Planetary crash leaves stellar ring
A violent crash between two planetary bodies has created a ring of debris encircling a nearby young star, say scientists, similar to the event that formed the Moon.
The host star, HD 172555, is located about 100 light-years away in the southern constellation Pavo, the Peacock.
Scientists using NASA's infrared-sensitive Spitzer Space Telescope found unexpected chemical signatures when they analysed the star's light.
Lead researcher Dr Carey Lisse, of Johns Hopkins University Applied Physics Laboratory in Maryland, recognised the fingerprints of melted glass and gases such as silicon monoxide - created by vaporised rock. He had seen similar signatures from the comet-sampling missions Deep Impact and Stardust.
"Without those two missions we wouldn't have had the confidence to say what it is we were seeing. Now we're aware of what's possible," says Lisse. "This was something that really stood out hard."
Scientists suspect an object about the size of Earth's moon crashed into a Mercury-sized body orbiting HD 172555 about 1000 years ago, creating an unusual debris ring circling about as far as the distance of Jupiter from the Sun.
A similar event early in the history of our solar system is believed to have positioned material for the Moon to form.
The crash around HD 172555, however, likely booted debris too far from its fledgling planet's gravitational grasp to coalesce into a moon.
"The impact that formed [Earth's] moon had to happen relatively slow," Lisse says. "It's much easier to see a big, spread-out mass."
The scientists' research appears in next week's Astrophysical Journal.
It follows a report this week of another potential cosmic collision that turned around a planet's orbit.
WASP-17, which orbits a star about 1000 light-years from Earth, is the first planet ever found that circles its mother star in the opposite direction to how the star spins.
WASP is the acronym for the Wide Area Search for Planets, run by a consortium of UK-based universities.
So far, the most likely explanation for the backward orbit is that the planet's path was reversed very early in its history as a result of an impact or near-miss with another body.
"It's one that the theorists need to do a lot of investigation on," says astronomer Professor Coel Hellier of Keele University in the UK.
"All the [computer] models that would result in having a retrograde [backward] orbit, you need some other body that is perturbing it."
While comets and other small bodies have been found orbiting backwards, WASP-17 is the first large body to do so, Heillier says.
The planet's odd orbit may explain its huge size.
WASP-17, which is the largest planet found so far, has billowed out to roughly twice the size of Jupiter, though it has approximately the same mass.
Scientists suspect extreme tidal and gravitational forces are to blame.
The planet orbits extremely close to its mother star, circling in just 3.5 days.
Mercury, the closest planet in our solar system to the sun, takes 88 days to complete an orbit.
Any debris from a crash that may have flipped WASP-17 would have long settled into new bodies or scattered away, Heillier says.
"If there was a collision, it was probably a billion years ago or something very early on in that planetary system. This system is quite old, two to three billion years old.'