"With lots of methane in the atmosphere, it becomes clear why Pluto's atmosphere is so warm," says Emmanuel Lellouch, lead author of the paper reporting the results.
Pluto, which is about a fifth the size of Earth, is composed primarily of rock and ice. As it is about 40 times further from the Sun than the Earth on average, it is a very cold world with a surface temperature of about minus 220 degrees Celsius!