PASADENA, CALIF. — 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.
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.
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.
A star like the Sun, with a surface temperature of about 10,000 degrees, emits most of its light in visible light wavelengths, but it also gives off predictable and well-known amount of x-rays, ultraviolet light, infrared, radio waves, and so on.
With the advent of infrared space telescopes in the 1980's, scientists began to notice that some stars were giving off much more infrared light than they should. This "infrared excess" couldn't be coming from the star itself, so something warm and fairly large had to exist nearby. That's the first step in detecting a planet-forming disk.
Disks left over from star formation tend to be huge (larger than the orbit of Pluto around the Sun), and the material is too cold to glow in visible light. But the dust and gas in the disk are still warm, so they contribute extra infrared light to the emission of the system as a whole.
Spitzer is getting so good at detecting this infrared excess, it's picking out new planetary systems by the hundreds. In a single star-forming area in the constellation Centaurus, Spitzer found over 300 stars with definite infrared excesses in a single go, at a distance of almost 14,000 light-years.
Although the large-scale detection of disks around young stars is interesting, scientists were hungry for more detailed clues about how planets form out of those disks. And by looking at some young stars a bit closer by (about 400 light years away), they got just that.
In the constellation Taurus lie four young stars with disks close enough to observe in some detail. Using a technique called spectroscopy, astronomers were able to sample the content of the dust for interesting chemistry and search for the same sorts of molecules that may have helped life get started in our own Solar System.
Spectroscopy works something like this: an instrument onboard Spitzer passes infrared light through a grating, which spreads it out into different wavelengths. In visible light, this is the same thing that happens when sunlight passes through a glass prism (or droplets of water in the sky) to make a rainbow. Take white sunlight, spread it out into its component wavelengths, and you've got the familiar visible light spectrum. It works the same way for any kind of light, including infrared light, even if the light is invisible to our eyes.
By taking very accurate measurements of how much light is present at each specific wavelength, you can decode patterns that tell you what atoms, or even molecules, the light passed through before it got to you. I like to think of it as remote sensing, but instead of using your eyes, you're using taste and smell. You can analyze the chemistry, even if you can't physically see the object.
Some of the first things Spitzer saw in the spectrum were the signatures of silicates, stuff that's similar to beach sand. Surprisingly, some of the silicates were in crystal form, and in truth, if we had to pick an Earth beach to compare with the material we saw in the disk, it would be the famous green beaches of Hawaii.
Several of the Hawaiian Island have green beaches (I've been to one on the Big Island) where the sand is colored by crystals of olivine, a green crystal commonly formed near active volcanoes. There is a suggestion that olivine also exists, probably in tiny grains of dust, in these planetary disks as well.
The olivine crystals (unlike those in Hawaii) are probably getting coated with ices over time, forming the seeds of snowflakes, or even tiny snowballs up in those disks. We saw the spectroscopic fingerprints of several different kinds of ices, including water, carbon dioxide, and methyl alcohol.
Those particular chemicals bear some consideration. Before any planets form out of the disk around the young stars, we already know that there will be water and organic molecules in the mix; a very good proposition if you ever hope to get life started.
But are we sure that planets haven't formed already? In at least one of the disks, there is preliminary, but quite convincing evidence that we might be looking right at the youngest planet ever seen.
Remember the fact that warm dust and gas produces an excess of infrared light? There's a way to look at that infrared radiation and see if the disk is smooth and uniform, or whether it has gaps.
In a very young system called CoKu Tau 4, the infrared excess has an inconsistency -there is only infrared excess at long infrared wavelengths, light that comes from very cold material. Any dust or gas close to the star would be at a higher temperature, and should be emitting higher energy (or shorter wavelength) infrared light. What happened to all the hot dust and gas near the star?
Some of it may have fallen onto the star, but what's interesting is that the entire disk did not -there's plenty of cooler, farther out material giving off infrared light. Something had to happen that cleared out the inner portion of the disk, but kept the rest of the stuff from falling in to replace it.
Honestly, the only thing we can think of that would do such a thing is a planet, probably a big one. The formation of a planet probably sweeps up nearly all the material in the path if its orbit, and what doesn't get pulled onto the planet gets boosted up into a farther orbit.
It may seem strange that the gravity of a planet could prevent more material from falling into the inner part of the disk, but it works the same way as a "gravitational assist" that NASA uses for spacecraft flying into our outer Solar System. Fly a satellite very close to a large object, like Jupiter, and you can give it more energy (and boost it to a higher orbit) by sling-shotting it around the planet. As material from the outer disk tries to fall closer in, the young planet passes by and give everything a gravitational energy boost, flinging the stuff back out again.
The gap in the disk around CoKu Tau 4 may very well contain the youngest planet ever detected. Our current guess is that the star itself is only about one million years old, so even if the planet got an early start on things, it's probably younger than that.
Of course, there may well be more than one planet too. We don't see any infrared emission in the inner region of the disk, which means that there is very little extended, diffuse dust and gas drifting around in there. But there could be larger things, maybe the size of the Moon, that are beginning to crash together and form rocky planets like the Earth.
At this point, we don't know whether Earth-like planets are forming around CoKu Tau 4 or not, but we do know that if they are, they will have a lovely environment to grow up in. The place is already rich with water and organic molecules just looking for a place to settle down and get going. We may not have found a place just like our own Solar System, but we've just received some very cute baby pictures from one of our sister stars.