Pluto repeats an anomaly when compared with the terrestrial planets (Mercury, Venus, the Earth and Mars) or the giant planets (Jupiter, Saturn, Uranus and Neptune). This icy world is more like a huge asteroid or the satellites of the outer planets; it has even been suggested that Pluto was a satellite that escaped from Neptune’s pull. The small size of Pluto combined with its distance make it very difficult to observe; however, two important discoveries have just been made. In 1976 new spectroscopic observations revealed that the surface of Pluto was covered, at least partially, by frozen methane.
The infrared spectrum enabled this methane to be identified, besides other kinds of ice, such as ice of ammonia or water. The surface of Pluto, covered by ice, is therefore bright and its reflective power (or albedo) is much greater than we would suppose, thinking that it was all covered by rocks. Knowing its distance from the Earth and its reflection coefficient we can deduce a diameter of between 2000 and 2700 kilometers; its mass is thus much less than first estimates.