The Sciences

Wrong way planets screw up our perfectly good theories

Bad AstronomyBy Phil PlaitApr 13, 2010 8:00 PM


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Stupid reality, always mucking about with our ideas. How dare it! In this case, reality is interfering with how we think planets form around stars. And the monkey in the wrench belongs to a handful of newly discovered planets that go around their stars the wrong way.

That's an artist's illustration of one of these planets. As you can see in the diagram, the star rotates left-to-right, but the planet orbits right-to-left. That's a bit of a puzzler, and here's why. First, how do we think planets form? If you look at my last post, you'll see a giant cloud of gas and dust collapsing in places to form stars. The stars form from little knots of overdense regions in the cloud. As the material collapses, any slight amount of rotation it has -- from eddies and vortices in the gas, say -- get amplified (think ice skater as she draws her arms in and spins faster). Random collisions of particles inside the cloud tend to drop more of the matter toward the center, along the equator of the spin, forming a flat disk there. The disk spins, rotating around its center like a DVD (though stuff toward the center goes around faster than stuff near the outer edge). The middle of the disk is where the star forms. Farther out, local eddies and vortices can form planets. But the important thing to note is that in this scenario, everything spins in the same way. If the disk appears to be spinning clockwise, say, then the star will spin that same way, the planets will orbit that same way, and the planets will spin that same way. We're pretty sure this is how things work because that's pretty much what's happening in our own solar system. This theory has been tested by observation and by increasingly complex modeling. Sometimes there are problems with it, but in general new ideas have been added that fix those problems, and over time we've been pretty happy overall with the idea that stars and planets form this way. However, a bunch of newly discovered planets have messed this nice idea up. They orbit their stars the wrong way! How do we know? That part is pretty cool. First, these are transiting planets. As the illustration above shows, from our point of view the planets pass directly in front of their stars every time they make an orbit. When that happens, they block a fraction (usually around 1%) of the star, and we see that as a slight dip in the light detected from the star. A lot of planets have been found this way, and it's a pretty good method of finding planets. Now picture the star in the image above. It's spinning, so the left side of the star in the diagram appears to be headed toward us, and the right side moving away from us. But that means there's a Doppler shift, a slight change in the color of the light from the star. Just like a car roaring past you makes that "EEEEEEeoooooooow!" sound, light changes pitch if the source is moving toward or away from us, and that change in pitch is seen as a shift in color. The light from the part of the star rotating toward us shifts a bit to the blue, and the side moving away shifts a bit to the red. That shift is very small, but measurable. But the planet messes that up. As it transits (moves in front of) the star, it blocks first one side, and then the other. If it orbits the star in the same direction as the star spins, it will first block the blueshifted side, and then a bit later the redshifted side. That change in the starlight can be seen and measured. But for some of these planets just discovered, it's all backwards! The redshifted side gets blocked first, and then the blueshifted side. That means the planet is going around the star the wrong way. The press release about this discovery has a nice video which makes this a bit more clear. Does this mean our theory is wrong? Well, not exactly. It probably means that overall the theory is solid, but that there are exceptions, modifications, we don't understand. Most likely the planets that form around other stars start off revolving around the star the same way, but then some sort of gravitational interaction with other forming planets knocks them off course. Some of these newly discovered systems do appear to have outer planets that could do the trick; the tug-of-war resulting from a close encounter could slingshot one of the planets into a retrograde (backwards) orbit. This would play hell with the system. The planet knocked backwards would migrate in close to the star, tossing other smaller planets either into the star or out of the system entirely. If that's true, then it means these weird planet systems won't have many planets, just the one backwards-revolving one and one or two outer planets. That's a nice prediction, in fact, and one that can be confirmed or falsified with more observations. And it's not like this is a rare event: fully 6 out of 27 systems appear to have these backwards-moving planets! That means that however these planets get knocked about, it has to happen fairly often. Obviously, we need to observe a lot more of these systems so we can get better statistics, and be able to see what similarities and differences they have with each other. That's the best way to figure out what the heck is going on. What does all this mean? Well, it means, as usual, that Nature is a bit more clever than we are, thinking up all sorts of ways of forming planets and systems of planets that didn't initially occur to us. But that's how science works. Things get complicated, so the first thing to do is simplify. Make your idea general. Then start adding complexity to it to explain what you actually see. As observation techniques get better, the idea has to get modified to account for new data. In this case, it's a pretty big modification, but that's not surprising: we're new at this planet finding thing. We're bound to get plenty of surprises for a while, until we have a better grasp of the situation. Surprises are good: they help us test the theory, they help us understand reality a little better, and they help us learn a little bit more. But they're also fun. Who wants a Universe we understand completely and utterly? How boring that would be! Science is all about peeking around the next corner and seeing what's there. And there are always more corners. Always. ESO/L. Calçada

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