Pluto Colder Than Previously Believed

Pluto's surface temperature varies widely because of its erratic orbit, which can range from 30 to 50 Astronomical Units (AU). An astronomical unit is the average distance from the Earth to the Sun.

It is believed that interactions between nitrogen gas in Pluto’s atmosphere and nitrogen ice on Pluto’s surface cause the lower temperature. As Pluto moves away from the Sun, the nitrogen gas freezes and falls back to the surface as ice. When the planet moves toward the Sun, the opposite happens. This leads to what Mark Gurwell, an astronomer at the Harvard-Smithsonian Center for Astrophysics, calls an “anti-greenhouse effect”. Instead of being absorbed and warming the surface, sunlight striking Pluto is used to convert nitrogen ice on its surface into gas. This cools the surface similar to the way in which sweating cools human skin by having evaporating water carry away excess body heat.

Pluto's surface temperature varies widely because of its erratic orbit, which can range from 30 to 50 Astronomical Units (AU). An astronomical unit is the average distance from the Earth to the Sun.

Charon, unlike Pluto, has no atmosphere. Its surface temperature is what astronomers predicted based on its reflectivity and geological makeup.