Also, we don't know where in its orbit the planet happens to be right now. If we're lucky, it's relatively close to the sun, and it will be detectable. If we're

Also, we don't know where in its orbit the planet happens to be right now. If we're lucky, it's relatively close to the sun, and it will be detectable. If we're unlucky, it's relatively far away from the sun, and it could easily be undetectable by current observatories because the orbit is so eccentric. [Source: I saw Mike Brown give a talk on this topic at the Cal Academy of Sciences, where he hoped he was lucky...]