Yep, there's still the Oort Cloud out there (somewhere).
Fred Hoyle also wrote a novel called Tenth Planet, which I must have read but have forgotten (his early novels were great, and got made into fine tv sci fi, but his later ones were utterly gaga!).
The mythological concept of "tenth planet" comes from Pythagorean astronomy. 2500 year ago, 8 astronomical bodies were known, mercury, venus, earth, moon, sun, mars, jupiter, saturn. But, Pythagoras though there had to be 10, since that's a "perfect number". So he added the "central fire" (which produced light, which the sun only reflected!). And, the 10th planet, a "counter earth", always on the opposite side of the central fire to us, and therefore always completely invisible (how convenient!).
Pythaogreanism was a religion, there were all sorts of crazy rules, and joining was like becomming a monk or nun. But, as we all know, Pythagoras invented triangles, and where would we be today without those?
For more info on Pythagoreanism, see
http://users.ucom.net/~vegan/pythagorean.htm .
(For anyone seriously interested, there's some excellent factual articles on this website, as well as the crazy "new age" stuff!).