First known exoplanets have few counterparts

Planets and pulsars, the whirling cores of dead massive stars, appear to be an unlikely match. Most of these pulsating stellar corpses can’t nurture fledgling planetary systems, researchers suggest.

Scrutiny of 151 pulsars turned up no evidence for planets, researchers report in the Aug. 10 Astrophysical Journal. Planet nurseries must therefore rarely appear in the wakes of supernova explosions out of which pulsars are born, Matthew Kerr and colleagues propose.

“I was fairly pessimistic about finding any,” says Kerr, an astrophysicist at the Australia Telescope National Facility in Epping. “But you never know until you go and look.”

The first planets confirmed beyond the solar system were found around a pulsar designated PSR B1257+12, researchers announced in 1992. Despite the discovery of over 1,500 worlds since then, astronomers have found only one other pulsar, PSR B1620-26, that harbors a planet.

Any planets around a pulsar either improbably survived the detonation of their sun or formed out of debris raining down in the wake of the explosion. If the backwash of gas and dust forms a disk encircling the pulsar, then planets might form the same way they do around young stars. The lack of pulsar planets suggests that such disks are rare.  

One caveat is that all 151 pulsars are young and energetic, notes Alex Wolszczan, an astrophysicist at Penn State and codiscoverer of the first pulsar planets. Young pulsars blast the surrounding space with radiation. “You run the risk that you evaporate a disk even before planet formation starts happening,” he says. Expanding the study to include older pulsars — an effort that Kerr has already started — would be more definitive.

The two known pulsar planet systems appear to be oddballs. The pulsar that hosts the first confirmed exoplanets has a relatively weak magnetic field, which might have helped a planet-building ring to form. The second pulsar shares its planet with a companion star; the planet was probably snatched from the pulsar’s neighbor.

“People were so enthusiastic about those two serendipitous discoveries, they believed we would have found more surprises by now,” says Stephen Thorsett, president of Willamette University in Salem, Ore., and codiscoverer of the second pulsar planet system. “The fact that they aren’t found is a pretty strong statement that they aren’t there.”

Editor’s Note: This story was updated on September 15, 2015, to correct the name of the Australia Telescope National Facility.