[av_video src=’http://youtu.be/Xtw4cEjFBDs’ format=’16-9′ width=’16’ height=’9′]
Yesterday I discussed how larger particles are trapped by a pressure wave within the protoplanetary disk, but I failed to say what a pressure wave is. John Baez‘s description of it as a traffic jam is a pretty good one. As he wrote in a comment:
Here in Los Angeles you get waves in the traffic where the traffic goes fast, then bunches up and slows down, then loosens up and goes fast… if you ask “what’s making this happen?”, the answer is that there’s just a built-in instability in the motion of cars when the traffic has a certain density – these waves form spontaneously, without any obvious cause.
When this effect happens with gas it’s typically called a pressure wave (the variation in traffic density is similar to a variation in pressure). When it occurs with particles, it is known as a density wave. If you’ve ever been in a traffic jam you’ve noticed that while you slowly make your way through it the overall traffic pattern remains the same. Usually when you’re caught in a traffic jam you eventually find the source (construction, minor accident, etc.) but sometimes you enter a traffic jam and go slowly through it without ever seeing a cause. At some time earlier something started it, but now there is just the traffic. The jam itself is now the cause of the jam. Planet formation is not the only place where such an effect occurs. A similar effect occurs with spiral arms in a galaxy. Individual stars are not locked in a particular spiral arm, rather they move around the galactic center passing through one spiral arm after another. But as a star moves toward a spiral arm, that arm’s mass gives it a little gravitational boost to edge them into the arm. When a star begins to leave an arm, the gravitational pull of the arm slows it down just a bit. As a result stars tend to cluster into the traffic jam of spiral arms, and the arm patterns sustain themselves even while individual stars move through them. You can see this in the video above.