Planet-Forming Disk Spotted Around Dead Star

John Roach
for National Geographic News
April 5, 2006
A disk of potentially planet-forming debris has been found around a pulsar about 13,000 light-years from Earth, scientists announced today.

The debris is most likely material that has fallen back toward the star after a supernova, or star explosion. The material could clump together to form planets, astronomers say, but such planets would be unlikely to harbor life.

"This disk looks remarkably like those also seen around ordinary young stars in which planets are known to form," said Deepto Chakrabarty, a physics professor at the Massachusetts Institute of Technology in Cambridge, who led the discovery team.

"But for the first time we may be seeing the start of planet formation in the very different, harsh environment that exists around an old dead star like a pulsar," he added.

Pulsars form during the collapse and death of stars that are between 10 and 20 times more massive than our sun, Chakrabarty explains.

Chakrabarty described the discovery in a press teleconference this afternoon. A paper detailing the debris disk will appear in tomorrow's issue of the journal Nature.

Pulsar Planets

When a massive star collapses, the inner core, which is about 10 percent of the original mass, is compressed into a dense object only 10 miles (16 kilometers) across called a neutron star.

The other 90 percent of the mass is ripped away in an explosion.

Neutron stars that rotate rapidly give off regular pulses of radiation, which is why they are called pulsars, Chakrabarty said.

Scientists theorized that some of the ejected material could have falled back toward the pulsar and stabilized in a disk around the star.

Using NASA's Spitzer Space Telescope, Chakrabarty and his colleagues found one of these fallback disks around a pulsar named 4U 0142+61 in the constellation Cassiopeia.

The researchers combined their data with observations from ground-based telescopes and concluded that the disk orbits the pulsar at a distance of about a million miles (1.6 million kilometers) and contains roughly the same mass as ten Earths.

Alexander Wolszczan is a professor of astronomy and astrophysics at Pennsylvania State University in University Park.

Wolszczan was not part of the discovery team but took part in the teleconference. He says that the debris disk almost certainly formed after the supernova explosion, as no pre-explosion disk could have survived a supernova.

The finding fits in with his 1992 discovery of three planets orbiting a pulsar called PSR B1257+12, the first planets of any type ever found outside our solar system.

Wolszczan says the new finding helps confirm a scientific theory on planet formation.

"Now we can more confidently say that planets around pulsars … are probably not that uncommon at all," he said in the teleconference.

Uninhabitable Planets

These pulsar planets are unlikely to harbor life, according to Charles Beichman, an astronomer at NASA's Jet Propulsion Laboratory (JPL), based at the California Institute of Technology in Pasadena.

"We are really looking at an environment that is very hostile," he said in the teleconference.

"This is not beachfront property, this is not Malibu. It's not what the search for habitable planets is likely to find as a very attractive place."

Beichman explains that, for planets to potentially harbor life, they need to be warm enough to support liquid water.

Such planets then need to have the elements of known life—particularly light elements such as hydrogen, oxygen, and carbon—and a source of energy.

"The environment of pulsar planets would be anything but habitable," he said.

While the planets could be close enough to the pulsar to have a warm temperature at the surface, lighter elements are likely to have been thrown off into deep space by the supernova explosion.

Also, the star would have burned up any available hydrogen before it died.

"The fallback material is likely to be very rich in basic elements—iron and things much heavier than what we are used to seeing in large abundance in the solar system," he said.

Nevertheless, he added, today's announcement improves overall understanding of the processes that lead to planet formation.

What's more, Beichman said, "If planets can form around a pulsar, they can form just about anywhere."

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