Celestia is not a game in the same sense that X3, Terran Conflict is but is a simulation application that allows you to traverse the universe. It allows you to view planets in both The Solar System and beyond. The planets or to give them their real name, Extrasolar Planets (Exoplanets) will be artists impressions or fictional planets that have added to the application. There is a whole universe of add-ons (sorry for the pun) to be explored and downloaded onto your computer.
For example, it is possible to download and explore the Star Wars planets of Moon of Endor and the desert planet of Tatooine by downloading the appropriate Add-on. All you need to do is to download the zip file that you wish to view and install it in the application`s extra folder. From there, run the program, selection Navigation then Select Object and put in HD 143761 which is the star thats been designated as the Star Wars planet in the add-on. Then select `Navigation`...`Solar System Browser` and select the planet you wish to visit, if you used the addon from the page I just linked, there are only two planets available but they should give you some fun.
If you`re more of a Star Trek fan, they are also catered for on the Motherlode page so warp speed over there. Picture 6 is a Ferengi battle cruiser with the Ferengi homeworld in the back ground. Zooming in on the cruiser doesn`t change the size of the planet and you have to select the planet to move to it. The crafts aren`t flyable as far as I can see.
There are scripts that you can run so you can view journeys through space and get information. The program comes with a guided tour of the solar system so sit back and enjoy.
The last release of the application was in 2011 and there`s no idea when the next release will come out but everythings fine with it at the moment.