An Upper Limit for the Diameter of Pluto.

Abstract

The determination of the diameter and mass of the planet Pluto is made difficult by its extremely small angular diameter as seen from the earth and by the lack of any known satellite. Kuiper (1950) measured an apparent diameter of O'/23 which leads to a diameter of 5900 km for the planet. The estimated mean error of this observation is about 5 percent. If, however, there were a need for certain corrections which were not applied, Kuiper suggested that a value as small as 4900 km might be obtained. The mass of Pluto can be estimated from its gravitational effects on the orbits of Uranus and Neptune. The conventional value is given by Brouwer and Clemence (1961) as 0.90 earth masses (360,000 reciprocal solar masses) although they stress the unreliability of the result which depends on old observations at times of conjunction of Pluto with Neptune or Uranus. A combination of the separate determinations of diameter and mass yields a mean density for Pluto of 50 gm/cm3 which is unreasonable and indicates a serious error in at least one of the mass or diameter values. It has been suggested by Alter (1952) that the surface of Pluto may reflect light in a semi-specular manner which