5 Replies to “Don’t Worry Pluto, I’m Not A Planet Either”

    1. Yeah, effectively. After the definition of a planet was established in 2006, it was determined that Pluto (along with Eris – also in the Kuiper Belt, Ceres – in the asteroid belt, and a few others) didn’t meet the “clearing the neighbourhood” criteria, so it is considered a “dwarf planet”.
      I think one of the strangest parts of the whole story is that there wasn’t a real definition of a planet before 2006. That seems kind of late considering we’ve known about planets since Greece. It was just something that everyone took for granted for 2000+ years.

      1. whoa I didn’t know any of this, but isn’t a planet a mass which rotates around the sun? So is Pluto rotating around our sun, even if it takes a long time for its 1 year to complete? Or is it too far out the Kuiper Belt, and therefor it doesn’t revolve around the sun, and is why it’s considered not a planet? Do you know the exact definition of what constitutes a planet or not? Fascinating. Thanks a lot!

  1. When the IAU met in 2006, they decided in order to be a planet, a celestial body had to pass three conditions. 1 – A planet must orbit the sun directly (distinguishing them from moons, which orbit planets). 2 – A planet must be large/massive enough to be a sphere (distinguishing them from asteroids, which don’t have enough gravity to be pulled into a sphere). 3 – A planet must clear its neighbourhood and be the dominant object in its orbit (this is the condition that Pluto and the other dwarf planets fail).

    Objects which pass 1 and 2 but fail 3 are considered dwarf planets.

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