80's children's fantasy book with a cat on the cover

Trying to remember the title/author of a book that had a cat on the cover, with some planets or moons in the background. Borrowed it from my elementary school library in the 80's. The main character was a kid who grew up without a mother, and somehow ends up finding a gate that leads off the world (don't think it was set on Earth). The ending of the book implied that the kid's mother had disappeared through one of these gates, and that the kid was going to go through the gate and look for his mother (cliffhanger ending). Don't remember what the cat on the cover had to do with anything.