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Crushed comets give star a dusty belt

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Two thousand comets a day collide around nearby star Fomalhaut creating a continually replenished dust belt in the outskirts of the star's system, according to a new paper recently published in the journal Astronomy & Astrophysics.

Fomalhaut and its dusty disk, as seen by Herschel. Credit: {link url="http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50254"}ESA/Herschel/PACS/Bram Acke, KU Leuven, Belgium{/link}

Fomalhaut is a young star. It is twice as massive as the sun and sits 25 light years away from us. In the 1980s, astronomers discovered that it was surrounded by large amounts of dust. The Herschel Space Observatory has now produced the best ever far-infrared images of the star system and given a team of astronomers lead by Bram Acke at the University of Leuven in Belgium chance to take a fresh look at the system.

Most intriguing is a narrow belt of dust and debris in the outer edges of the Fomalhaut system, that is a bit like the solar system's own Kuiper belt. Fomalhaut's belt is 140 times further from the star than the Earth is from the sun. The dust particles that fill it have temperatures between -170C and -230C.

In our solar system, the Kuiper belt includes Pluto and two other dwarf planets but mostly consists of smaller icy objects left over from the formation of the solar system. Belts like this tend to exist in planetary systems at locations where, for one reason or another, no planets formed. Fomalhaut's belt is much younger than the Kuiper belt, though. And it is more active too. The Fomalhaut system resembles our solar system in its most active phase, says Acke.

Fomalhaut system as seen by Hubble's High Resolution Camera (HRC) of the Advanced Camera for Surveys (ACS) in 2008. Credit: {link url="http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=43724"}NASA, ESA and P. Kalas (University of California, Berkeley, USA){/link}

The belt is off-centre with respect to its star, hinting at one or more planets close by interacting with it via gravity. The narrowness of the belt, confirmed by the new Herschel observations, also points to planets nearby that keep the dust and debris in place and stopping it spreading out.

But that's not the most interesting thing about Fomalhaut. Neither is the existence of a dusty disk around the star – they are probably as common as planets (which are very common indeed). "What makes Fomalhaut special is the presence of large amounts of very small dust," says Acke.

Dust in Fomalhaut's narrow belt absorbs light as if it were made of tiny particles, micrometers across, according to the new Herschel observations of the dust grains' "glow". But previous Hubble observations saw the dust belt reflect light as if it were made of much larger grains.

To resolve this, Acke and his colleagues suggest that the grains that fill Fomalhaut's belt are "fluffy". That is, they are small, as Herschel shows, but clump together loosely and reflect light as if they were bigger.

A fluffy dust particle from our own solar system, formed by a cometary collision. Credit: {link url="http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=50256"}NASA{/link}

But there was another problem: small grains like those in Fomalhaut's belt should get blown out of the system by strong winds from the star. Larger grains have enough gravitational staying power to resist getting blown away, but smaller ones do not. That there are still small grains in the belt suggests that they are being constantly replenished somehow.

The "fluffy" nature of the particles pointed to comets as the source of the dust, says Acke. So, along with his colleagues, he calculated how many comets would need to be destroyed to keep the belt full of tiny dust particles. "From the amount of small particles, we deduced that 2000 [1km-sized] comets are reduced to dust each day," he says.

The views expressed are those of the author and are not necessarily those of Scientific American.

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