[Renegade 13]

Possible, but I'd say it's very unlikely. Though if the large planet was VERY far away, it could happen. Actually, there could be more large planets that orbit close by even, if the orbit was highly elliptical. If you look at the article I linked to, you'll see that one of the previous KBO's they discovered beyond Pluto's orbit (Sedna) has a very highly elliptical orbit. When it was discovered, it was 13.6 Billion km away from the Sun (Earth is a mere 149.6 million km away on average). But when it's at its farthest point from the sun, it will be 141 Billion km away from the sun. Now if there was a large planet, even one the size of Jupiter for example, if it was that far away, and had a similar elliptical orbit that could take it closer to the sun further into its orbit, it's very likely we would never detect it until it was a lot closer to the sun.

So yeah, large planets orbitting within our solar system are possible, but they're probably either too far away to be detected currently, or have very elliptical orbits and just happen to be at one of the further points that orbit would take them on.

I hope that made sense!