Evidence for planets around other stars has been sought for many years. In the past, many of the closest stars had their positions (against background stars) measured carefully for signs of wiggles as they rotated about their center of mass (barycenter) with a planetary companion. This astrometric technique was simply not precise enough to do the job, so astronomers turned to a Doppler shift technique where stellar spectral lines were monitored for evidence of a frequency shift, again induced by the star's rotation about the barycenter. The leading U.S. group, headed by scientists Marcy and Butler, was searching for a Jupiter-sized object in a Jupiter-like (5 AU radius) orbit, but they were beat out by the University of Geneva (Switzerland) team of Queloz and Mayor, who found a multi-Jupiter mass planet orbiting at a fraction of 1 AU, which in our solar system would put it inside the orbital radius of Mercury!
This type of large exoplanet (i.e., a planet outside our own solar system) with such a small orbit was completely unexpected to say the least. Since this initial discovery, many more planets like it have been found, plus some further out in non-circular orbits. This has thrown the planetary system formation theorists into temporary disarray. It was thought that only rocky, terrestrial-type planets would be located close to a star. The theories predicted that the so-called "gas giants" would orbit farther out.
A wealth of new theories has been proposed to explain these recent detections. Some of these have very dire consequences for small inner planets like ours: massive Jupiter-like planets may come cruising through our part of the solar system, ejecting smaller planets into the star or out into space.
Can we live on such planets? Can anything else?
Of course, we humans cannot visit the surface of a planet 150 times more massive than the Earth. (Note: this is still less than half the mass of Jupiter). That is, assuming there is an accessible surface. These extrasolar planets are probably more like our gas giant planets. In any case, the gravity and perhaps the gas pressure would crush us. In addition, very few of these planets are near our comfort zone, temperature-wise. They mostly orbit too close to their stars, and thus their temperatures are too hot for us. We believe that liquid water may be a near-universal requirement for the development of life, so we wouldn't expect to find any other life on these planets. A zone in a gas giant's atmosphere might accommodate liquid water, possibly at high pressure, but the vertical winds do not make this a stable environment. So none of the exoplanets discovered to date or expected to be found for some time should be capable of harboring life.