A game of gravity. Go crash some planets! Make a planetary system of your own creation be stable (i.e. no planet is ejected, or collides with another body). This is of course exceedingly easy when your system comprises of a few Earth-mass planets, but dynamical instability can quickly set in when adding a lot of heavier bodies (from giant planets, all the way to stellar companions). The challenge is then to fit as many massive bodies as possible inside 2 AUs (twice the distance between the Earth and the Sun), teetering close to instability but lasting at least 500 years. Accordingly, the game rewards a daring player with more points (proportionally to the mass of each body added to the system).