(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.
The second possibility that we considered and wrote about was that perhaps a star had passed extremely close to our solar system at some point during the lifetime of the sun. “Extremely close” for a star means something like 20 times beyond the orbit of Neptune, but that is 500 times closer than the current nearest star. A star passing by that close would have been brighter than the full moon and would have been the brightest thing in the night sky for hundreds of years. Perhaps our early ancestors even temporarily lived under a dual-star sky. Sedna, before the rogue star came calling, would have been a normal Kuiper belt object with a looping orbit which would take it out to the distant solar system but then eventually back to Neptune (which had, presumably kicked it around earlier). But on one of its trips to the edge of the solar system, Sedna would have accidentally gotten too close to this interloping star, and the star would have given Sedna another little kick. Suddenly, Sedna would find itself on a new orbit which no longer went back to Neptune. The orbit would, of course, have to go back to the spot where Sedna had gotten the kick from the passing star, but the star would be long gone by then. This idea was a fun one, and, best of all, we could do a reasonably good job estimating the probability that something like this might have occurred. Looking at the number of star near us in the galaxy and fast they all move relative to each other, we found that the chances of such a rogue star encounter happening sometime in the past 4.5 billion years was around 1%. Not good odds to hang your theory on. (People often ask: can’t you just go back and find the star that did it and see if it is there? Sadly, there is no chance. The sun is 4.5 billion years old and it takes about 250 million years to orbit around the galaxy, so it’s gone around about 18 times. So has everything else in the vicinity. Everything is now so mixed up that there is no way to know for sure what was where back when.)
The third possibility was the one that we deemed the most likely. Instead of getting one big kick from an improbably passing star, imagine that Sedna got a lot of really small kicks from many stars passing by not quite as closely. The chances of this happening might seem low, too, but astronomers have long known that most stars are born not alone, but in a litter of many stars packed together. How tightly? In our region of the galaxy, there is currently something like one star per cubic parsec (don’t worry too much about these units here; suffice it to say that a parsec is a little less than the distance to the nearest star, so it is not surprising that in a box with edges about that length there is about one star). In the cluster of stars in which the sun might have been born there would have been thousands or even tens to hundreds of thousands of stars in this same volume, all held together by the gravitational pull of the massive amounts of gas between the still-forming stars. I firmly believe that the view from the inside of one of these clusters must be one of the most awesome sights in the universe, but I suspect no life form has ever seen it, because it is so short-lived that there might not even be time to make solid planets, much less evolve life. For as the still-forming stars finally pull in enough of the gas to become massive enough to ignite their nuclear-fusion-powered cores they quickly blow the remaining gas holding everything together away and then drift off solitary into interstellar space. Today we have no way of ever finding our solar siblings again. And, while we see these processes occurring out in space as other stars are being born, we really have no way to see back 4.5 billion years ago and see this happening as the sun itself formed.
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.