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Is the sun encountering a black hole?

Scientists continue to wonder if the sun is keeping compamy with another star -- a dark body whose gravity tugs at the solar system. Although fre scientists take the idea too seriously, a scattering of studies has been unable to rule it out, although it has narrowed the possibilities. The most likely candidate is a supermassive black hole.

E. R. Harrison of the University of Massachusetts first suggested the possibility of an unseen solar companion to explain an anomaly in pulsar data.

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Pulsars are objects that emit regular pusles of light, radio noise, or X-rays.

They are believed to be spinning Neutron stars -- that is, stars that have collapsed in a final stage of evolution and whose substance consists largely of neutrons. As such stars age and spin more slowly, the rate of their pulsing also slows in a typical way. However, Harrison noted that, for half a dozen of the known pulsars, the spindown seems too gradual. Furthermore, all of these anomalous pulsars lie more or less in one direction as seen from Earth, the direction toward the center of our Milky Way Galaxy.

Harrison suggested that the pulsars actually are quite normal. The change in their pulse rate merely appears to be more gradual than expected because the solar system is being accelerated toward those pulsars by the gravitational pull of an unseen stellar companion.

Since Harrison threw out this idea two years ago in the journal Nature, several astrophysicists have looked more closely at the prospects. H. F. Henrichs and R. F. A. STaller of the University of Amsterdam showed it to be unlikely that any body massive enought to produce the effect could be a permanent companion of the sun and yet have remained detected. This left open the possibility of a passing neutron star on black hole (a star that has collapsed to the point where its gravity is too strong for anything, even light, to escape).

Serge Pinealth of the University of British Columbia, reinforced this conclusion, showing such a neutron star or black hole encounter to be plausible theoretically. J. Kirk of the Max Planck Institute for Physics and Astrophyics showed from a study of expected effects on comet orbits that the passing object should be moving faster than 20 kilometers a second. Now, in Nature, Daniel Wilkins of the University of Vienna has looked again at comet orbits and finds no evidence even for a fast moving neutron star. But he adds that an encounter with a "supermassive black hole" (350 times as massive as the sun) moving at 100 kilometers a second at a distance about 15,000 times farther from the sun than is Earth remains plausible.

Thus, although the constraints have narrowed, Harrison's suggestion still can't be ruled out. And no one has yet taken up his challenge to shoot down his his notion decisively by finding an anomalous pulsar in the opposite hemisphere of the sky.