Smoke Ring

Contributed by
Dec 15, 2005
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137 light years from Earth sits a star very much like the Sun. Its mass and size are about the same, though it's a little bit cooler and a hair more luminous.

But there are two big differences. One is that this star is young, maybe only 30 million years old. That's barely even a baby! The sun is 150 times that old, at a middle-aged 4.55 billion years. This young, the star hasn't quite settled down yet; it's not steadily fusing hydrogen into helium in its core. But it's close.

The other difference was revealed by a recent observation by the Spitzer Space Telescope. We've discussed Spitzer before, yes indeed we have. Spitzer is sensitive to infrared light. One very common source of this is warm dust. Dust in space is a complex topic, because there is more than one kind, and more than one source. But one repository of it is a dirty solar system. Asteroids, for example, smack into each other and the result is dust. Comets lose dust too when they pass near the Sun.

So we expect to see dust in other solar systems. Sure enough, it's been seen in many places. But how do we know that?

Well, you can learn a lot about an object simply by taking its temperature. If you take an object -- say, a rock, or a piece of metal, or even a human being-- and warm it up, it tends to give off most of its light at a characteristic wavelength. Very hot objects (like a star) give off most of their light at visible wavelengths. Very cold objects, like interstellar clouds, emit radio waves. But warm things give off infrared. And by carefully measuring just where that peak emission is (by examining the spectrum of the object, essentially breaking up its light into colors), you can fairly accurately take the temperature of the object.

Which brings us to our young star, which, by the way, sports the name HD12039. When Spitzer took a look at the system, it saw the spectrum of the star (which peaks in the visible region of the spectrum), but it also saw a bump in the infrared. That is a clear indication of dust, and lots of it! The picture below shows this (click on it for a fuller explanation, too).

Now, think about this. If the dust were really warm, it must be near the star, right? And if it's really cold, it must be far away. A wide disk of dust would be at all different temperatures, because of the great range of distances from the star in the disk. You'd see hot dust, warm dust, cold dust, all emitting light at their various wavelengths. That's the middle graph in the picture.

But a ring of dust won't look like that. Confined to a small region around a star, it won't give off light indicating it's close to the star, or too far. You get a more narrow bump in the spectrum, which is the lower graph in the picture.

That's just what the astronomers found when they looked at HD12039. The spectrum gave them the temperature range in the dust, and that in turn tells them how far the dust is from the star: no closer than about 600 million kilometers (400 million miles) and no farther than 900 million km (550 million miles) away. That's actually a fairly narrow range, centered roughly at the same distance from the star that Jupiter is from the Sun.

Normally, dust would spread itself out, so having it in a narrow ring like this is a bit weird. The most obvious solution is that there are planets orbiting HD12039. They can "sweep up" the dust, and actually confine it to that skinny annulus. Saturn has moons which do the same thing.

So think about this: here we have a newborn star, just a few percent of its lifetime out of the womb, so to speak. It sits at a distance from us of 800 million million miles. At that mind-numbing distance, any planets circling it are far too small and faint to see... yet we know they're there. They left their calling card in the form of a narrow hoop of ground up rock, which itself betrays its presence because it's warm. And not too warm, and not too cold. Most likely, this ring won't last more than a few million years before various forces disperse it. Some small percentage of young stars have them, and it's only by looking at lots of them that we happened to catch this one. Soon enough, this one will be gone.

And all this we've learned -- and so much more -- just because we want to. We decided to look at these things, and they reveal themselves to us. One of the best ways to learn about nature is to simply observe it. Combined with our mechanical tools, and our remarkable, curious monkey brains, we've managed to puzzle a lot of it out.