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Alien dust kicked up by baby planet collisions

In the search for other planetary systems like Earth that are capable of hosting extraterrestrial life, scientists have come across some very alien systems indeed. But the latest ones have researchers truly perplexed.
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In the search for other planetary systems like Earth that are capable of hosting extraterrestrial life, scientists have come across some very alien systems indeed. But the latest ones have researchers truly perplexed.

New observations have found evidence for planet formation around stars much more massive than the sun, as well as dusty debris – thought to be leftovers from collisions between rocky planetary embryos. There's a twist: The dust has a completely different chemical makeup from the composition of our own solar system.

These new findings were presented this month at the 215th meeting of the American Astronomical Society in Washington, D.C.

The strange dust that shows different chemical markers than what we find in our own neighborhood is in a star system about 500 light-years from Earth.

The parent star, known as HD 131488, is surrounded by warm dust in a region called the terrestrial planet zone, where the star heats dust to temperatures similar to those found on Earth.

Infrared imaging and spectroscopic measurements of the system, performed by the Gemini South Telescope in Chile, showed the unusual chemical composition.

"Typically, dust debris around other stars, or our own sun, is of the olivine, pyroxene, or silica variety – minerals commonly found on Earth," said Carl Melis, who led the research while a graduate student at UCLA. "The material orbiting HD 131488 is not one of these dust types. We have yet to identify what species it is – it really appears to be a completely alien type of dust."

Melis and his team think that the most plausible explanation for the presence of the warm dust is a recent collision between two rocky planetary-mass bodies.

Cold dust
While the warm dust is located at a distance from HD 131488 comparable to that between the Earth and the sun, Melis and his team also found cooler dust about 45 times farther out. This distance is analogous to the Kuiper Belt in our own solar system. The detection of both cold and warm dust around a young star is unusual, the team noted.

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"Although dusty telltales of planetary formation processes in the outer regions surrounding young stars have often been seen with infrared-sensitive space telescopes, for some reason stars that have large amounts of orbiting warm dust do not also show evidence for the presence of cold dust," said Benjamin Zuckerman, also of UCLA. "HD 131488 dramatically breaks this pattern."

The cooler dust likely did not result from planetary collisions, and is instead probably the leftovers of planet formation that took place farther away from the sun, Melis said.

HD 131488 joins five other stars with masses a few times larger than the sun that show evidence of dust in their terrestrial planet zones. All five are young stars, around 10 million to 30 million years old (the sun, by comparison, is 4.5 billion years old).

"This finding indicates that the epoch of final catastrophic mass accretion for terrestrial planets, the likes of which could have resulted in the formation of the Earth-moon system in our own solar system, occurs in this narrow age range for stars somewhat more massive than our sun," Melis said.

The team plans to further study HD 131488 to try and determine what strange minerals make up its dust, as well as look for evidence of rocky planet formation around other stars.

Massive stars
More dusty evidence of planet-forming disks around other stars has also been found around stars that are much heftier than the sun.

Xavier Koenig of the Harvard-Smithsonian Center for Astrophysics and his colleagues examined the star-forming region called W5, which lies about 6,500 light-years away in the constellation Cassiopeia.

They looked at over 500 type A and B stars, which are stars about two to 15 times more massive than the sun. Around one-tenth of those stars, the astronomers found evidence of dusty disks. And of those, 15 disks showed signs of clearing that suggest that newborn Jupiter-sized planet are sucking up some of the dusty material as they form.

"We think we've found evidence for planets forming around stars twice the mass of the sun, and even bigger stars," Koenig said.

Because these stars are only 2 million-to-5 million years old (our sun is middle-aged by comparison), and the fierce stellar winds from them are expected to rapidly destroy any dust disks around them, the new findings indicate that planets must form quickly or not at all – at least around more massive stars.

The unfriendly nature of these possible planetary systems combined with the short lifetimes of these stars (around 10 million to 500 million years) means that these systems are unlikely to harbor any higher forms of life.

"These stars aren't good targets in the hunt for extraterrestrials," Koenig said.

But these systems will help astronomer to learn more about planet formation across the broad range of stellar systems in our galaxy.

"We want to understand what kind of planets form in what stellar systems," Koenig said. And if the evidence his team found holds up, "this will tell us that planet formation is both a natural and common result of star formation," he added.