From all my discussions with practicing astrologers (mostly Modernists), David's summaries here are the definitive situation. In Modern Astrology, the outer planets are full-blown domicile rulers. There's no such thing as "co-rulership" of a Sign. Uranus alone rules Aquarius. Everything about Aquarius that has been blamed on Saturn is actually the result of the nature of Uranus - the rejected rebel, the misunderstood genius - feeling alone in a crowd - but at one with the Universe. That's not really Saturn at work.We have three ways of dealing with the outer-most planets - 1) Mods fully accept them as full fledged Domicile rulers of , , and /2) Hybrids partly accept them as "influences" in the chart, perhaps as "co-rulers"/and, 3) Trads reject them entirely.
Some Modernists bluntly refer to the Hybrid as a messy pasting of the new on top of the old, or they quote the Bible about not pouring new wine into old jugs, or sewing new cloth to patch the old.
The greatest living physicists say that there is always a clean, elegant aesthetic to the Universe.
The Hybrid (co-rulers, etc.) does seem like a clumsy cobbling of old and new together. And Traditional is just plain "Flat Earthism".
The most progressive astrologers I've listened to anticipate a determination of two additional domicile rulers to separate the doubling up of Taurus and Libra under Venus, as well as Gemini and Virgo under Mercury. And voilà: A single domicile ruler for each of the 12 signs. Nice and elegant.
Just how this will come about still seems under hot debate. But the consensus is that somehow astrological research will realize that a major asteroid is actually a domicile ruler and/or another planet will be discovered. When I mention Chiron as a possibility, some say it doesn't really "fit" any of these four Signs. However, Barbara Hand Clow, in her impressive book Chiron (1993: Llewellyn), makes a good attempt at proving Chiron rules Virgo. That would be one down, one to go.
I scanned a thread here that suggested the Earth is the actual ruler of Taurus. I like that. I have nothing to back it up with, but it sounds "right" somehow. That would wrap the whole rulership thing up. And there would be no more boundary disputes, no two rulers claiming the same territory. We've got way too much of that destroying our world right now. It would be nice to have the astrology of our Solar System all straightened out at least.