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As the list of known planets beyond the solar system grows, it seems increasingly likely that many billions of planets inhabit the Milky Way. Most of the planets discovered so far are big and close to their parent stars, so they are unlikely homes for life as we know it. But Earth-like planets should turn up soon, giving new energy to the search for life among the stars.

Rings encircle a young planet in this artist's concept.

Are we alone in the universe?

This question has captured the imaginations of scientists, theologians, and philosophers for millennia. And while we don't know the answer yet, we do know that our solar system is not unique.

Current theories of how planets are born indicate that planet formation is a natural and almost necessary result of the process of star formation. There are billions of stars similar to our Sun in our own galaxy. How many of these other stars host planetary systems? And how many of these planets might harbor life? Such questions have motivated astronomers to conduct many searches for extrasolar planetary systems over the past decade. These searches have discovered hundreds of giant planets orbiting nearby Sun-like stars, as well as a large number of intriguing puzzles.

Detecting extrasolar planets is not simply a matter of aiming a telescope at nearby stars and taking pictures. The amount of light from a planet is minuscule compared to the amount of light from its parent star. Seen in optical wavelengths (the kind our eyes can detect), a typical star is about a billion times brighter than its planets.

So instead of trying to detect planets directly, most astronomers take advantage of the fact that the star and planet both orbit an imaginary point between them called the center of gravity. Astronomers deduce the planet's presence by observing the tell-tale signs that the star is orbiting a center of gravity. Either they watch the star change position with respect to background stars (called the astrometry method), or they measure the change in the line-of-sight speed of the star as it orbits (called the radial velocity method). Another technique looks for transits -- a tiny drop in a star's light caused by a planet passing directly between Earth and the star.

So far, the radial velocity technique has been the most successful. Its first catch came in 1995, when Swiss astronomers Michel Mayor and Didier Queloz discovered a planet orbiting the star 51 Pegasi. The planet, which is about half the mass of Jupiter, takes only 4.2 days to orbit the star. And the planet is much closer to its star than our models of planet formation (based on our own solar system) predicted. Since then, other teams have discovered many other "hot Jupiters."

All of these systems were found by radial-velocity measurements, which are most sensitive to massive planets that orbit close to their parent stars. Using this technique, astronomers are just now detecting systems with "Jupiters" like our own, which at its distance takes nearly 12 years to orbit the Sun, or with planets that are close in but that are much less massive than Jupiter. It may take many more years to find planets similar to Earth, which exert a much smaller gravitational tug on their parent stars.

The many "hot Jupiters" indicated that current theories of planet formation may be incomplete. Jupiter-like planets may always form far from their parent stars in nearly circular orbits. However, in the systems discovered to date, the giant planets may have migrated inward soon after they formed. As astronomers discover more planetary systems, they will adapt their theories to understand how the planets formed and evo