Question

How are planets detected around other stars?

Most of the planets around other stars have been discovered from extremely precise measurements of changes in the speed of the star they orbit. The planets themselves are invisible, but they exert a gravitational pull on their star. As the planet goes around in its orbit, the star wobbles slightly, and these minute motions can be detected. This is usually called the Doppler spectroscopy method of discovering a planet. A few planets have been discovered by two other means. One is to measure the slight drop in brightness of a star as one of its planets moves across its face as seen from Earth. This is called a transit observation. The other method uses the so-called microlensing effect. If a planet passes directly between the Earth and a distant star, there is brief brightening of the distant star due to bending it its light by the gravity of the intervening planet. To see such a microlensing effect, however, requires precise alignment of the planet with the distant star. As of now, approximately 120 extrasolar plants have been discovered.

David Morrison
NAI Senior Scientist
July 23, 2004