Planets Collided - Recently - Around a Star Called HD 172555

"Two Worlds Collide in Deep Space"
Space.com (August 10, 2009)

"Two distant planets orbiting a young star apparently smashed into each other at high speeds thousands of years ago in a cosmic pileup of cataclysmic proportions, astronomers announced Monday.

"Telltale plumes of vaporized rock and lava leftover from the collision revealed its existence to NASA's Spitzer Space Telescope, which picked up signatures from the impact in recent observations.

"The two-planet pileup occurred within the last few thousand years or so - a relatively recent cosmic timeframe. The smaller of the two bodies - a planet about the size of Earth's moon, according to computer models - was apparently destroyed by the crash. The other was most likely a Mercury-sized-planet and survived, albeit severely dented...."

The planets weren't, from one point of view, in 'deep space.' They were orbiting the star HD 172555, about 100 light-years from Earth. The star - and its planets - are around 12,000,000 years old, compared to the Solar system's 4,500,000,000 years.

Astronomers figure that they hit at around 22,400 miles an hour, or 10 kilometers per second: fast enough to produce a great deal of heat. At this point, there's quite a bit of "amorphous silica rock" (melted glass), tektites, and clouds of silocon monoxide gas orbiting HD 172555 - along with the surviving planet.

Yeah, it's Kind of a Big Deal

This sort of collision is, assuming that current theories of how planetary systems form, fairly common. The Hellas Basin on Mars, what we're discovering about Mercury, the odd way Venus and Uranus, and the currently least-unlikely explanations for how Earth's moon got there all indicate that there were a lot of collisions around here. Happily, that's about 4,000,000,000 years in the past now.

And, although they're presumably fairly common, these collisions happen fast. Spotting debris from one that happened maybe a few thousand years ago is a rare opportunity - particularly since it's so close. Cosmically speaking, that is.

The Space.com article gives a pretty good look at what's been observed, and why it's important.

Background and related articles:

• "Oh, My! When Worlds Really Collide"
  Space.com (September 23, 2008)
  • (BD+20 307, about 300 light years away)
  • Background on that planetary system at: "The Wayback Machine? Nearby Solar System Looks Like Home"
    Space.com (July 25, 2005)
• "How the Moon Was Made: Cosmic Collision"
  Space.com
  video 2:40

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Related posts, at

• "A Serious Search for Other Worlds, Life, and - Maybe - Extra-Terrestrial Intelligence"