In SimEarth, the player can vary a planet's atmosphere, temperature, landmasses, etc., then place various forms of life on the planet and watch them evolve. Since it is a software toy, the game does not have any required goals. The big (and difficult) challenge is to evolve sentient life and an advanced civilization. The development stages of the planet can be reverted and repeated, until the planet "dies" 10 billion years after its creation, the estimated time when the Sun will become a red giant and kill off all of the planet's life.