Planets do not emit their own light, so they are lost into the light emitting from their associated "sun." To make matters more challenging, exoplanets are relatively far away from us--the closest of the planets found to date is 10 light-years away from Earth. In comparison, our is only eight light-minutes away.
One of the most successful approaches to exoplanet discovery is to measure the change in the frequency of the light emitted from a star. If a star has an associated planet system, the planets will move around the system's center of mass. This motion induces a change in the light's frequency, known as the Doppler effect; when the star is moving away, the light measured on Earth is shifted toward longer wavelengths (or lower energy), and when the star is moving toward us, the inverse occurs and the shift is toward the shorter wavelengths (or greater energy). The larger the planet, the larger the motion of the star.
A Jupiter-like planet induces a 13 meters per second, or m/s, movement of a star the with the mass of our sun. The discovery, in 1995, of the first planet, called 51 Pegasi b, observations using the W.M. Keck Observatory in Hawaii measured the movement of the star with a precision of five m/s. 51 Pegasi b has half the mass of Jupiter, and this was quite a technical feat for high-resolution spectroscopy; recent measurements have reduced that to three m/s.
At this point, astronomers have used the technique to find more than 75 such Jupiter-mass planets. To hunt for smaller, Earth-like, planets requires a much greater precision in the spectroscopic measurement. That will have to wait until some of the newer instruments are launched, such as NASA's Next Generation Space Telescope and Space Infrared Telescope Facility. --Ilana Harrus
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