Quiz: How Many Planets in the Solar System?

If you said 9, you’re not correct… not anymore:

(hat tip, Wes)


The solar system has 12 planets.

That is the conclusion, to be announced today, of an international panel formed to devise a scientific definition of a planet and settle an increasingly intense dispute over whether Pluto qualifies. The panel suggests retaining Pluto and immediately adding three new planets to the nine that are familiar to any schoolchild: Ceres, currently considered a large asteroid; Charon, now considered a moon of Pluto; and Xena, a recently discovered object that is larger than Pluto.

But the group's proposal also makes clear that many more objects in the solar system -- perhaps dozens of them -- could qualify as planets after further study.

There are 53 objects that meet the panel's criteria and probably many more to be discovered… The total number of planets, Brown said, could easily climb above 100.


The key issue is how a planet is defined. One way to define it is anything big enough to have a spherical shape* that orbits the sun. But it’s also reasonable to define “planet” to refer only to the largest satellites in the solar system, i.e. those that coalesced in to large masses as the solar system was forming (the "evolutionary" view). Looking at it this way would basically give you only 8 planets (excepting Pluto).


*Physics Factoid: There is a limit to how big or long an object can be and still retain its shape. Once an object gets big enough, its own gravity forces it into a spherical shape. It’s impossible to build a bridge to the moon, for example, because nothing can be that long and stay linear. After it gets to a certain length it will compress itself and never get longer. And if you keep adding mass it will turn into a sphere eventually.