Trillions of comets collide around Fomalhaut

Posted by Kate Taylor

A ring of dust around the nearby star Fomalhaut is created by the collision of thousands of comets every day.

Just 25 light years away, Fomalhaut is a fairly young star, twice as massive as our sun and discovered in the 1980s to be surrounded by large amounts of dust.

Now, astronomers using the Herschel space telescope have taken a close look, and discovered a denser belt of dusty material, about half of which is water ice, on the outer edges of the system.

The belt's a long way from the star itself, at more than 100 times the distance of the Earth from the sun. This makes it very cold, at around -200 Celsius.

"This disc is similar to the Kuiper Belt in our Solar System, which lies beyond the planet Neptune, but is much, much younger," says Dr Bruce Sibthorpe of the UK Astronomy Technology Centre.

"As well as relatively large objects, such as Pluto, our Kuiper Belt also contains millions of much smaller objects."

The dusty belt is both narrow and off-centre relative to the star, implying that there could be planets orbiting close to it. Back in 2008, the Hubble Space Telescope took what may be a picture of one.

Infrared observations indicate that the dust's made of very small grains, just a few thousandths of a millimetre across.However, Hubble images show that it scatters light in the same way as much larger particles - implying that they're actually 'fluffy' particles consisting of smaller grains stuck together.

But as such grains would inevitably be blown out of the system by the intense light from Fomalhaut, it now seems that there must be a continuous re-supply - most likely produced by the continual collisions and disintegration of larger asteroid-sized objects.

To produce the amount of dust seen by Herschel requires the equivalent of around 2,000 kilometer-wide comets to be destroyed every day.

"I was really surprised," says Dr Bram Acke of the University of Leuven in Belgium. "To me this was an extremely large number.”

Such a large number of collisions, he says, implies that there are trillions of comets in the ring in total, containing enough material to make over one hundred Earths.