Impacts between planets are common events during the early stages of a stellar system's formation. Now astronomers have evidence of a big impact between two rocky planets in a high-speed collision.
Collision Between Two Terrestrial Type Planets
NASA's Spitzer Space Telescope has found evidence of a rocky collision between two worlds orbiting a distant star. The first planet was at least as big as the Moon while the other was at least as large as Mercury. The collision annihilated the smaller object, vaporizing and melting huge masses of rock.
The planets were orbiting a star called HD 172555, which lies in the southern constellation of Pavo the Peacock. The star and its stellar system formed about 12 million years ago, still youthful compared to the 4.5 billion-year age of our solar system. The collision between HD 172555's planets occurred within the last few thousand years.
"This is a really rare and short-lived event, critical in the formation of Earth-like planets and moons," says Carey M. Lisse of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "We're lucky to have witnessed one not long after it happened."
The Spitzer Space Telescope's Findings
NASA's Spitzer Space Telescope used a spectrograph to break the light coming from the star into its spectrum, which reveals chemical signatures.
"I had never seen anything like this before," Lisse says. "The spectrum was very unusual."
Three main components were discovered that marked a collision between two rocky planets. The first was amorphous silica, or melted glass. Amorphous silica also exists on Earth as obsidian rocks, which are pieces of shiny black volcanic glass, and as tektites, which are pieces of refrozen lava. The second signature Spitzer found was silicon monoxide gas, created when rock was vaporized. The last telltale sign was the rocky rubble that escaped the vaporization and melting.
"This collision had to be huge and incredibly high-speed for rock to have been vaporized and melted," Lisse says.
Spitzer has witnessed asteroidal collisions before, but this was the first time it found evidence of a high-speed impact with such vaporization and melting. The objects must have been traveling at least 10 kilometers per second as they hit.
"Almost all large impacts are like stately, slow-moving Titanic-versus-the-iceberg collisions," says Lisse, "whereas this one must have been a huge fiery blast, over in the blink of an eye and full of fury."
Planetary Collisions in Earth's Solar System
Earth and the moon came in to being through just such a violent collision. An object the size of Mars hit Earth 4 billion years ago, melting the surface of Earth, with the orbiting debris eventually coalescing to form the moon.
"This is about the same scale of impact we're seeing with Spitzer," says Geoff Bryden of NASA's Jet Propulsion Laboratory in Pasadena, California. "We don't know yet if a moon will form or not, but we know a large rocky body's surface was red hot, warped and melted."
Collisions in the early history of our solar system also stripped Mercury of its outer crust, tipped Uranus on its side, and spun Venus so that it rotates backward as compared to the other planets. As our solar system aged, collisions became fewer and far between, but they can and do still occur, as was recently seen with the July 2009 impact on Jupiter.
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