Over the years, a number of stars have been identified as having unseen, non-stellar companions of the size of Jupiter. And where there is smoke, the argument is that there may also be fire. A distant observer would only be able to detect the presence of Jupiter and perhaps Saturn, on the motion of the Sun through space. The planets would cause the sun to wobble by a few hundredths of a second of arc, as the Sun's path is traced out in space. This wobble would occur with a period equal to the orbital periods of the planets.
There is also the peculiar case of the pulsar PSR1257+12. According to Penn State astronomer Aleksander Wolszczan, there are irregularities in the regular, milli-second flashes from this rapidly spinning neutron star that indicate as many as three planets orbiting it and causing distinct Doppler wiggles in its emissions. He proposes two Earth-mass planets orbiting with periods of 67 and 98 days, and a 25 day wobble due to a Moon-sized body. For more information, see Sky and Telescope, May 1992 and May 1994.
The actual number of detected planets is a 'moving target' which increases every few months as more of the long-term observations of specific stars begin to turn up signs of velocity shifts or position wobbles. As of May,2000 the number stands at 40 known planets. For an up to date list, you should visit Extrasolar Planets Encyclopedia by Jean Schneider at the Harvard-Smithsonian Center for Astrophysics.
Copyright 1997 Dr. Sten Odenwald
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