Why Isn’t Pluto a Planet?


So y’all know that astronomy is very near and dear and close to my heart, and with all the HOOPLAH about Pluto in the news lately, I thought I’d share some cool facts about Pluto and dwarf planets and why they’re categorized the way they are.

And also, to be clear, this is the categorization given to Pluto by the International Astronomical Union (wiki), and is the way astronomers refer to it. Obviously, Pluto is culturally and sentimentally significant to a lot of us, having been discovered over 80 years ago now. Many of us grew up learning about Pluto and its discovery; it has its own astronomical symbol (image), and even the Magic Schoolbus went there! This classification just indicates that we now understand more about Pluto than we did before. If you are not an astronomer and wanna keep calling it a planet from the bottom of your heart, than by all means go ahead! Humanity’s history and fascination with it has given you that right.

Pluto was first discovered in 1930 when astronomers made a miscalculation in Neptune’s orbit. They thought they had discovered an anomaly which would indicate a large object in our solar system past Neptune. It was in searching for this large object that they discovered something in the general area which had a large proper motion (image), meaning it was very close to us, at least compared to the stars around it. Many names for it were discarded until a Miss Venetia Burney, an 11-year-old from Oxford, suggested the name to her grandfather, who passed it on to astronomers in the US.

Pluto was at first considered a planet, as at the time there was no concrete astronomical definition. In 2006 the IAU named three criteria for what it takes to be a capital-p Planet, which excluded Pluto. Some opposed this exclusion, but the argument was that if we were to include Pluto in our definition of a planet, we would also need to include several other objects in our solar system, more and more of which are being discovered every day. As a sort of consolation prize, the new classification “plutoid" (wiki) was created, referring to all dwarf planets beyond Neptune’s orbit.

Here are the three criteria for planets:

  1. the object must be in orbit around the Sun
  2. the object must have enough mass to become spherical
  3. the object must have “cleared the neighborhood” in its orbit

Pluto fulfills two of these three criteria, being a spherical object obviously in orbit around our Sun, but the last point is where dwarf planets fail the test.

A clear path of orbit for a planet indicates that it is past its last stages of formation, and is the gravitationally dominant object (wiki) in its orbital zone, meaning that all objects in its path are controlled by that planet. Pluto fails to meet this criteria because it is a member of the Kuiper Belt (wiki), a sort of inner-tube-shaped ring of asteroids beyond the orbit of Neptune.

Pluto also has another oddity which indicates that it did not form in the same way as the rest of the planets in our solar system: its orbit. Pluto’s orbit has a large eccentricity (~0.25), making it so elliptical that it at times is closer to the Sun than Neptune. This orbit is sometimes describe as “crossing” Neptune’s, but in reality the two will never be in contact with each other. Pluto’s orbit is also in a different plane than the rest of our planets’; when viewed from the side it seems to come in at an angle (image). This defies our current understanding of how planets were formed in our solar system.

And Pluto isn’t the only dwarf planet either. We have discovered FIVE dwarf planets in our solar system so far, with presumably more on the way. They’re all smaller than our Moon, and they’re all, in my opinion, just as interesting and quirky as Pluto. They are:

  1. Haumea (with its moons Namaka and Hi’iaka). It’s known for its oval EGG shape due to its rapid rotation.
  2. Makemake (no known moons). Its orbit out of all the dwarf planets in our solar system is the most out of line, with an incline of almost 30 degrees.
  3. Eris (with its moon Dysnomia). It was almost named Xena (yes, THAT Xena!) and it’s moon almost became Gabrielle.
  4. Ceres (no moons). Its story closely follows the story of our dear Pluto, having been discovered in 1801 and originally classified as a planet, until several more large asteroids were discovered in the area. It’s the largest object in the Asteroid Belt and the closest dwarf planet to us.
  5. And there’s also… Charon! Since the moon is roughly half the size of Pluto, they are sometimes described as being in a binary system. This seems to fit, as neither of these objects orbit each other; rather, they orbit a center of mass in the middle, which is also orbited by Pluto’s other moons Nix, Hydra, Kerberos, and Styx.

So that’s why Pluto is a planet, but still not a Planet, and some other dwarf planets that were also kicked out of the Planet Club. Feel free to spread this around as space is REALLY, REALLY AWESOME and it’s fun to learn about it!

If your heart is still set on Pluto, though, keep an eye on the news around July of next year, 2015, as the NASA space probe New Horizons (wiki) will be doing a flyby to hopefully take lots of cool pictures and reveal some more about our beloved icy friend, as well as some other objects in the Kuiper Belt.


starkening asked:

PLEASE RANT TO ME ABT IM3 IM JUST MAD ABT HIM BLOWING UP HIS SUITS (tony would never in a billion years do that nobody can convince me otherwise) AND THE MANDARIN NOT BEING REAL WHAT ELSE THOUGH

thedirectorstark answered:

well ok since i’m being enabled…….

hey krusca. dis for u too

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holy shit thedirectorstark hits the nail on the head again

also the thing about tony brushing harley off really bugged me i literally had to draw an entire comic to fix it and to support the headcanon of how im3 was told in the viewpoint of an unreliably narrator (tony) this is literally the only way i can watch im3 now the pacing and tonality of the movie felt so erratic to me


AND THE PEPPERONY IN THIS MOVIE oh my go d im3 is literally why i think that ship wouldn’t work long term im3 was teh nail in the coffin for me and you just explained it perfectly bless u (i am so so bothered that pepper actually DIED or would have if it werent for extremis she was treated to against her will how would tony be ok with that how would pepper be ok with that you cant just get over shit like near death bc of boyfriend’s stupidity announcing his address to terrorists on tv that easily ok there are so many emotional components to this movie that were overlooked or ignores probs bc we cant fit all that into a 2 hour movie but we can fit killing off maya hansen a significant female mastermind anYWAYS) 

read this this is literally why i do not like im3 it had so much potential it couldve been greAT but no they used wipe transitions and made everything into a joke thanks 4 nothing marvel