…Sedna?..Nibiru?..Nemesis?..if they found it, where is it?
It’s out there somewhere. They don’t know exactly. They are presuming it’s there based on the behavior of objects in the outer solar system. Once some locates it precisely we’ll all find out.
If they find it, they’ll know where it is.
Anyway, it’s somewhere around here. A bunch of smaller trans-Neptunian things have orbits that are off to one side. The idea is that this is caused by something big on the other side.
Edit: What **dolphinboy **said.
Interesting. What causes its eccentric orbit, compared to the relative flat ellipticals on a single plane of the Inner Planets and the Gas Giants?
BTW, not Sedna. Sedna is this little cutie. That was found a long time ago, and I don’t anyone has misplaced it in the meantime. It’s one of the small things off to one side.
The report I read said that this planet is expected to be a gas giant, comparable to Uranus or Neptune in size. Not some little frozen snowball 
It’s similar to the Oort cloud in that regard. Something that is inferred from the dynamics of other bodies rather than being observed directly.
Space is big. Like, really really big. We know the general area of space we expect it to be in, but that’s still more than enough space to lose even a gas giant planet in.
I get an untrusted connection warning from Firefox (expired security certificate).