It was written and produced by William Orbit and Anthony Preston, with additional songwriting provided by Spears, Dan Traynor, and Ana Diaz. "Alien" is a mid-tempo dance-pop song, which lyrically discusses Spears' feelings of loneliness. "Alien" received generally favorable reviews from music critics, who appreciated its production and recognized it as being among the more personal offerings from the record. The track is part of the setlist of Spears' ongoing residency show, "Britney: Piece of Me".