Hot Jupiters

Many of the extrasolar planets known are so-called hot Jupiters. This just means that they are Jupiter sized planets very close in to the central star. The reason that so many of these types of planets have been found is that the method used to discover extrasolar planets is most sensitive to heavy planets close in to the central star. Most extrasolar planets cannot yet be directly imaged because they are very small compared to their stars. Instead astronomers "see" that they are around a star by the gravitational influence they have on that star, causing it to wobble ever so slightly around the centre of mass of the whole system. Since the gravitational force depends both on the mass of the object and how close it is, massive planets close to the star have the biggest effect and so are the easiest to detect.