Pluto usually orbits beyond Neptune at an average distance of 5,900 million km from the sun, though its orbit is so eccentric that at perihelion it is closer to the sun than Neptune (as in 1979–99). Pluto is smaller than earth’s moon (diameter about 2,250 km), but it has its own large satellite (Charon), and four other small moons. From the time of its discovery it was regarded as the ninth (outermost) planet of the solar system, but in the 1990s its unusual characteristics led astronomers to question its planetary nature. In August 2006 the International Astronomical Union formally declared Pluto to be a dwarf planet rather than a planet proper. Pluto was visited by NASA’s space probe, New Horizons, in 2015.