A. J. Dessler and C. T. Russell (Eos, Forum, October 28, 1980) are behind the times. Pluto already disappeared into Never-Neverland and has returned again! Dessler and Russell committed several blunders in their analysis that were further obfuscated by their failure to adhere to such fundamental AGU standards as showing error bars and publishing references.
Nevertheless, I have unearthed an old, dusty issue of Science, wherein one finds that Ash et al.  report a value for Pluto's mass that probably accounts for the third last data point in Dessler and Russell's graph. But Ash et al.'s value reflects their assumption that the density is 3 gm/cm3. They actually measured a negative mass.