Planetarium Uchuu Kyoudai: Itten no Hikari

プラネタリウム 宇宙兄弟 ～一点のひかり～

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. (Source: Anime News Network)