5) It's not the biggest object out there in the black
But just because it's brighter doesn't mean it's alone!
Astronomers figured for decades there must be other objects out there near or beyond Pluto, but they are so far away and so faint that they're incredibly difficult to spot. However, automated telescopes and computers make searching a lot easier, and now we know of hundreds of objects in the deep solar system.
As we found more and more, some started to rival Pluto's size. Most folks -- including me -- predicted it was only a matter of time until a larger object was found.
And sure enough, in January 2005
Mike Brown and his team at Caltech found an object eventually named Eris. It's about 2500 km (1500 miles) across, making it comfortably larger than Pluto. A moon (
named Dysnomia) orbiting Eris was discovered, and its orbit gave the mass of Eris as being 27% bigger than Pluto as well.
You may think that astronomers in the IAU suddenly decided out of the blue that Pluto was no longer a planet, but in reality Eris is what spurred them into action. If there was one object out there bigger than Pluto, then there must be more. A lot more, since space out there is pretty roomy! They were faced with a choice: call Pluto and Eris planets (not to mention several other objects already known) and wind up with a solar system possessing potentially hundreds or thousands of planets, or simply make up a rule that excludes those objects.
And you know how that turned out.
Again, I won't go into planet definitions here, but I will say that Eris is almost certainly not the biggest object out there either. Calculations cannot rule out a mass as large as the Earth, as long as it's way, way out-- so far that its gravity doesn't appreciably affect Neptune, or else we'd have noticed by now. So there could be some pretty big objects out there. And if there are, some day we'll find them.