Where does the solar system end?

Where does the solar system end, and the rest of the universe begin?  And how big does a planet have to be to be a planet? 
Just a few years ago, Pluto had it's planetary status pulled by (earthbound) astronomers, on the grounds that it is too small to be considered as such: it is, instead, a planetoid.

Today comes news of two astronomers insisting that there is, in fact, a ninth planet: and it is large enough to be classified as such, being four times as big as Jupiter.  Its status as a planet would be confirmed easily then, if it can pass two tests --- first, and most obviously, if the astronomers can convince the rest of us that they are right, that it is exists: and secondly, if it is close enough to the sun to qualify for planetary proximity.

Yours, wishing Tyche good luck,
N.