DETECTING EXOPLANETS -- ASTROMETRY AND DIRECT OBSERVATION Astrometry refers to the method of detecting planets around other stars discussed in the previous notes. One looks for a star (these would be only near stars) that has, not just straight line motion, but a wobbly path. This indicates that an invisible companion is in orbit around it. By observing the wobble we can deduce the mass of the companion. Astronomers tried this for many decades, but were hampered by turbulence in the earth's atmosphere. Because of the refraction of light as it goes through air, the image of a star on a photograph is subject to random variation that can easily cover up the small motions of the wobbly path. This problem was finally solved with the launch of the Hubble Space Telescope . Photographing the sky from above the atmosphere, Hubble began in 1995 providing clear evidence of planets around near stars. The first objects seen were (and are) ambiguous and mysterious. They lie in the region around one-tenth of the mass of the
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