News Analysis

When worlds collide – how the moon was born

Rachel Courtland

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.

 

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