Planetarium Uchuu Kyoudai: Itten no Hikari

プラネタリウム 宇宙うちゅー兄弟きょーだい ～～一いち点てんのひかり～～

The anime's original story features Mutta and Hibito during one summer when they were children. During the summer when Mutta and Hibito made their promise to go to space, they met a young girl who also had a wish.