New Evidence for Planets Orbiting a Pulsar

Article excerpt

After 2 years of additional study, a radioastronomer reports "irrefutable" evidence confirming the existence of two planets orbiting a dense Milky Way star. Estimated at about three times the mass of Earth, these planets would be the first identified outside the solar system.

Alexander Wolszczan of Pennsylvania State University in University Park reported his initial planetary finding in 1992, after studying radio emissions from a compact star some 1,300 light-years from Earth. Now dubbed PSR B1257+12, this dense resident of the Virgo constellation is a millisecond pulsar - a neutron star that rotates like clockwork hundreds of times a second. A millisecond pulsar acts like a lighthouse beacon, aiming radio waves toward Earth at precise intervals.

But in studying the pulsar with the 305-meter radio telescope at Arecibo Observatory in Puerto Rico, Wolszczan detected small fluctuations in the arrival time of the star's radio signals. Some of the waves arrived about three-thousandths of a second sooner than predicted, while others arrived about three-thousandths of a second later.

The variations suggested that the pulsar wobbles, moving closer to and farther from Earth in a quasiperiodic fashion. Wolszczan and a colleague concluded that the best explanation for the wobbling would be the gravitational tug supplied by two or possibly three unseen planets orbiting the pulsar (SN: 1/11/92, p.20).

Because it's unlikely that current telescopes could detect the dim, faraway planets directly, Wolszczan searched for other ways of verifying his finding. In the Jan. 23, 1992 NATURE, a team of astronomers, including Frederic A. Rasio, now at the Institute for Advanced Study in Princeton, N.J., suggested a strategy, Given the special relationship between the orbital periods of two of the proposed planets, their mutual gravitational tug should produce a tiny additional wavering of the pulsar's radio signals. …