Astronomers in the United States reported in March 2000 that they had discovered the smallest planets yet found outside the solar system. Although the researchers did not see the planets directly, they inferred their existence by measuring variations in the light from the planets' parent stars. The finding of relatively small bodies was important because it showed that the methods for locating extrasolar (beyond the solar system) planets were becoming more sensitive. The discovery, by astronomers Geoffrey Marcy of the University of California at Berkeley and San Francisco State University and Paul Butler of the Carnegie Institution of Washington, D.C., was the latest in a string of findings of such planets. It capped a five-year period in which the number of known planetary systems grew from 1—our own—to more than 30.

Astronomers in 2000 expected to find planets around even more stars. The discoveries up to then had confirmed the longstanding belief of astronomers that planets are common in the universe. The planets they had found, however, had very surprising properties, forcing astronomers to rethink their ideas about how solar systems form. And perhaps the most important question of all remained unanswered: Are there other planets similar to Earth that are capable of supporting life?

The idea that planets might orbit other stars has a long history. Some ancient astronomers believed that the sun and the planets all orbit around the Earth, and that the “fixed stars,” as they called them, were merely points of light dotting a celestial sphere. In 1543, the Polish astronomer Nicolaus Copernicus published his revolutionary theory that the sun is the center of the solar system and that the Earth is one of the planets that circle the sun. The Italian philosopher and monk Giordano Bruno took Copernicus's idea further. Bruno claimed that the universe is infinite, that the stars are other suns around which other planets orbit, and that life is widespread throughout the universe. For these and other beliefs unacceptable to the Roman Catholic Church, he was burned at the stake in Rome in 1600.

Despite Bruno's fate, many scientists after Copernicus speculated that planets around other suns may be common. Modern astronomers found support for that idea in their observations of disks of gas and dust surrounding young stars. The material in a disk, according to current theories of how our own solar system formed, gradually comes together to form planets.