Planets that are visible because they reflect the light of the star their orbit.

Searching for Extrasolar Planets

The worlds orbiting other stars are called extrasolar planets (extra means beyond and solar means sun—planets beyond the sun). Because our equipment and methods are still fairly crude, most of the extrasolar planets discovered have had masses equal to or greater than Jupiter and are often very close to their star. As telescopes and technology continue to improve, we should be able to find planets that are the mass of Earth and smaller. With current technology, finding extrasolar planets directly is not possible. Our instruments lack the sensitivity to see extrasolar planets, but that will change with new technology.

Planets that are visible because they reflect the light of the star their orbit. Even so, they are only about one one-thousandth as bright as the star itself, or less. Currently, our telescopes are unable to distinguish this small amount of light from the tremendous amount radiated by the star itself. Also, because stars are so far away, the distance between the star and its planet appears very small—too small for current telescopes to resolve. The problem is similar to the way distant headlights appear as a single bright light, even though the light is being made by two distinct lights. Astronomers are developing techniques that one day may help them see enough detail to overcome the challenges posed by distance and differences in brightness.

Currently, astronomers are restricted to using indirect techniques to locate an extrasolar planet. These techniques use changes in light coming from the star to infer a planet’s presence and orbit. In this session you will study two techniques astronomers currently use, transiting and spectral, and another, astrometry, that may be possible in the near future.