Pluto's distance from Earth makes in-depth investigation difficult. Many details about Pluto will remain unknown until 2015, when the New Horizons spacecraft is expected to arrive there.

Spectroscopic analysis of Pluto's surface reveals it to be composed of more than 98 percent nitrogen ice, with traces of methane and carbon monoxide.

The Hubble Space Telescope places Pluto's density at between 1.8 and 2.1 g/cm³, suggesting its internal composition consists of roughly 50–70 percent rock and 30–50 percent ice. Because decay of radioactive minerals would eventually heat the ices enough for them to separate from rock, scientists expect that Pluto's internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of ice. It is also possible that such heating may continue today, creating a subsurface ocean of liquid water.

The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Once Charon's gravitational effect on Pluto was measured, estimates of Pluto's mass fell to 1.31×1022 kg—less than 0.24 percent that of the Earth. Observations of Pluto in occultation with Charon were able to fix Pluto's diameter at roughly 2390 km. With the invention of adaptive optics astronomers were able to determine its shape accurately.

Pluto's atmosphere consists of a thin envelope of nitrogen, methane, and carbon monoxide, derived from the ices on its surface. As Pluto moves away from the Sun, its atmosphere gradually freezes and falls to the ground. As it edges closer to the Sun, the temperature of Pluto's solid surface increases, causing the ices to sublimate into gas. This creates an anti-greenhouse effect; much like sweat cools the body as it evaporates from the surface of the skin, this sublimation has a cooling effect on the surface of Pluto. Scientists using the Submillimeter Array have recently discovered that Pluto's temperature is 43 kelvins, 10 K colder than expected.

See also: Terraforming, Pluto Redefinition as a Dwarf Planet

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