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.
"This collision had to be huge and incredibly
high-speed for rock to have been vaporized and melted," said Carey Lisse
of the Johns Hopkins University Applied Physics Laboratory in Maryland, lead
author of a paper describing the findings in the Aug. 20 issue of the
Astrophysical Journal.
Researchers believe the planets were moving at about 22,400
mph (10 kilometers per second) before the crash. The violent
wreck released amorphous silica rock, or melted glass, and hardened chunks
of lava called tektites. Spitzer also spotted large clouds of orbiting silicon
monoxide gas created when the rock was vaporized.
"This is a really rare and short-lived event, critical
in the formation of Earth-like planets and moons," Lisse said. "We're
lucky to have witnessed one not long after it happened."
Infrared detectors on Spitzer found the traces of rocky
rubble and re-frozen lava around a young star, called HD 172555, still in the
early stages of planet formation. The system is about 100 light-years from
Earth. One light-year is the distance light travels in a year six trillion
miles (9.7 trillion km).
A similar fender-bender is thought to have formed
Earth's moon more than 4 billion years ago, when a body the size of Mars
rammed into Earth.
"The collision that formed our moon would have been
tremendous, enough to melt the surface of Earth," said co-author Geoff
Bryden of NASA's Jet Propulsion Laboratory in California. "Debris from the
collision most likely settled into a disk around Earth that eventually
coalesced to make the moon. This is about the same scale of impact we're seeing
with Spitzer - we don't know if a moon will form or not, but we know a large
rocky body's surface was red hot, warped and melted."
In fact, such violent encounters seem to have been common in
our own solar system's early history. For example, giant impacts are thought to
have stripped Mercury of its outer crust, tipped Uranus on its side, and spun
Venus backward.
As recently as last month, a small space rock slammed
into Jupiter, making a large black bruise.
In general, rocky planets like Earth coalesce and grow when
small rocks crash and clump together, merging their cores.
The system around HD 172555 is a relative baby at only 12
million years old, compared to our solar system's age of 4.5 billion years.
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