As unusual as this theory sounds, apparently Saturn may have been considered as a culprit as well:

But researchers haven’t been able to explain until now what could have happened to that extra planet, with both Jupiter and Saturn named as potential culprits for doing the kicking out.

So the team decided to look at the trajectories of Callisto and Iapetus – two of the regular moons orbiting Jupiter and Saturn respectively. Using computer models, they investigated the likelihood of the moons having the same orbit as they do today if they’d been involved in a mass planetary ejection 4 billion years ago.

“Ultimately, we found that Jupiter is capable of ejecting the fifth giant planet while retaining a moon with the orbit of Callisto,” said Cloutier. “On the other hand, it would have been very difficult for Saturn to do so because Iapetus would have been excessively unsettled, resulting in an orbit that is difficult to reconcile with its current trajectory.”

Does this seem plausible to you?  And if there was a rogue planet, do you think we could pick it up via radio telescope?  Feel free to comment with your ideas.

thanks to NASA for the pich