A space adventure simulator comes with level editor. You fly in a free space, do what you want: complete missions, trade minerals, fight pirates or rob the traders. Upgrade your ship, get better equipment and travel to the fathers planet in the solar system. The distance scale is proportional to the astronomical data.
The game comes in two editions: the release version with missions (1 solar system)
And the tech-demo where you can change the game settings and design the solar systems. (beta)
Links are on the website.
Note that the game runs on win-xp platform as it is quite old, you might want to use a VM.