Decades ago, they used to search for slight changes in the positions of stars as they moved across the sky. As a planet orbits a star, the star's position will wobble by one part in 1000 of an arc second or so over many years. Unfortunately, many of the stars for which some effect was seen could never be confirmed. Then about 5 years ago or so, astronomers detected a change in the frequency of pulses from a pulsar, and were able to use the Doppler Effect to determine what the pulsar's change in velocity was. This led to the discovery of two planets orbiting the pulsar.
As they moved, they caused the velocity of the pulsar through space to change slightly, and this speed change caused a measurable Doppler shift in the pulsar's frequency. In 1996, astronomers used spectroscopic data to search for a similar Doppler shift in the light from several bright nearby stars. They discovered periodic speed changes of about a few hundred meters per second in several candidates, and when they used supercomputers to fit the velocity curves, they found evidence for several massive planets. It is expected that this Doppler shift technique will be extended to many more stars in the next decade, so that we will be able to see smaller and smaller objects, perhaps even as small as the Earth, orbiting close to many stars.
Copyright 1997 Dr. Sten Odenwald
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