Using the Spitzer Space Telescope, astronomers have found a planetary collision 100 light-years away, near a star that goes by the name of HD 172555. Evidently, an object the size of the Moon smashed into one the size of Mercury. NASA has an awesome animation of the event below:

According to the Bad Astronomy Blog at Discover.com, astronomers found the collision when they were looking at spectra from HD 172555. In addition to hydrogen, helium, and the other usual suspects, the spectra showed evidence of an unusually large amount of amorphous silica, as in glass. From blogger/astronomer Phil Plait:

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!

I should note that while the animation is impressive, it’s somewhat misleading. Plait explains:

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.