Who was Carl Sagan's doctoral advisor?

Carl Sagan's doctoral advisor was Gerard Kuiper. Sagan completed his doctorate in astronomy and astrophysics at the University of Chicago in 1960, with Kuiper serving as his dissertation director.

Kuiper was a leading planetary scientist whose research helped establish modern planetary science. During his graduate studies, Sagan worked on theoretical studies of planets, including questions about planetary atmospheres and surfaces—subjects that shaped his later career in planetary exploration.

Sagan also worked with other prominent scientists, including physicist George Gamow and chemist Melvin Calvin. Those collaborations can cause confusion, but they were not substitutes for his formal doctoral advisor. Kuiper is the single canonical answer expected for this question.

Kuiper later became associated with the Kuiper belt, the distant region of icy bodies beyond Neptune. Although the belt was named for him, its existence was not directly confirmed until decades after his death.