AFAIK that hasn't been done yet so you'd have to make your own engine.
The engines are made to create games that hasn't been created, aren't they?
Not really, good game engines are made alongside a game, not prior to the game, sometimes they're even made after the game in more or less a "Ok, this game is selling quite well, lets try to re-use as much as possible of it to make a sequel and get more cash" way and occassionally studios will allow others to use their old technology in exchange for some money.
If you want to push technology for a game you have to make your own engine (or get a sourcecode license for an engine so you can update it) because any engine that is allready on the market will be old by the time your game is done.
The big problem with your idea is the planet size, Jupiter is huge, 6.1419×10^16 m2 , so if you use 1 byte for every 1m2 of surface (very low detail, enough for a space MMO where you view the planets from a low orbit) you'd still need 61.42 PetaBytes of data, if you want weather effects, rivers, etc you need far more detail than that aswell.
This basically means that you need a procedural terrain generator. (Allthough i don't see why you'd want such a large world, even something as "small" as earth would be overkill for an MMO since even the truly great MMO backends will struggle if you try to push them past 100k players in a single world)