The exoplanet naming convention is an extension of the system used for naming multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the name is normally formed by taking the name of its parent star and adding a lowercase letter. A provisional IAU-sanctioned standard exists to accommodate the naming of planets that orbit two stars, which are known as circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.