The Sciences

When worlds collide

Bad AstronomyBy Phil PlaitAug 10, 2009 4:31 PM


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The Spitzer Space Telescope has detected signs of an interplanetary smashup, and oh, what a colossal event it was... apparently, 100 light years away around the young star HD 172555, an object the size of the Moon slammed into a planet the size of Mercury! First, the way cool animation they created portraying the event: Whoa. So how did they figure this out? Astronomers used Spitzer to take infrared spectra of HD172555, a young star only a few million years old. By breaking up the light into a spectrum, you can determine what elements and molecules are in the light's source (I've written about this sort of thing before). When astronomer Carey Lisse looked at the spectrum from the star, he got a shock: it was a mess. His team finally figured out what they were seeing: amorphous silica, and lots of it. In other words, glass. Glass? From a star? The most likely explanation is that the glass is in the form of tektites, which are blobs of glassy material that form when something big hits something else big. The silica gets fused into glass. But that means that there was a pretty big impact that must have happened at that star, and that in turn means that two planet-sized objects must have had a very bad day. This was supported by the detection of other chemicals consistent with the aftermath of a massive collision. The best fit to the data suggest that one object was planet-sized and the other Moon-sized, meaning the collision would have been at very high speed -- several kilometers per second -- and launched an unimaginable amount of material into space. Furthermore, it couldn't have happened too long ago, or else the material would have dissipated and wouldn't have been seen. It looks like this was a recent event, then, occurring maybe only a few thousand years ago! And there's another thing that I find personally very cool. Remember, HD 172555 is only 100 light years away. That is extremely close on a galactic scale (our galaxy is 100,000 light years across, so this star is our next door neighbor). It seems incredibly unlikely that this is a rare event in the galaxy, since this happened so close by and so recently. It's far more likely that it happens all the time in young solar systems, which means it happened to us, too. Well, a long time ago, like 4 billion years ago when planet-sized objects were more common and still forming in our solar system. But it lends credence to the idea that planetary-scale collisions do happen. We know this already, of course, because we're pretty sure the Moon formed when a Mars-sized object smacked into us over 4 billion years ago, and we see evidence of vast impacts on other bodies in the solar system as well. But to see evidence of it around another star, with different conditions, means this sort of thing is common, and is an inevitable step in the process of solar system formation. The Universe is a violent place, but every day it looks more likely that violence is necessary for us to be here at all. I'm not sure there's a lesson in there worth extrapolating, but it's certainly an interesting thing to be reminded of. P.S. The animation above is cool, but not a perfect representation of what happened. For example, the shock wave ring travels around the planet as shown, but when the ring converges on the point opposite the collision point, there would be a huge explosion and a vast plume of material launched into space. No one ever puts that in their animations, and I think it would be very cool! I need to get people who create physics-based simulations to make one that's accurate, so it can be used in situations like this.

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