Are there other planets in the Milky Way?
Yes, perhaps hundreds of millions of them. Over the past decade, astronomers have discovered more than 100 planets orbiting around stars other than our Sun. So far, they primarily have only studied relatively nearby stars (closer than 200 light-years), but unless there is something special about our local neighborhood, planets should be found around more distant stars as well.
Not all stars have planets, however. According to some estimates, fewer than 5 or 10 percent of nearby stars have planetary companions. Some stars are too hot and bright, and any planetary material would have long since boiled away into vapor. Some stars may have the wrong composition, or be in crowded star clusters, where the gravitational influence of other stars prevents planets from forming. The planetary systems astronomers have found so far are quite different from our own solar system. Instead of multiple planets in nice, neat, nearly circular orbits, astronomers usually see only a single massive planet, much larger than Jupiter, following a highly elliptical path, often orbiting very close to its star. It would be difficult for any smaller planet to maintain a stable orbit in such a system, because of the gravitational influence of the large planet. In only a few cases do astronomers detect planets in large, slow, circular orbits, like Jupiter and Saturn. Some astronomers think that "well-behaved" systems like these may also have smaller planets in closer orbits, like Mercury, Venus, Earth, and Mars. Current measurement techniques are not sensitive enough to tell whether such "terrestrial" planets are there, but special orbiting telescopes are being planned to look for them in the next 10 or 20 years.
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