NOBODY has ever seen an extrasolar planet--although 20 have now been identified--much less another solar system. Even when they are orbiting nearby stars, the feeble light reflected from planets is overwhelmed by the flood of illumination from the host star.

So astronomers identify planets indirectly, by measuring the way they influence the motion of their parent stars. As an object orbits a star, its gravitational pull causes the star to wobble back and forth. That motion creates a periodic displacement, or Doppler shift, in the spectrum of the star as seen from the earth. The light from a receeding star shifts toward the red; that from a star approaching, toward blue.

Using sensitive spectrophotometers, such as those operated by the Lick Observatory and AFOE planet search programs, researchers examine spectra for the tiny displacements that could denote the presence of planets. The Doppler technique can reveal the orbit and the minimum mass of an orbiting body, but no details of its nature. But, despite the technique's sensitivity, the faint signals from planets the size of our Earth are still too small to be detected.

Doppler diagram

Image: San Francisco State University

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