Ring around the Pulsar: Planets may form in a harsh environment

Astronomers over the past 2 decades have discovered several hundred planet-forming disks around young, ordinary stars. But now, researchers report evidence of one of these rings of debris in a most unlikely place—circling the ultradense cinder of a star that died in a supernova explosion.

SUPERDISK. A disk of rubble and gas, possibly capable of forming planets, surrounds a pulsar known as 4U 0142+61, in this artist’s concept. The pulsar lies about 13,000 light-years from Earth. Chakrabarty et al., JPL/NASA

The finding suggests that these stellar blasts might create an environment in which planets can coalesce. The presence of the disk may also shed light on the still poorly understood events that trigger supernova explosions.

Researchers found the disk around a rapidly rotating neutron star, the dense core left behind when a star 8 to 20 times as massive as the sun collapses under its own weight and explosively ejects its outer layers. This particular core, known as an anomalous X-ray pulsar, broadcasts X rays of high intensity.

This radiation heats the surrounding gas and dust, causing it to glow at infrared wavelengths. The Spitzer Space Telescope, which orbits Earth, detected the infrared radiation. Although Spitzer lacks the resolution to create an image of the disk, the pattern of light that the telescope recorded provides the first clear evidence of a disk around an exploded star, says codiscoverer Deepto Chakrabarty of the Massachusetts Institute of Technology. The disk contains about 10 times as much mass as Earth, he and his collaborators report in the April 6 Nature.

Although pulsars emit large amounts of high-energy radiation, creating a harsh environment, the new disk nonetheless resembles those found in milder conditions around young, planet-forming stars, Chakrabarty says.

The finding rounds out a 14-year-old puzzle about planet formation outside the solar system, says Charles Beichman of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. In 1992, researchers analyzing radio signals from an elderly pulsar announced that they had found evidence of three unseen planets around the dense body (SN: 1/11/92, p. 20). Those planets might have formed in a disk that dissipated over the pulsar’s lifetime, estimated at a billion years.

The disk discovered by Chakrabarty and his colleagues surrounds a pulsar that’s only about 100,000 years old. That’s around the time when planet formation around any star is likely to begin, notes Beichman. Taken together, the 1992 discovery and the new finding provide information on some of the early and late steps in planet formation around pulsars.

But how does an exploded star create a disk in the first place? More than a decade ago, Stan Woosley of the University of California, Santa Cruz and his colleagues proposed that when a massive star explodes, it doesn’t always have enough oomph to permanently shed all its outer layers. Some of the ejected material would then fall back toward the exploded star.

Most of that material would spiral onto the pulsar, but a small amount could end up as a swirling disk like the one now detected by the Spitzer telescope, notes Woosley.

The disk confirms that fallback is a feature of supernovas, Woosley says. Fallback could build up both the spin and the mass of a pulsar. That’s important because the extra rotation could power gamma-ray bursts, extraordinarily energetic outpourings of radiation that have been linked to some supernovas, he adds.

Woosley says that the extra mass might also transform a pulsar into an even more exotic object, a black hole.

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