I'm sure OP's already made up their mind but I'll just throw some out there as well.
Neverwinter Nights - Mostly lonely. Kinda dreamy and cool fantasy world. Great gameplay. Great stories. Amazing mods. Only problem is, you're not really alone of course, this being a Bioware game. You'll be traveling with party members. And they do talk but not as much at all as the more recent Bioware games. According to your demands, this game is IMO, the best one that fits your needs.
Halo: CE - Yeah, it's not an RPG, and it's not (entirely) open world, but to this day, I still say that Halo: CE has the best campaign of all the main-entry Halo's. This game does the best job of conveying a kind of dark peaceful sense of solitude in a beautiful alien world out of all the Halo games, hands down. And seriously, the gameplay is rock solid. After this game, other Halo's improved many many things about it, but in the end, I'll never forget my time on that beautiful cold alien installation in the middle of nowhere by myself.
Minecraft - The biggest thing that this game is missing for you is quests, period. There are none except maybe get to The End. Besides that though, it's a slow peaceful game. Very meditative. Just load up Mystcraft and/or some of your fave mods and have a hell of a time. It's very, very lonely, serene, and relaxing.
The Talos Principle - Kind of has an open world but it's not an RPG. Rather, it's a philosophical puzzler. And a bloody good one at that. Croteam knocked this one out of the park. The world's incredible and empty. And besides Elohim and maybe Milton, you're alone 24/7. Really great story. Great puzzles. The expansion's puzzles though IMO are way too brutal so don't feel bad if you gotta cheat through them just to see the expansion's story. I got through all of TTP without a single walkthrough but the expansion puzzles were really out there and I finally just said, "fuck it" and cheated all the way through it to progress the story.
The Beginner's Guide - This one's surreal as hell but in a very good way and kind of relaxing too. It's a short linear game with literally zero challenge but it's all about the journey for this one. JUST BE CAREFUL though because this game will mindfuck you good. Not in a brutal or sexual way I mean, but with some really hard questions and scenarios it will bring up. It's slow soft mindfuck but it's a mindfuck nonetheless. My advice is, when you get to the end, don't think about it TOO hard or you're really gonna bust a circuit.
Honorable Mention: Far Cry 2
This one's an FPS and doesn't technically have an "alien" world but it fits everything else on your list. The thing about Far Cry 2 is, you need to TAKE. IT. SLOW. That's how you enjoy it. You take the time and enjoy the details. Take some time getting to your next desired place. Think it through. Savor all the battles you run into instead of trying desperately to avoid them just to get to your goal faster. Consider your loadout and basic approach strategy. You do these things and assuming you like FPS', I almost guarantee you'll love the free, dark, easy-going grittiness of FC2.