Planet X

Planet X is a fictional planet featured in the Godzilla film series of the Shōwa period. They appeared in the 1965 film Godzilla vs. Monster Zero (also known as Invasion of Astro-Monster).

Description
Planet X is the home planet of the Xians. It is the main setting for the space battle between Rodan, Godzilla and King Ghidorah in Invasion of Astro-Monster. Planet X is located somewhere beyond the orbit of Jupiter. It is the first location outside of Earth which kaiju are shown to visit, and is to this day the only extraterrestrial location from which Godzilla has visited. It is also the place where Godzilla makes his famous "victory dance" after defeating King Ghidorah.

Planet X seems to have an atmosphere. It also seems to have a similar gravity as Earth's, since Godzilla, Rodan, and King Ghidorah seem to walk, jump, and fight without any gravitational interference. It seems to also have oxygen and carbon dioxide since the monsters can breathe on the planet.