Just a shoot 'em up. Defend the planets. Do not allow the UFOs to hit them with their plasma-balls and do not let them collide.

Defend your planet from pesky alien invaders that arrive from all around the galaxy. Blast the enemies before they impact your planet. Once the planet's lifeline is gone your planet explodes and the game is over. Avoid contact with your ship and the enemies or your ship gets damaged and can explode, upon which your planet will take time to build another new ship.