The planet Mars has an unofficial flag that is approved by and used by The Mars Sciety and the Planetary Society. The flag is rountinely flown at both Mars Analog Research Stations - FMARS in Devon Island and at MDRS in Utah. If you look closely you can even see the flag on the shoulders of the EVA spacesuits. It is not official in the legal sense because there is no government or other authority to adopt such a flag. In addition, the Outer Space Treaty states in Article II that "outer space, including the Moon and other celestial bodies, is not subject to national appropriation by claim of sovereignty, by means of use or occupation, or by any other means."
The flag of Mars is a tricolour with vertical bars of red, green and blue and is designated to portray the "future history" of Mars. The red bar which lies closest to the mast sybmolizes Mars as it is today. The green and blue symbolizes stages in which Mars is undergoing the process of terraforming. It was Kim Stanley Robinson's popular trilogy Red Mars, Green Mars, and Blue Mars that provided the concept for the flag and the design was originall proposed by NASA engineer Pascal Lee to Dr. Zubrin during a summer 1993 expedition to Deveon Island in Canada as task force leader for the Mars Society's Mars Analog Research Station Projects.
The flag now flies over the Flashline Mars Arctic Research Station (FMARS) on Devon Island, and is displayed in several places on the Mars Desert Research Station (MDRS) campus in Utah. It has also flown in space, carried aboard the space shuttle Discovery by astronaut John M. Grunsfeld on STS-103 in 1999.