While most people don't like watching TV reruns, most of us have a few shows that we will stop to watch if they come on. My parents had a favorite episode of M*A*S*H, "5 O'Clock Charlie," that they loved to watch and recount. I have a few favorite movies, like Airplane!, that I love to watch over and over, because I often catch something new. And my daughter is just a few years past the phase when she would shout "again!" from the back seat of the car, her request to replay her favorite song from the CD we'd be playing.
In the world of astronomy, information and research is often conveyed by hour-long talks ("colloquia"). But it is extraordinarily rare to hear a "re-run;" with several thousand astronomers to choose from and only one or two colloquia a week, why should the department spend money to bring back a scientist to give the same talk she gave just a year before? But, like with entertainment, we often can get more out of hearing a talk for a second or a third time.
Today, I had the lucky opportunity to get an astronomy colloquium re-run. We had a speaker here at Texas that I saw a couple of years ago at the University of Arizona. Hal Levison is an astronomer at the Southwest Research Institute in Boulder, Colorado. He studies planetary system dynamics (this means he studies how planets, asteroids and comets move in solar systems). And he was talking about the history of our own Solar System.
When we look at our Solar System, we see a nice, ordered structure. Four rocky planets, an asteroid belt, four big gassy planets, and a belt of icy asteroids. If you put the Solar System in a computer and see what will happen a billion years from now, you'll see that the Solar System will look about the same. Maybe a few asteroids have hit some planets, and some comets have come and gone. But Earth will still be in the same orbit; Mars won't suddenly go veering off into deep space. Has our Solar System always been this way?
Detailed studies of asteroids and the icy Kuiper Belt Objects (like Pluto and its family) suggest not -- they suggest that the big planets have moved around. And the moon shows scars from what was a surge of impacts about 3.9 billion years ago. What happened?
Levison and his collaborators took a wild guess, and then tested it with computer models. They guessed that, 4 billion years ago, Saturn, Uranus and Neptune were much closer to the sun than they are now, and fairly close to the planet Jupiter. And they guessed that, outside of Neptune, there were thousands upon thousands of Pluto-sized and smaller icy bodies. And then they let the computers do their thing.
As time passes on, the icy bodies sometimes swing past a big planet and get tossed around. And after about half a billion years, the big planets slowly move a little bit because of the combined effect of a lot of these encounters. (One person may not be able to push a car around, but add hundreds of people together, and you can move the car quite a ways.)
Eventually, Saturn and Jupiter get to a state called resonance, where Saturn goes around the sun once for every two times Jupiter orbits. In physics, resonance is a special thing -- you get feedback. Just like a small whine picked up by a microphone can feed back and give you a piercingly loud noise, the resonance of Saturn with Jupiter started tossing Uranus and Neptune around the outer Solar System. The ensuing chaos of giant planets rumbling through the thick belt of icy Plutos sent rocks and comets and ice everywhere -- deep into space, into the Earth and Moon, into the sun -- everywhere. And then, as quickly as it started, Jupiter and Saturn moved out of resonance, and everything settled down. Uranus and Neptune survived, but were much further from the sun, and 99% of the icy belt that had been the outer Solar System was lost to deep space. In fact, the Solar System looked just like it does today -- nothing has happened since then.
This is a neat idea that explains lots of things we don't understand about the makeup of our Solar System. And so it was nice to see it a second time. Some ideas were clarified in my mind, some things I mis-heard last time I caught this time, and a few new ideas were introduced. And I learned where there are still some things to be explained and some interesting avenues of future research. Maybe 5 years from now we'll find that this idea just doesn't work. Or maybe it will be confirmed. I'll await the next re-run to see.