Gliese 581 c was thought to be habitable when it was discovered, and it was thought to be in the habitable zone. However, later studies by the von Bloh team said that Gliese 581 c circled too close to the star to be in the habitable zone. Gliese 581 c may also suffer a serve greenhouse effect, making it as hot as Venus. If that is true, it is uninhabitable. Also, red dwarfs like Gliese 581 are not very friendly to life as we know it, since they are very stellar active (meaning they emit strong solar flares and such).