I figure I share some knowledge and open the window for everyone on the meaning of words as best as I can. As well I had took some Astronomy in my High School years.
"A place, that isn't a place."
SOL is also in the center of the bubble on the galaxy map that which reads 0:0:0 considering this is space we are not just looking at Longitude and Latitude we have to think 3 Dimensional as well.
This can also refer to a place that is not a place. Usually when you create a place you need to give it a name and becomes a place to go to. Asteroid belts were never given name in game as example.
"Minor" which is a word used to relate no-name
https://www.merriam-webster.com/thesaurus/no-name can also refer these examples.
Which you relate that to space
https://en.wikipedia.org/wiki/Minor_planet
https://en.wikipedia.org/wiki/List_of_minor_planets_and_comets_visited_by_spacecraft
Now the tricky part of the clue "A door that is also a key"
This one stump me a little but I figure I share what I found. The door can be anything an opening to something but the Key you need and it's already at the door so instead of focusing on the whole sentence I look at the word "Key"
as it's the very thing you need to go to to open the door.
I looked up Key and it's relation to space and I found 1 match
A Minor Planet name Chiron (which by luck was giving the symbol that resembles a Key) and the funny thing about this planet is it orbits the sun between Saturn and Uranus and has not been discovered in game yet or on the list of planets. But does exist in RL.
https://en.wikipedia.org/wiki/2060_Chiron ( the key symbol is just above the picture posted)
I don't own a SOL permit so I can't go myself but hopefully anyone with access can look into it.