Why not just start with a smaller area of "Jupiter", You could load the areas in levels instead of a huge open world. People could be presented to a map of a planet and then click the areas they wish to travel to and then load that area. Then suddenly a lot of engines would be fine for the job. Just a suggestion.
Well... my vision is to get rid of the actual MMO style.No maps(Self made by players and sold to one and each other... keeping it a bit of secret!).No buttons.No clicks.No minimap.No leveling.No experience.Maybe I will reinvent the character walk.And definitely No! "portals"(but I will make portals available... only that will be required like 10 Mages to hold the portal open or gather all the Mages from all the realm to help enchant a portal linked to another part of the continent, the greater the distance the greater the power needed... and a lot of materials wasted to do magic, magic dust and wands! The reason I have this in mind with the open world.. is cause I just want to get away from the "hit the invisible wall" type of games.
But the idea of starting small is very wise of you.I was thinking of that.
But please I need help with idea's of some engine that I do not know of.
A name please!
I just wanted to take a minute to mention that if you are making a game, you have to consider making it fun. Everyone has different tastes but if you are going to invest "years" creating something, it might be wise to perform market research to decide if something like you describe is desirable to actually play.
You mention getting rid of experience and leveling. In my mind the character progression and trying to achieve a more powerful character is what makes MMO type games "fun".
One of the aspects I consider to be the most tedious when it comes to such games is travel times. You are talking about increasing travel times by a very large margin. If you make a fairly mundane planet like Jupiter, what is to stop people from becoming very bored and fatigued by the environment?
For instance imagine I was at the bottom of a crater and you decide due to movement speeds and scale, that it is going to take me 4 hours or so to travel out of this crater. Do you think someone is going to bother to keep playing?
One of the main reason for the "invisible" walls is to provide the illusion of a larger game world, while reducing the available area to something that can be interesting/practical. There is no way you will have the time or budget to be able to make an area the actual size of Jupiter interesting.
If I were in your shoes I would opt to use Unity for my game engine. Not because it would be able to handle all of what you describe, but because it would provide a solid technical foundation that would allow me to create something fairly quickly.