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Baby pictures from a new planet

By Michelle / June 3, 2004


A few years ago, it was pretty big news if someone found good evidence of a planet-forming disk around a young star. Now, it's becoming a bustling cottage industry. With infrared space telescopes looking into the heart of giant dust clouds, the birthing ground of stars and planets, we're identifying not just a few, not just dozens, but hundreds of young planetary systems at a go.

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Upon closer inspection, some of those disks appear to have either formed planets as little as a million years ago, or are in the process of forming them right now. But as amazing as it is that we're finding these young systems, even more mind-blowing is the fact that, by using the technique of spectroscopy, we can taste and smell the chemistry of these disks. There, as if waiting for time and evolution to start tinkering with things, are the very same building blocks of life that got us started billions of years ago.

Seeing with heat

This amazing new detection of young solar systems comes to us courtesy of the Spitzer Space Telescope, an all-infrared mission launched by NASA in August of 2003. Spitzer has the ability to detect and image infrared light, which we usually think of as heat radiation. This lower-energy light is invisible to our eyes, but it extremely useful to astronomers.

As it turns out, there is plenty of material in the universe that isn't hot enough to glow in visible light. Stars, such as our own Sun, put off lots of visible light because their surfaces are very hot - usually several thousands of degrees. Planets, on the other hand, tend to be significantly cooler.

The Earth doesn't give off any visible light of its own, and when you see a planet like Venus or Jupiter in the night sky, the light you see is reflected sunlight, not anything coming from the planet itself. But what if you had eyes that were sensitive to infrared light - like the thermal imaging cameras we use to see in the dark? Planets are certainly warm, and glow all by themselves in infrared light.

Spitzer is too small a telescope to be able to image the heat from individual planets (NASA is working on that), but it can detect the large disks of warm dust and gas that are the first step in the process of making a solar system.

A star forms when a large cloud of dust and gas collapses under the force of gravity and the pressure inside the cloud drives the temperature up enough to ignite nuclear reactions at the very center. The process of collapsing the cloud into a star isn't 100% efficient, and around the young star there's usually a lot of material left, which settles into an orbiting disk of dust and gas.

Over time, instabilities in the disks, maybe shock waves or some such thing, cause the disk material to form clumps, which begin to pull in even more material with their gravity. Over millions of year, these clumps grow bigger, and clear out the disk around the star. Particles of dust in the disk either get scooped up by one of the clumps, or thrown farther out into the young system by a gravitational boost from one of the growing planets.

So how do you know which stars in the sky have planet-forming disks when you can't image them directly? The first clue is that the stars with disks have too much infrared light coming from them. There's a fairly simple law of physics that dictates how much light a star emits in each wavelength, depending in its temperature.