Another possibility (unless they've already ruled it out) is that it's a rogue planet passing through the Oort cloud. I don't know if thousands of them are any

Another possibility (unless they've already ruled it out) is that it's a rogue planet passing through the Oort cloud. I don't know if thousands of them are any more plausible than thousands of gravitationally bound planets, but supposedly rogues outnumber star-bound planets. They would be spread pretty thin depth-wise and hard to detect.