What do the Planetary Spirits look like?

[narfellus]

I posted this elsewhere, but the topic has more relevance here. These beings have been called all sorts of stuff over the ages. Some are beneficial to humans, some aren't. Some like us, some don't. I think that only competent magicians or lucky sorcerers are able to evoke and bind them to the material plane, but do they take a physical form, dropping their vibration enough to become tangible? Do they have a consistent form, or are they mutable? Do they take on aspects that the summoner wishes to see, or aspects that magicians over thousands of years have shared and verified. This is just a topic i am curious about, although i'm sure someone will tell me to just summon one myself and find out!