"Planetarium" (プラネタリウム Puranetariumu) is Ai Otsuka's tenth single, which was released on 21 September 2005. The single had great success and became Otsuka's second #1 hit on the Oricon charts, selling 316,425 copies. In 2005, Planetarium sold 215,114 copies, making it the #41 best-selling single of the year. However, it still charted into the next year and was the #99 best-selling single in 2006, selling 100,555 copies. Planetarium is her second best-selling single after "Sakuranbo."
"Planetarium" was featured on Ai Otsuka's studio-album Love Cook. It was also used as an OST for the Japanese drama "Hana Yori Dango".