Doinaka is a planet in Episode 124. Keroro is curious about this planet as a child, despite the train are prohibited for children alone, Keroro, Giroro, and Zeroro rides on the train, and hides in the bathroom before departure. They discovered Viper (old) after chased by a conductor robot. The three finally arrived in Doinaka, but it seems to be empty, no aliens living here, but in the present, Doinaka is now populous.
Doinaka is home to the Space Cerberus, which may mean it also the home planet of the other "space" creatures mentioned throughout Keroro Gunso/SGT. Frog.