The best Open World Game alternative to Space Engine is No Man's Sky. It's not free, so if you're looking for a free alternative, you could try TerraGenesis. If that doesn't suit you, our users have ranked more than 25 alternatives to Space Engine and three of them are Open World Games so hopefully you can find a suitable replacement.
No Man's Sky is an action-adventure survival video game developed and published by the indie studio Hello Games.
Starfield is the first new universe in 25 years from Bethesda Game Studios, the award-winning creators of The Elder Scrolls V: Skyrim and Fallout 4. In this next generation role-playing game set amongst the stars, embark on an epic journey to answer humanity’s greatest mystery.
Starfield is an RPG with space as background, not space simulation
Engage in space exploration and terraform new worlds in this fun and exciting planet building simulator rooted in real science. TerraGenesis dynamically animates entire planets with changing biospheres, all based on real data from NASA.
You can't terraform Jupiter, Saturn, Uranus, and Neptune.
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You can explore all the planets you want and the concept is very similar to Space Engine, albeit unrealistic (no gas giants, no tidally locked planets etc. it's more "fantasy" than Space Engine).