1. Planetology
  2. There is something out there

There is something out there

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Is it real, or is it cat hair?

Seven years ago this week I was preparing one of my favorite lectures for The Formation and Evolution of Planetary Systems, a class I frequently teach at Caltech. “Preparing” is probably the wrong word here, because this lecture, called The Edge of the Solar System, was one I could give even if instantly wakened from a cold deep sleep and immediately put on stage with bright lights in my eyes and an audience of thousands and no coffee anywhere in sight. The lecture explored what was known about the edge of our main planetary system and the ragged belt of debris called the Kuiper belt that quickly faded to empty space not that much beyond Neptune. Conveniently, one of my most active areas of research at that time was trying to figure out precisely why this ragged belt of debris had such an edge to it and why there appeared to be nothing at all beyond that edge. I could wing it. So instead of preparing the lecture, I really spent that morning doing what I did whenever I had a few spare moments: staring at dozens of little postage-stamp cutouts of pictures of the sky that my telescope had taken the night before and my computer had flagged as potentially interesting. Interesting, to my computer, and to me, meant that in the middle of the postage stamp was something that was moving across the sky at the right rate to mark it as part of the Kuiper belt. I was not just lecturing about this debris at the edge of the solar system, I was looking for more of it, too.

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.
Sedna is the Inuit goddess of the sea, often depicted with the body of a seal, long hair, and no fingers.

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?
Sedna takes 12,000 years to go around the sun on its elongated orbit, and it never comes close to any of the planets.

Many objects out in the Kuiper belt have shockingly elongated orbits like Sedna. For almost all of these objects, this characteristic makes sense. These small leftover pieces of debris have been kicked around by planets throughout their existence. Whenever they come too close to one of the planets (usually Neptune, since it is the closest to these objects), they get a gravitational kick that can send them on a looping orbit to the distant outskirts of the solar system. But – and this is the key part here – unless they get kicked all the way out of the solar system, they always come back to where they were kicked. If you get kicked by Neptune, you can go zooming off into the unchartered regions far beyond the Kuiper belt, but you will come back to see Neptune again. When we look at the Kuiper belt, we see the results of all of this kicking clearly: the Kuiper belt objects that come closest to Neptune are on the most elongated orbits. Those far away are more free to go about their circular orbiting lives.

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.
NEXT WEEK:
How we are trying to answer the mystery of Sedna’s origin, why we have so far failed, and why 5 year olds make more sense than 7 year olds.

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  1. Planetology
  2. There is something out there