Page last updated at 13:20 GMT, Monday, 20 April 2009 14:20 UK

Rocky clues from dirty dead stars

By Jonathan Amos
Science reporter, BBC News

Rocky debris (Nasa)
As rock is drawn in towards the star, it would be torn apart

A survey of dead stars has found that many are probably surrounded by the rocky remains of asteroids and planets.

The discovery comes from a Spitzer space-telescope study of white dwarfs - the fading embers of burnt-out stars.

Astronomers showed at least one in 100 of these objects has "surface" layers that are contaminated with elements such as calcium and magnesium.

They say the elements could only have come from rocky debris that was constantly raining down on the stars.

"We used Spitzer to study these white dwarfs and look for elements that shouldn't be there and we found that a very significant fraction of them had closely orbiting dust right on top of them," said Dr Jay Farihi of the University of Leicester, UK.

"We see this phenomenon 1-3% of the time, so we infer that main sequence stars similar to our Sun - or perhaps just a little bit bigger - have terrestrial planets 1-3% of the time."

Dr Farihi told BBC News: "In our galaxy, a very rough estimate would be that there are five million white dwarfs with left-over rocky planetary material."

Pollution signature

Just how many stars out there might harbour terrestrial planets, like an Earth or a Mars, is an open question because the technology needed to detect them directly is only now becoming available.

Until astronomers can routinely identify them, indirect methods must be used to infer the planets' existence - such as with the Spitzer study.

Spitzer (Nasa)
The orbiting Spitzer telescope looks at the cosmos in the infrared

Spitzer is the US space agency's (Nasa) infrared orbiting observatory.

It is tuned to see the longer wavelengths of light emanating from cooler objects, such as the compact white dwarf stars that have ended their main hydrogen- and helium-burning phase and blown off their outer layers.

These objects tend to be about the mass of our Sun but encapsulated in a body no bigger than the Earth. Their strong gravity pulls any heavier elements that are present to the core, leaving just the very light elements of hydrogen and helium to float on top.

But, occasionally, scientists do see the signatures of heavier elements persisting at the surface; and the only way to explain this is if material is continually being drawn down on to the white dwarfs.

The model constructed by Dr Farihi and colleagues to explain Spitzer's latest observations suggests planets in orbit around the dead stars are disrupting smaller objects, hurling them towards the dwarfs.

Planet hunting

Close in, these rocks are broken up by gravitational tides (the same phenomenon which led to the creation of Saturn's rings) to form dusty discs that feed the pollution.

"We see heavy elements," said Dr Farihi. "Calcium is the main one, but we also see magnesium and iron.

"These are typical of rocks. They're not typical of, say, interstellar clouds which are rich in hydrogen and volatile elements such as carbon and helium. It's not a definitive conclusion but it's pretty supportive of the idea that this material is rocky."

Dr Farihi said dirty white dwarfs would be excellent places to go looking for Earth-sized planets once the detection technology matures. However, he cautioned that worlds that circle dead stars would not be very interesting places.

"They're not going to harbour water and they're not going to be great sites for habitability. But they may have been habitable previously and perhaps life did get started on them. It's speculation, and the outlook for such things is bleak," Dr Farihi said.

The researcher was presenting the discovery here at the European Week of Astronomy and Space Science conference at the University of Hertfordshire.

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