Wavering radio signals hint at an unseen planet orbiting a pulsar.

British astronomers have found tantalizing evidence that a planet-size object orbits a pulsar some 33,000 light-years from Earth. If their finding proves correct, it will mark the first detection of a planet outside the solar system. A confirmation of this provocatives result would also challenge accepted theories about the formation of pulsars.

Previous reports of planets orbiting stars other than the sun have not withstood further scrutiny. But the British team maintains that the long-term consistent nature of their data bolsters their finding.

Early last year, Andrew G. Lyne and his colleagues noticed a peculiar, periodic pattern in the arrival times of radio emissions from PSR1829-10, a Milky Way pulsar they had discovered in 1985. In reexamining earlier data, they discovered that the timing of radio signals from this star, unlike that of the 39 other pulsars, they had identified, had fluctuated about every six months since close monitoring of PSR1829-10 began in 1987.

Six weeks ago--after Lyne and his co-workers at the University of Machester rejected other explanations for the star's variability--they arrived at a startling conclusion: The emission pattern indicates that the pulsar and a much smaller body orbit each other, and that the "companion" object meets the criteria for a planet. The unseen object, perhaps only 10 to 15 minutes the mass of the Earth, follows a nearly circular path about 105 million kilometers from the pulsar -- roughly the distance between Venus and the sun--and each orbit takes six months to complete, the researchers report in the July 25 NATURE. "If there is a companion, the work... will challenge some, perhaps several, fundamental aspects of our view of the evolution of stellar and planetary systems," writes David Black, of the Lunar and Planetary Institute in Houston, in an accompanying commentary.

The pulsar and its planetary companion, Lyne says, would orbit around a common center close to the pulsar, as is the case with Earth and the sun. When the pulsar travels on the far side of its orbit, its radio pulses take longer to reach Earth; on the near side, they arrive sooner. This hypothetical scenario, Lyne aseerts, would account for the six-month changes in the timing of the signals.

While the discovery of a possible planet outside our solar system seems dramatic in itself, the presence of a planet near a pulsar poses a special puzzle, Lyne says, particularly if the planet predated the pulsar's explosive birth. According to standard theory, the catastrophic events leading to the formation of a pulsar--a rotating neutron star--should destroy a planet or eject it from the star's gravitational grasp.

For example, if the red giant star that likely gave rise to the pulsar suddenly shed more than half its mass to form the new pulsar, the planet could not remain bound. In any case, the ensuing shock wave spewed out by the collapsing red giant would blow the planet apart. But Lyne and his colleagues suggest that the pulsar may have formed more slowly and less violently than the standard theory permits, and that this would enable the planet to remain intact. "If this 'kinder, gentler' process works, the new discovery will have led to a revolution in [pulsar] astrophysics," writes Black.

Lyne and his colleagues have also developed an alternative theory, which assumes that the planet did not predate the pulsar. In this model, which Black calls "more likely equally intriguing," a neutron star spinning especially rapidly would slow down by forming a disk around its equator, and material in the disk could condense into a planet. Researchers believe the solar system planets arose from a disk that once surrounded the sun. A planet that formed in this way could have several features similar to those of the object inferred by the British team, Black says.

Direct observations of the proposed planet seem unlikely, since the object probably reflects light dimly at all wavelengths, lyne says. Black says he's reluctant to call the object a planet, because this substellar mass and its environment would bear little, if any, resemblance to known planets.

Lyne says his group will continue monitoring PSR1829-10 to corroborate their findings and to examine whether subtler changes in the radio emissions indicate the presence of a second, more massive planet. The team will also look for periodicity in emissions from other pulsars in the hope of uncovering evidence of other planet-like objects.