There's something out there -- part 2

The view from Sedna

(Be sure to read part 1)

Seven years ago, the moment I first calculated the odd orbit of Sedna and realized it never came anywhere close to any of the planets, it instantly became clear that we astronomers had been missing something all along. Either something large once passed through the outer parts of our solar system and is now long gone, or something large still lurks in a distant corner out there and we haven’t found it yet.

Of all of the planets, comets, asteroids, and Kuiper belt objects in the solar system, Sedna is the only one that tells us this astounding fact so glaringly. The orbit of every single other object in the entire solar system can be explained, at least in principle, by some interaction with the known planets. Sedna alone requires something else out there.

But what?

In our 2004 paper announcing the discovery of Sedna (give it a try; though it – like all research papers – has some technical details that might not make sense, I believe it to be relatively readable), we suggested three possibilities. Our first idea was that perhaps there was an unknown approximately ...