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Planets Might Form from Dead Stars

Posted on: Wednesday, 5 April 2006, 13:25 CDT

By Deborah Zabarenko

WASHINGTON -- Planets outside our solar system might form, phoenix-like, out of the debris circling a dead star known as a pulsar, researchers reported on Wednesday after finding the makings for a planet near such a body.

This could mean that planet-formation could be more common than previously thought, said Deepto Chakrabarty of the Massachusetts Institute of Technology.

"What's remarkable here is this process of planet formation, which we associate with the birth of stars, seems to also be able to occur at the end of the stellar lifetime, sort of a renaissance of the system, in some sense," Chakrabarty said in a telephone interview.

Most planets are thought to form around young stars, like the sun, from debris left over when the star formed. This cosmic dust and gas spreads out around the star in a wide ring known as a protoplanetary disk, and the planets are made when gravity causes the debris to clump together.

This occurs typically when stars still have plenty of nuclear fuel at their cores. However, research by Chakrabarty and his colleagues found a disk around a pulsar, the remnant from a spent star which ran out of nuclear fuel and collapsed, tossing out debris as it died.

OLD DEAD STAR

Using NASA's Spitzer Space Telescope, an orbiting observatory that tracks infrared light, the team spied the disk around a pulsar about 13,000 light-years from Earth, in the constellation Cassiopeia.

A light-year is about 6 trillion miles, the distance light travels in a year. The pulsar and its disk are part of the Milky Way galaxy, which contains Earth.

These findings, published in the journal Nature, connect with an earlier discovery of so-called extra-solar planets around a pulsar more than a decade ago.

"At the time those planets were discovered, it was a puzzle how they could have formed, although the one way that astronomers know to form planets is out of some sort of disk of debris," Chakrabarty said. "No one had ever seen a disk around a pulsar, around an old dead star."

A pulsar is the densest known kind of object in the universe, packing the mass of a sun-like star into a ball about 10 miles across, he said. A single teaspoon of its mass would weigh about 2 billion tons (2.032 billion tonnes).

Anything around it would be inhospitably bathed in intense X-ray radiation and particle radiation of the sort that can only be achieved on Earth in a particle accelerator. The researchers were surprised that the dust disk could remain there without being promptly torn apart.

But infrared emissions from the disk showed solid material, such as dust grains, that add up to the mass of 10 Earths, Chakrabarty said. And dust grains have been the seeds of planets in other planetary systems outside our own.

No planets are in evidence around the pulsar the scientists studied, he said, but the fact that the disk has survived is significant: "It says that this planet formation process seems to be a much more robust process than we initially realized."

Source: REUTERS

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