Now I'd like to try to explain about how planets get named and answer Hokulele's question " Which came first, the name of the planet, or the association with personality traits?"
The naming of the planet came first.
Astrologers didn't persuade the Astronomical Union to call the planet, in this case, the one discoverd in 1930, Pluto - they wanted to name it after a Greek god, like the other planets, but I think it was suggested by a 12 year old girl from Oxford. It's also a coincidence that the first two letters of Pluto are the initials of Percival Lowell, at whose observatory Clyde Tombaugh discovered it.
After it was discovered, astrologers charted it in people's horoscopes, and to their surprise, the planet seemed to symbolize traits that could also be ascribed to the Greek god (also known as Hades). stood for - wealth, but also an ability to dig below the surface, (he was king of the underworld) as in what we now call psychology. In the myth, Pluto abducts Persephone in the autumn and keeps her underground for 6 months, only releasing her to her mother Demeter in the Spring. Just as the cycle of nature regenerates itself when leaves fall to fertilize the ground ready for the new growth in the spring, the planet Pluto was seen to represent the psychological trait of transformation and regeneration, and it was given co-rulership of the sign Scorpio. Pluto was also found to indicate secrecy, control and sexual passion.
I know that it seems as though astrologers could have just made up these correlations just to fit the traits of the mythical god, but if we look at many, charts, the evidence becomes more convincing. As a single example, look at what went on in the life of Brad Pitt during 2005-2006, when Pluto transited his Sagittarian Sun (Pluto transits take up to 2 years). He went through a complete transormation of his love interest and partner, much of this done in secret, at least at first, from Jennifer Anniston to Angelina Jolie, and started travelling (Sagittarius) and dispensing his wealth in countries that needed help. I know that one example doesn't prove anything, but I'm sure that there are others if we look for them.
As for the question of the difference between Pluto in a chart of a contemporary person and Pluto in the chart of say Henry VIII, I did explain this back earlier in the thread. Before the date when planets are discovered physically, of course their associations still exist, but they are not completely understood by the mass of humanity. For example the power of mass psychology and control over people wasn't really understood in a way that we could pinpoint to the actual planet, until exhibited by fascism, especially Hitler, in the 1930s
