Well That's Done It
Turns out that 2003 UB313 (aka “Xena”) has a moon (aka “Gabrielle”) and is significantly larger than Pluto.
So do we call this a “10th Planet”? Here’s the thing: both 2003 UB313 and Pluto are part of a band of millions of small, icy bodies, called the Kuiper Belt. There are most likely more and bigger ones to find.
On the other hand (since it was never really an issue before), there’s not a well-agreed-upon definition of “planet”. So there’s no reason to say that a sufficiently large KBO couldn’t be considered a planet, in principle.
What’s ultimately going to have to come out of this is:
- a firm definition for “planet”
- if Pluto doesn’t pass muster, a decision on whether to grandfather it in anyway
The issue’s been looming for a while, but now we’re going to be forced to deal with it. We’ll see what happens.
Intuitively, I think KBOs should not be considered planets (even Pluto), but I’ve not really thought much about an appropriate definition for “planet”.