X27991 is a planet in the Sparks Nevada, Marshal on Mars universe.
In the Original Graphic Novel's Tales of the United Solar System Alliance episode, Gene Peeples and his crew are sent to X27991 to evaluate the possibility of a treaty and alliance with the queen of the spider people who live on the planet. After going through several cultural rites, Ensign Lewis Bambrill accidentally starts a revolution with the Fly people who are kept as slaves of the spider people. Peeples declares that X27991 is not prepared for an alliance with the Mars-Earth Coalition.
Later, in Carbonite-mare, The Red Plains Rider travels to X27991 in search of Cactoid Jim, who had been frozen in carbonite and sold to aliens on planet XK9B by Jib Janeen, the Jupitarian spy. She found out that Jim had been sold in a yard sale to a representative for Chesh Glutterfane, and arrived to rescue him via gladiator-style combat. Glutterfane had moved into the Spider Queen's palace after she was ousted from it in the Fly People revolution.
The Red Plains Rider, with help from Glenn, ousts Glutterfane from his palace and arrest him. The ruler and political state of X27991 is currently unknown.