Is The Solar System Unique?

摘要

Since the first discovery of a planet orbiting another star in 1992, some 280 alien solar systems have been identified. Most look quite unlike ours, and for good reason. Planets are mainly spied by the way their gravity makes their host star wobble as they orbit. The smaller the planet, the smaller the wobble: lightweight planets like Earth produce effects too small to detect with current technology.rnMost known extrasolar planets, or exoplanets, are gas giants similar in size to Jupiter or Neptune, but orbiting close in, within a few AU (Earth-sun distances) of their stars - 6 or 7 per cent of sun-like stars seem to have such satellites. The prevalence of giant planets orbiting at greater, Jupiter-like distances from their parent star is unknown. They would take a decade or more to complete an orbit, and few gravity-wobble surveys have been watching long enough to detect them.