(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.
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 earth-sized planet circling the sun about twice the distance of Neptune. Sedna could have gotten too close to this Planet X and been given a kick which would have flung it out into a far corner of our solar system. But, like always, nothing can kick you into a far corner and make you stay there. You always come back to the spot where you were kicked. So Sedna’s new orbit would be one that came in as close as this Planet X and went far into the outer solar system – just like Sedna’s orbit. Back in 2003 I had liked this idea a lot. Our search of the skies had only begun a few years earlier, so the prospect that there might be an earth-sized planet awaiting discovery seemed pretty exciting indeed. It was, admittedly, a long shot, but discovering planets always is.
If Sedna got put on its peculiar orbit by the interactions of all of these stars 4.5 billion years ago, it is now a fossil record of what happened at the time of the very birth of sun. Everything else in the solar system has been kicked and jostled and nudged by planets big and small so there is no way to trace them back 4.5 billion years. Sedna, on the other hand, has been doing nothing but going around and around the sun in its peculiar elongated orbit every 12,000 years. After almost half a million of those orbits, Sedna remains lonely and untouched by anything else. By watching the orbit of Sedna we could be watching 4.5 billion years in the past.
All of these possibilities are exciting! A new planet! A rogue star! Fossils from the birth of the sun! And in the years since Sedna’s discovery other astronomers have chimed in with their own ideas, including the possibility that Sedna was kicked by something large out in the Oort cloud (small planet? Brown dwarf? Nemisis? Who knows) and, in the most imaginative spin, that Sedna was kicked by the sun. The sun? Yes, because, in this hypothesis, Sedna used to orbit a different star, and the sun got close and kicked Sedna around and stripped if away. Sedna would then be the first known extra-solar dwarf planet. Or something like that.
Sedna is telling us something profound, but what? With only a single object, there is absolutely no way to know. It would be like finding a fossilized skeleton of a T. Rex and trying to infer the history of the dinosaurs. If you had just that one skeleton you would know just what to do: head back out into the desert and start digging. When we found Sedna, we, too, knew what was next: head back out into the night and keep looking. Until we found more, we wouldn’t know what this profound bit of the solar system was trying to scream so loudly in our ears.
Part 3: The search for more Sednas.