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

Category: Science