I didn’t find more objects in the Kuiper belt every morning I looked, but that previous night seven years ago had been a good one. I quickly found two of the typical debris chunks moving slowly across the sky, and I was about ready to walk over to give my lecture, when, with only about a minute to spare, the outer solar system seemed to change before my eyes.
There, on my computer screen, was a faint object moving so slowly it could only have been something far more distant than what I was just going to walk into the classroom and declare to be the edge of the solar system. Maybe. The object was so faint that I didn’t know whether to believe it was real or not. If you look at enough sky – and, really, I had – you are bound to find some chance alignment of blips of noise or variable stars or cat hairs that looks just like something real.
I went into the classroom, delivered the lecture as I knew it, but stopped short at the end.
“Here is the way I was going to end this lecture,” I told them.
I proceeded to talk about how nothing existed beyond the edge of the Kuiper belt (yes, yes, you sticklers, the Oort cloud is way out there, but that is not supposed to start up until 100 or 200 times further out than the edge of the Kuiper belt).
“But I’m not sure I believe this anymore,” I said.
I told them about that morning’s blip. I couldn’t promise them that it was real, but I told them that if it was, the solar system might be very different place than I was just telling them.
That little blip, far more distant than what was supposed to have been the edge of the solar system, was indeed real. It was Sedna.
A few weeks later, after confirming that Sedna was real and determining its unprecedentedly strange orbit around the sun, I came back, told the class all about it, and wrote down a few simple equations on the blackboard to show just how strange the orbit is and also the many different ways it might have gotten that way.
“Come back and take my class again next year, and I’ll have it all figured out,” I confidently told them.
That was seven years ago. Any poor student taking my advice would have sat through the last six years of lectures and still not learned what put Sedna where it is, since I still don’t know the answer.
What makes Sedna’s orbit so strange?
The exception to this rule is, of course, Sedna. Sedna has one of the most elongated orbits around, but it never comes anywhere close to Neptune or to any other planet. Indeed, the earth comes closer to Neptune than Sedna ever does. And the earth is not in danger of being kicked out of its orbit by Neptune anytime soon.
Something had to have kicked Sedna to have given it its crazy orbit. But what?
The answer is: something large that is no longer there, or that is there, but we don’t know about yet.
This answer is astounding. 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 (and, again, for you Oort cloud sticklers out there, the known galactic environment). Sedna alone requires Something Else Out There.
What is it? Seven years out, we still don’t know. The hypothesized culprits have included passing stars, hidden planets, Oort cloud brown dwarfs, and, of course, Sumerian-inspired alien conspiracy theories. Whatever it is, it is bound to answer profound questions about the origin and evolution of the solar system, as well as inspire many new questions we had never known to ask.