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Scrutinizing Pulsar Planets.

In 1992 the first planets outside of our solar system were discovered where they were least expected: orbiting a pulsar, PSR B1257+12 in Virgo. How and when planets might form around the ultradense cores of exploded stars remains unclear. But PSR B1257+12 is apparently not alone. Scientists have identified four other pulsars with the small shifts in their signals that could indicate the presence of more extrasolar planets.

A recent paper, however, casts doubt on the existence of two planets that supposedly orbit the pulsar PSR B0329+54 in Camelopardalis. In the July 1st Astrophysical Journal Letters, a team of researchers led by Maciej Konacki (Nicolaus Copernicus University, Poland) claims that the planets long thought to orbit the well-known pulsar are "unlikely to be real." Scientists first speculated that a planet might account for an apparent three-year periodicity in the pulsar's timing data in 1979. In 1994 Russian astrophysicist Tatiana V. Shabanova asserted the existence of the three-year planet as well as an Earth-mass planet with a nearly 17-year period (S&T: January 1996, page 10).

But Konacki and his colleagues combined more than 10 years' worth of data from the Jet Propulsion Laboratory with data from observatories in Germany and Poland, and they found none of the distortions in the pulsar's pulses that would be expected if either planet really orbited the spinning neutron star. They suggest that any anomalies are due to the nature of the relatively young, rapidly evolving object. "Timing noise often appears periodic when you look at it over a finite interval," says coauthor Aleksander Wolszczan (Pennsylvania State University). "But when you look at more data, it goes away." According to Wolszczan, seismic activity or precession of the star's spin axis are typical in pulsars like PSR B0329+54 and could account for the noise in the timing data.

Wolszczan, a codiscoverer of the first planets around PSR B1257+12, is no stranger to controversy over the existence of planets around pulsars. In the December 12, 1997, issue of Science, a research team lead by Klaus Scherer (Max Planck Institute for Aeronomy, Germany) challenged Wolszczan's interpretation of PSR B1257+12's timing anomalies. Working with results from the Pioneer 10 mission, Scherer's team noticed that a 25.3-day solar-wind cycle, caused by the Sun's rotation, interfered with the Doppler data transmitted by the spacecraft. Since the period of the pulsar's innermost planet closely matches that of the solar-wind cycle, Scherer and his colleagues wrote the planet off as "an artifact of the heliosphere."

However, a preliminary analysis of Wolszczan's most recent measurements suggests that the 25.3-day periodicity is frequency independent and thus not related to solar interference. Wolszczan remains confident that this conclusion will remain valid. Meanwhile, evidence continues to support the existence of the pulsar's second and third planets.

That a roughly 10-Jupiter-mass body orbits the pulsar PSR B1620-26 in the globular cluster M4 has been largely accepted, but researcher Stephen E. Thorsett (University of California, Santa Cruz) refrains from calling the object a "planet." PSR B1620-26 is tightly orbited by a white dwarf. The third object is thus the lightest and most distant member of a "hierarchical triple" system and does not fit the accepted definition of a planet, says Thorsett. The body probably formed around another star in M4, he argues, only to be captured by the binary system's gravity in the dense, high-energy environment of the globular cluster. Whether the body is a planet or a brown dwarf remains unclear.

Lastly, the putative planet around the high-energy gamma-ray source Geminga is now believed to be an artifact of a change in that unusual pulsar's spin period, says Boston University astronomer John Mattox, who announced its provisional discovery two years ago.
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Publication:Sky & Telescope
Article Type:Brief Article
Date:Oct 1, 1999
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