Pluto is the ninth and smallest planet. It takes 248.59 years to orbit the Sun. Between 1979 and 1999, Pluto was closer to the Sun than Neptune, approaching to within 30 AU of the Sun. Pluto only has one moon named Charon. It orbits Pluto every 6.4 days. Like Earth's moon, it always keeps the same face towards Pluto. Pluto has a surface temperature of 50 Kelvin, which is too cold to have a substantial atmosphere. Infrared spectroscopy has shown that Nitrogen and Methane are present, but it is hard to tell the difference between the solid and gaseous forms of these chemicals at the temperatures and pressures found on Pluto.