Well to begin:
Pluto isn't the last "planet" either, there are millions of objects in the kuiper belt, and far many more planets beyond that.
What are the interpretations of uranus, neptune and pluto? Have you ever considered that most interpretations are only abstract meanings that don't really say anything, and most are borrowed things from other planets...?
Lets analyze both types of meanings of, for example,
neptune: rulerships (things neptune rules) and attributes (psichological aspects). Going to use some of the significations posted here:
http://www.skyscript.co.uk/neptune3.html
a) On the psichological side, neptune, considered to be
"mistery",
"secrecy", etc. Most of the time, this doesn't seem to mean much regarding an individual, and most of the times it just describes abstract emotions that are common to any individual, rather than describing the psichological reality of the individual itself. Too many words packed together to make it seem "pretty", but not really saying muc
h about the individual's actual personality.
EX: It challenges the individual to differentiate between subtle and barely conscious essential truth, on the one hand, and tantalising mirage, on the other - since it presents both. It evokes vision, which when positively expressed manifests as idealism; when negatively expressed, as escapism.
EX:Neptune conjunct Mars. The conjunction of Mars and Neptune gives the personality strong psychic magnetism. Mystery and secrecy shroud the affairs governed by Mars. You have a definite talent for leadership because people will respond to you and follow you instinctively. Often, goals and energies are directed to spiritual matters.
b) Then you can just look at the borrowed meanings from Venus. So far, for the past 2000 years Venus has ruled over: beauty, arts, sexual desire.
Lets look at some of neptunes "original" rulerships (actually borrowed from venus). Lets see how many venusian things we can find???
Ex: Neptune is aesthetic; amorphous; attuned to the finest nuances of beauty, feeling and mind; blissful; compassionate; diffuse and at times subjectively unclear or confused; dreamy; enthusiastic; ethereal; highly emotional; imitative;People signified by Neptune include all those engaged in aesthetic, artistic, inspirational, literary, occult and psychic vocations (including mediums and mystics), or those connected with water.