A vast mess of frozen lava and vaporised rock has been
found orbiting a nearby star, evidence of a cataclysmic collision
between planet-like bodies outside our solar system.
Such collisions
are thought to have created Earth's moon and left other scars
in the solar system, but it's not yet clear how common they are
around other stars.
Hints of past violent collisions abound in our solar system.
Many suspect the moon
formed from the debris created when a Mars-sized object smashed
into the Earth. Other smashups may have
pulled off most of Mercury's crust, tilted Uranus on its side,
and caused Venus to spin backwards.
Now a team has found evidence of an intense impact surrounding
the star HD 172555, which sits some 100 light years away in the
southern constellation Pavo, or Peacock.
This is the first time materials like volcanic glass and
vaporised rock have been found orbiting a young star that is old
enough to have formed planets, says Carey Lisse of Johns Hopkins
University in Laurel, Maryland. The star is 12 million years
old.
Preliminary evidence also suggests two other stars show similar
hints of cataclysmic impacts, Lisse says. "We're now trying to
figure out whether we've found a new class of rare but very
exciting systems," Lisse told New Scientist.
Volcanic glass
Lisse and colleagues examined the spectrum of HD 172555's
infrared light, captured with
NASA's orbiting Spitzer
Space Telescope. Bumps and dips in the spectrum can reveal the
chemical composition of the star as well as the object that
surround it, and the team found two unusual features.
One peak matched up with volcanic, silica-based glasses. This
material is similar to obsidian, a dark glass created in volcanic
eruptions, and tektites, cooled chunks created when liquid rock is
ejected during impacts, cooling and hardening in flight.
Another peak revealed large quantities of silicon monoxide gas -
a byproduct of vaporised rock. The team also found a vast amount of
cold, dark dust and rubble.
The total amount of debris adds up to a mass between that of
Pluto and the moon, and points to a cataclysmic impact in which two
bodies slammed into each other at a relative speed of at least 10
kilometres per second. But it is difficult to say for sure what
sorts of bodies might be responsible for the collision.
A smashup sometime in the last million years between two large
rocky bodies, each perhaps rivalling the size of Mercury or the
moon, might explain the debris.
Other moons?
Studying systems that have suffered large collisions could
reveal more about how the solar system formed, says George Rieke of
the University of Arizona in Tucson, who was not affiliated with
the study.
"The interesting science twist is that we are actually finding
traces of the kinds of major collisions that we think shaped the
Earth-moon system," Rieke says.
But it is not clear whether the collision around HD 172555
created debris that would have coalesced into an object like the
moon. "This could just mean that two things smashed each other to
smithereens," Rieke told New Scientist.
The fact that only a few stars have so far shown evidence of a
violent collision, even though hundreds seem to be surrounded by
dusty discs, suggests the violent impacts that seem to have shaped
our solar system might be fairly rare. "It implies at some level
moons like ours may be pretty uncommon," Rieke says.