Hostile birth.
A protoplanetary disk surrounds pulsar 4U 0142+61 in this artist's rendering.


Planets, Planets Everywhere

The more astronomers look for planets outside the solar system, the more they seem to find them--and in the unlikeliest of places. The latest evidence came Wednesday at a NASA news conference, where a team of astronomers announced that the Spitzer Space Telescope has identified what appears to be a disk of planet-forming rubble surrounding a spinning neutron star known as a pulsar.

Pulsars are like the giant radioactive ball bearings of the universe--hard, dense, almost perfectly round, and spewing out massive amounts of x-rays. The super-compact remnants of giant stars that went supernova, they are made mostly of neutrons, with the mass of the sun pressed into a sphere no more than 16 kilometers wide. Just a teaspoon's worth of a pulsar would weigh over 2 billion tons. They also generate tremendous magnetic fields, trillions of times more powerful than Earth's. Not a pleasant place for planets to dwell, but that seems to be where more of them are popping up.

The Spitzer data, collected in infrared light, show a flat disk of dust orbiting the pulsar, named 4U 0142+61 and located about 13,000 light-years away in the constellation Cassiopeia. Once a star with a mass between 10 and 20 times the sun's, 4U 0142+61 probably survived no more than 10 million years before collapsing some 100,000 years ago. Its crushing mass created the tiny neutron core and then recoiled explosively into space.

Some of the fallback from that explosion eventually settled into a protoplanetary disk closely circling the pulsar. That disk orbits 4U 0142+61 about 1 million miles out and probably contains about 10 Earth-masses of material, according to the team, which publishes its data today in Nature.

"Now we can say [planets around pulsars] are not uncommon," says team member Aleksander Wolszczan of Pennsylvania State University in State College, who also discovered the first three known exoplanets orbiting another pulsar, PSR B1257+12, in 1992. "Planet formation must be a much more robust phenomenon that we thought."

Robust and fast, says Re'em Sari, a planetary scientist at the California Institute of Technology in Pasadena. "They would be small in size, but planetary embryos could have emerged around that pulsar within 100,000 years. And now there's much clearer evidence of how such planets could form."

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