pulled the rug out from under me and decided to actually define the term planet officially for the first time. By 2006 I’m just a “dwarf,” “Plutoid,” or “trans-Neptunian object.” So what is a planet? At one point in history, the sun was considered a planet. The Greeks named seven planets: Mercury, Venus, Mars, Jupiter, Saturn, the sun, and the moon. Eventually the Earth became a planet that orbits the sun, and the moon was understood to orbit the Earth. Now the associate director of IAU (the International Astronomical Union) Gareth Williams and astronomers like Mike Brown might argue that celestial bodies such as myself and Pluto aren’t planets, but I actually agree with scientists Kirby Runyon and Alan Stern who say not only are we planets, but that you earthers are missing out on about one hundred more awesome celestial bodies by denying them planethood. What I’m saying is that Pluto is a planet and I am too.
In fact, when I was first discovered I was initially called the tenth planet of the solar system- but certain scientists said that was inaccurate. You see, back when Pluto was discovered in 1930, there really wasn’t a working definition of the term planet. It was basically assumed you would recognize one if you saw it. So then here I come along and I’m 27% more massive than Pluto. I’m the ninth most massive object directly orbiting your sun. Obviously I’m a planet, right? Astrophysicist Ethan Siegel says no. According to Siegel, “The simple fact is that Pluto was misclassified when it was first discovered: it was never on the same footing as the other eight worlds…” (“Welcome Back, Pluto?” Wall, space.com) He argues that since Pluto hasn’t ‘cleared its neighborhood’ of other orbiting bodies it doesn’t get to count as a planet. But by that criteria, Earth actually wasn’t a planet for the first 500 million years or so of its existence because its orbit included a swarm of debris. (Stern, Grinspoon, “Welcome Back Pluto”) Further, if Earth were to be moved out past Pluto near the asteroid belt it’s too small to clear its neighborhood out there too. Therefore, by the current definition it would cease to be a planet. Also, Earth, Mars, Jupiter and Neptune share their orbits with asteroids even now, so by the current definition they aren’t planets either.
Scientist Kirby Runyon has proposed a geophysical definition of planethood that boils down to a spheroid “sub-stellar mass body that has never undergone nuclear fusion” (Azumbuja, Scientists Fight). Basically it needs to be roughly round in shape and isn’t star. This differs from the IAU’s definition because it doesn’t rely on what may or may not be orbiting nearby, it’s just focused on what the actual object is in and of itself. Critics of this definition argue that this would create too many planets, possibly even including our own moon and several of the moons orbiting gas giants like Jupiter and Saturn. While it’s true that number would sit around 110 total planets (so far!), aerospace engineer and planetary scientist Alan Stern says it’s all for the better. He thinks the more planets invite greater inspiration of curiosity and more opportunity for exploration and study. In fact, a common question he is asked is, ”Why would we go study places like Pluto if it’s not a planet?” It’s clear the impact of bestowing the title of “planet” to a celestial object has on human interest in that object. While it might be hard to memorize all of those 110 planets, there is actually very little reason to so with the internet so easily accessible and apps so readily available on smartphones, like star charts and space encyclopedias. There is no real reason to limit the number of ‘true planets’ to eight.
Then there is the cultural nostalgia for Pluto. Astronomer Mike Brown posted on Twitter (where he goes by @plutokiller) that, “Nostalgia for Pluto is really not a very good planet argument, but that’s basically all there is.” I say he’s wrong, that the sentimentality for Pluto is an excellent reason to keep it a planet and the weight the term carries is a very important reason to do so. The term planet gives a relevancy to a celestial body that it few other words do. It inspires curiosity and intrigue. People want to know what that planet is like, what is its composition? How long is it’s orbit? What are it’s moons like? Does it have seasons? Lots of people know that it likely rains diamonds on Neptune and Uranus. Many also know that Pluto may be up to one third water or that it has five moons and is sometimes considered a ‘binary planet’ because one of its moons, Charon, is almost as massive as Pluto itself. But perhaps you didn’t know that I, Eris, actually orbit closer to the sun that Pluto at certain times and that all of the objects in the Asteroid Belt together are roughly equivalent in volume to me… Or maybe you’ve never even heard of me because I’m not considered a planet! According to astronomer and historian Owen Gingerich, “Planet is a culturally defined word that has changed its meaning over and over again.” (“Planet-or-Not Debate,” Drake, NationalGeographic.com) But while the meaning may have changed, the weight the term carries has not. So while Pluto was only a planet for 75 years, the nostalgia that Planet Pluto carries is relevant and keeps it current in the hearts and minds of adults who remember the wonder and curiosity the little frozen world inspired, cast so far from your sun.
Therefore I say that Pluto and I are planets. The term planet carries psychological weight, which may cause the study of the solar system and beyond to be limited by what society thinks isn’t important. The current definition was only voted on by a small number of professionals involved in astro sciences and has continuously been hotly debated since the IAU demoted Pluto in 2006. By the current definition, which includes the criteria of clearing your neighborhood around your orbit, several ‘true planets’ technically aren’t or wouldn’t be under slightly different circumstances. Pluto carries cultural significance that shouldn’t be dismissed, even as understanding in planetary science is always growing and adapting. Further, while some may say it’s important to keep the number of planets down to something manageable like eight, it is an arbitrary number and excludes many important and interesting celestial bodies. There is no good reason to limit the number of planets, and 110 reasons to expand the idea of what a planet could be and inspire curiosity and imagination. Really, can’t we all just get along? I know that’s ironic coming from the planet named after the goddess of discord and strife, but there are some things we should agree on. For example, “Eris is the tenth planet!” There, wasn’t that easy?