Several methods are being used at the same time by different groups. The spectroscopic method, used by Paul Butler at San Francisco State University, and at the AFOE system at the Harvard/Smithsonian Center for Astrophysics measures the spectrum of the star, and carefully measures the locations of the spectral lines. Over the course of 5-10 years of nearly daily monitoring, the slight velocity changes caused by an orbiting planet...amounting to only a few tens of meters per second, can be picked up by sophisticated computer programs which search the data for all possible planetary orbits to find the best matches.
George Gatewood at the Allegheny Observatory has been using the older astrometric approach and has turned up several planets around the star Lalande 21185 by looking at 50 years worth of star position data. The orbiting planets cause the position of the star to wobble across the sky as it moves. According to his announcement in the Bulletin of the American Astronomical Society:
An initial pass to the data indicates that the disturbing masses would usually be more than 0.8 arcsec from this frequently observed star. Thus companions of significant luminosity are effectively ruled out. The approximate nature of a relatively short period component is suggested by data obtained, over the last 8.5 years, with the University of Pittsburgh Multichannel Astrometric Photometer (MAP). We find: a low eccentricity orbit, a period of approximately 5.8 yrs, a semimajor axis of 0.0022 arcsec, and a probable mass of 0.9 times that of Jupiter. Residuals from this orbital analysis, and from data obtained from the Allegheny Observatory's plate series, indicate the presence of at least one additional component, but its exact nature is unclear. A possible solution involves a second companion with a period of approximately 30 years, however the period is not well established and the residuals to this solution also indicate a remaining acceleration. The source of the latter may be a substantial underestimation of the period of the second mass or it may indicate the presence of a yet longer period 3rd planetary body.
In 1995, astronomers studying a pulsar PSR1257+12 detected the faint but unmistakable time signature of two small planets orbiting the pulsar and causing a periodic change in the pulses received by radio telescopes. Radio signals from the constellation Virgo led Penn State professor of Astronomy Alexander Wolszczan to discover the first planets ever known outside our solar system. He discovered the planets in 1991 and confirmed their existence in 1994.Pulsar Planets Page. As Wolszczan analyzed the data collected over a 486-day period, he looked for a pattern in the arrival time of the pulses, searching millions of bits of information. Using the supercomputer facilities at the Cornell Theory Center, he determined that the pulses were arriving every 6.2 milliseconds. Wolszczan detected unusual complexity in the pattern of the pulses' arrival times: they periodically arrived early and bunched together and then spread apart, as they began to arrive later than the predicted time. This behavior suggested that the pulsar's motion is affected by the presence of other orbiting objects. Instead of moving steadily, PSR1257+12 was being pulled around a point in space called the barycenter---the center of mass of a system---by the gravitational interaction with these objects.
Direct 'eclipses' or transits by a planet across the disk of the star have been detected photometrically by The first extrasolar planet to show transits across the disk of its star has been detected with the T8 0.80 meter Automatic Photoelectric Telescope (APT) at Fairborn Observatory. The APT is operated by Tennessee State University in Nashville. Photometric observations (above) of HD 209458 from the night of 1999 Nov. 7 UT taken with the T8 0.80m APT at Fairborn Observatory showing ingress of the planetary transit. The measured transit depth is 0.017 mag or 1.58%. The error bar shows the predicted time of mid transit and its uncertainty computed from the Keck radial velocities.
Copyright 1997 Dr. Sten Odenwald
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