Shock and Ahhhh in Circumstellar Disks

There’s some big news coming up tomorrow about exoplanets (planets around other stars) which I think people will find quite tasty. But, before we get to that story (which will be released tomorrow afternoon), let’s talk about the ingredients of planets. It turns out making them requires quite a complex mix of elements and processes. When it works right, it’s quite satisfying because you get new worlds to explore.

First you have to start out with some starbirth. This star has to be forming with a dusty disk around it. Planets are born in these swirling pancake-like disks. They start out as mere grains of dust swimming around in the disk before lumping together to form full-fledged planets. During the early stages of planet development, the dust grains crystallize and adhere together, while the disk itself starts to settle and flatten out. This occurs in the first millions of years of a star’s life.

Okay, so we know that these circumstellar disks provide the raw materials for planets, moons, rings, asteroids, and comets. What are they? Typically you have gases (hydrogen, oxygen, etc.) You also have ices made of water, methane, and ammonia (to name the most abundant). And, you have elements such as iron, carbon, and silicon, and as has been detected recently by Spitzer Space Telescope, crystals of materials such as silica, which — if exposed to a significant amount of heat to melt them — can form other crystals called cristobalite and tridymite (which are often found around volcanoes here on Earth).

So, what could be causing the heating that would melt and crystalize silica?  Astronomer WIlliam Forrest of the University of Rochester in New York, used Spitzer to studied material around other star systems and found the chemical signatures of these crystals, which need a quick, high-temperature environment to form in. “Cristobalite and tridymite are essentially high-temperature forms of quartz,” said Sargent. “If you heat quartz crystals, you’ll get these compounds.”

In fact, the crystals require temperatures as high as 1,220 Kelvin (about 1,740 degrees Fahrenheit) to form. But young planet-forming disks are only about 100 to 1,000 Kelvin (about -280 degrees Fahrenheit to 1,340 Fahrenheit) — too cold to make the crystals. They need a hot environment followed by rapid cooling.

So, how do you create the ovens of crystal formation in such chilly places?  Shock waves. These fast-moving (supersonic, in fact) pressure waves are likely to occur when clouds of gas in the disks collide. Those clouds are swirling around at high speed and when they smack into each other, it could raise the temperature enough to create those crystals.  You can also get such shock waves when giant planets like Jupiter are formed.

The planet-building process is a long one, taking millions of years at a minimum (if our own solar system is any example). It starts with the cloud of gas and dust swirling around in space, with a protostar in the center. As things heat up, things fuse and stick together to form these crystals. In the outer regions, where there’s not so much heat, ice cFrom smaller crystals you get bigger crystals and clumps of rock that fuse together over long periods of time to make even bigger clumps. Eventually, if you slam enough of them together, you get asteroids that clump together to make small worlds, which clump together to make larger worlds… and… you get the idea.

And, we see the evidence for similar processes here in our own solar system. Spherical pebbles called “chondrules” have been found in ancient meteorites that we know formed in the early days of the solar system. They bear evidence of shock processes similar to the ones that astronomers have found evidence for at distant stars.It’s pretty likely that, like those distant stars and their planets, our own solar system experienced the heating events that kick-started the recipe for planets more than 4.5 billion years ago. This is the kind of research discovery that helps astronomers put together larger, much more satisfying (and “ahhh”-inducing) scenarios for planetary formation.

And the hunt for planets continues. For the first time in history, we have enough observational firepower, both in space and on the ground, to make some big discoveries of planets AND hone in on the details of planet-forming processes. So, check back tomorrow (Thursday) afternoon and I’ll have some more big news on the planet-discovery front!