Why Isn’t Pluto a Planet?

To be a planet a solar object needs to meet 3 criteria:

• It needs to orbit the sun.
• It needs to have enough mass to crush itself into a sphere, to look like a planet.
• It needs to gravitationally dominate the area of its orbit.

As you can see, the last one is the one where Pluto fails.

So Pluto doesn’t dominate it’s orbit…what does that mean?

Each planet in the solar system (through the influence of its gravity) dominates any other object that is in the same basic orbital path as it. That means it either sucked up and merged with everything in the same orbit, or it captured those things into stable orbits such as moons/rings.

As we got better and better at finding things in our solar system, we started to find other objects in similar orbit to Pluto, objects completely independent of Pluto and many of them quite large. It was turning out that Pluto was actually just an asteroid in a very large asteroid belt.

Why not just keep it a planet anyway?

At first they tried to! But then they started finding objects closer and closer to the size of Pluto. If these objects weren’t planets it was hard to call Pluto one. And if we counted every decent size object in the belt (over 70,000 of them)…we’d have a lot of planets…not many of them even conforming to the proper definition of planets! But still scientists basically just called it a Planet for old time’s sake. But then…they discovered an object in the asteroid belt even bigger than Pluto. That was the last nail in the coffin, the choice was then to either add a second planet that wasn’t actually a planet just to cover up the past mistake…or demote Pluto.

Has this ever happened before?

Yes actually. In the early 1800′s people started discovering objects within the asteroid belt between Mars and Jupiter. Of course the 1st one discovered was thought of as a planet (and was named Ceres). But as observations of other objects in the belt grew, it was eventually determined that there were many such objects in an asteroid belt in the region. Ceres remained in textbooks as a planet for half a century, but eventually was demoted to dwarf-planet status in the same way Pluto recently was.

So what is Pluto? Just an Asteroid?

Sort of but not really. When this all happened to Ceres it was eventually given the title dwarf-planet when it was demoted since it adheres to the first 2 criteria but not the last. Pluto was also given dwarf-planet status, and there are now many more known dwarf-planets out in Pluto’s region; including Eris, which is actually larger than Pluto and also has its own moon (named Dysnomia).