Closer Look at the Equation

Perhaps the most important term in the equation is L, the lifetime of a civilization. If civilizations last for many millions of years, then our own Galaxy could be teeming not only with life, but also with civilizations. If L is rather small (hundreds or thousands of years), then the Galaxy may be a lonely place indeed. Civilizations, like candles, may be lit frequently, but if they do not burn for very long, only a few others will be burning with us.

Let's look at some of the other key terms.

Close Encounter

Astronomers have two basic ways to look for planets around other stars—extrasolar planets. Both involve an understanding of gravity and mass. When we talked about the solar system, we said that the planets orbit the sun. That is approximately true. In detail, all of the masses in the solar system (the Sun, Jupiter, and all of the planets) orbit around their common center of mass. As a result, the sun is not exactly fixed, but wobbles back and forth in space every 11 years as Jupiter (most of the mass other than the Sun) orbits.

This wobble can either be detected directly (with a technique called astrometry) by watching a star wobble around in the plane of the sky, or indirectly. The indirect method (so far) has been the more successful, and involves the use of the Doppler shift. As a star wobbles back and forth (alternately coming at us and moving away), its spectral lines shift from shorter to longer wavelengths and back again. Of course, the planets discovered by either method must be massive; if not, the star wouldn't wobble enough.

For example, a planet 2.4 times the mass of Jupiter has been discovered at a distance of more than 2 A.U. from the star 47 Ursa Majoris (47Uma).

Want to get the latest on the search for extrasolar planets? Point your Web browser to

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