Pluto Cut to Clear Solar System Salary Cap Space


People have been asking me about the decision by a council of astronomers to downgrade Pluto from the status of planet. Here are a few of the important issues at play as far as I can tell.

First, why does this matter? It matters because there are alot of objects in the solar system and scientists categorize them. Remember the discovery of the 10th planet? Turns out that there are at least two additional planets in the solar system if the definition includes Pluto, and more are likely to be discovered in the outer edges of the solar system. Having a clear definition of planet that is meaningful and differentiates the true planets from other large objects in the solar system is therefore useful.


Second, what about Pluto got it demoted? Is it too small? Turns out that three criteria are used to define a planet.
1) It orbits the sun. I think this is interesting, because it means that this issule of the definition of planet really only applies to our solar system. There is technically no definition for planets in other star systems.
2) It's gravitational pull is significant enough to form a giant sphere. Note that planets are all round. Giant, oddly-shaped asteroids need not apply. Pluto qualifies here.
3) It's gravitational influence is significant enough to clear out all other large objects in its orbit.
Here's where Pluto falls short, and it ultimately is about being "too small." Again, consider the asteroid belt. No asteroid is big enough to pull the rest of the asteroids into its own gravitational pull, hence we observe a belt. Pluto is also considered part of a belt, the Kuiper belt, which circles the outer edges of the solar system, and also includes "the 10th planet."

Pluto will fall into the new category, "dwarf planet," which includes objects that are big enough to meet criterion 2, but not 3.

This NY Times article covers the topic nicely.