The Trans Neptunian Objects are remnants from the early Solar System formation. They are small icy bodies orbiting the Sun beyond Neptune's orbit. From 1992 onwards, hundreds of new objects, some of them a thousand kilometres in diameter, have been detected. The visible frontier of the solar system has been pushed back, and a new region unveiled: the Kuiper Belt.
Although the reclassification of Pluto is the most visible consequence of these discoveries, it is also one of the least fundamental. The study of these trans-Neptunian objects, at the edge of the solar system, has now become one of the most active branches of planetology. What we are finding beyond the orbit of Neptune is providing us little by little with the key to subjects as diverse and fundamental as the troubled history of the solar system, its ‘architecture’, the origin of comets, and even the formation of lunar craters.