Sun is written in the key of G♯ Dorian.
According to the Theorytab database, it is the 11st most popular key among Dorian keys and the 48th most popular among all keys. The G♯ Dorian scale is similar to the G♯ Minor scale except that its 6th note is a half step higher (E♯). Chord progressions in Dorian have a characteristic sound due to the major quality of the chord built on the 4th scale degree.
See the G♯ Dorian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!