The study of extrasolar planets has become one of the most active fields of research in astronomy since the discovery in 1995 of several giant planets orbiting nearby stars similar to the Sun. There have been many unexpected results, and the progress has been remarkable. More than 100 planets are now known, many of them in systems of two or more bodies orbiting the same star. In one case it has even been possible to detect the atmosphere of the planet as it transits between us and its star.