Triton, the major satellite of Neptune, and Pluto, the outermost planet, are too small and distant to permit direct measurement of their size, and indirect arguments must be used to estimate diameter and albedo. These two objects invite a comparative study because they have essentially the same brightness (absolute visual magnitude V(1, 0)≃ -1) and current distance from the Sun (30 AU). In addition, the presence of methane frost on the surface, and of a possible methane atmosphere, have been detected for both objects from IR spectroscopy1-4. On the basis of these similarities, Triton and Pluto were selected as targets for possible detection of thermal IR emission. We report here new upper limits to the thermal IR emission from these objects that permit us to set significant upper limits on their diameters and to demonstrate that both are high albedo objects. These results clearly exclude the possibility that Triton is the largest planetary satellite, and they are consistent with the small size of Pluto deduced from other data.
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