Regarding Cecil’s column http://www.straightdope.com/classics/a1_159b.html
While I can’t disagree with Cecil’s answer, I’d like to point out, that at the time that Jupiter Effect was published, Pluto’s mass was assumed to be MUCH more than it is known to be now. It wasn’t until the discovery of Pluto’s moon, Charon, in 1977, that it was possible to accurately determine the mass of Pluto. In absense of any other measurements, Pluto’s mass was set at being near to the large mass assumed by Percival Lowell in his hypothetical “Planet X” reasoning, which launched the search for a planet that resulted in the discovery of Pluto.
For a time Pluto was assumed to have the mass of Neptune, or Uranus, in a body the size of the Moon, leading to the belief that Pluto was the densest object in the Solar System.
Since 1974 two things have changed: With a satellite orbiting Pluto it is possible to determine the mass of Pluto quite accurately by measuring the period of it’s satellite’s rotation; And secondly it’s been determined that Percival Lowell’s original calculations, pointing to an eccentricity in the orbit of Neptune, leading him to hypothesize that there was another large planet to be discovered orbiting the Sun outside of Neptune’s orbit, were in error.