Being able to read a chapter of this every month has definitely made my life better. Thanks for the chapter :D
If I had to guess what this planet is, I would guess that it the a world that Ridley dreamed about when she was a child. In other words, Alice, as well as the rest of the tea party people, are Ridley's imaginary friends. I think Ridley likely forgot about these dreams over the course of growing up.
The Japanese chapter title "
懐の星" roughly translates to "planet of [something held within in the heart]"; it's kind of hard to explain the nuance in English, but the word expresses "carrying" something in your heart and caring for it deeply at the same time.
Also, goodness, Watari really likes Ridley. "Alice kinda looks like me. I think she really likes you." This is way too sweet.