You're correct. There are 11 planets (plus a couple of planet-sized moons) as well as innumerable asteroids and something called the "Ort cloud" where the comets come from (very long period comets, such as Halley's comet). We think that the Ort cloud extends about 3 light years away from the solar system (maximum)... and anything beyond that limit has a chance of being caught by gravitational forces from other stars and moved.
Nibiru/12th planet material is a source of great hilarity for astronomers and will soon be featured on the "Bad Astronomy" site
www.badastronomy.com (Phil Plait will be on Art Bell later this month and wants to have a page up in time
(explaining in simple terms why the idea's so ludicrous) for that show.)
This is the most famous debunking page: www.planet-x.150m.com...
It made Crank.Net's list of worst science pages: www.crank.net...
So why's it such bad science? In a nutshell, it proposes a Mysterious Big Bad Planet (we're talking a BIG one) is lurking out beyond the Ort cloud
-- or maybe it's a brown dwarf star or maybe just a big monster comet (depends on which version you read.)
It's supposed to be sneaking up on us (unannounced by astronomers (who are supposed to be in some secret evil government cabal... all of them... the
minute you get your first job they apparently swear you into the secret cabal...) Apparently every telescope on the planet ALSO has some sort of
secret label on them saying "if you report any strange planet, aliens will pop out of the woodwork and pound you to death with sauerkraut", for no
astronomer or stargazer has ever reported this thing.
It's supposedly going to move from the Ort cloud region to the Earth in the next 365 days. This means it's got to be approaching the Earth at
near-light speeds (warp six, Mr. Sulu!.)
(well, actually, about 1/2 AU (half the distance from the earth to the sun) every day from now till then.)
THEN... when it gets near Earth (after accelerating all this time), it will suddenly put on the brakes and do a leisurely stroll around the Earth/Sun
and then will suddenly slingshot back into the outer dark (at Warp six again, presumably.)
Anyway, those pages are a good read if you're into some nice explainations of Silly Science.
But no, we're not going to be attacked by some rogue celestial body -- and the calculations that suggested a 12th planet in the first place were
based on inaccurate data. Better measurements of the outer planets by Voyager proved that there weren't any oddities in the orbits after all.