Dutch-American astronomer known especially for his discoveries and theories concerning the solar system.

Kuiper graduated from the University of Leiden in 1927 and received his Ph.D. from that school in 1933. That same year he moved to the United States, where he became a naturalized citizen (1937). He joined the staff of Yerkes Observatory of the University of Chicago in 1936, twice serving as director (1947–49 and 1957–60) of both Yerkes and McDonald observatories. Kuiper founded the Lunar and Planetary Laboratory at the University of Arizona in 1960 and served as its director until his death.

After conducting research in stellar astronomy, Kuiper shifted his focus to planetary research in the 1940s. In 1944 he was able to confirm the presence of a methane atmosphere around Saturn's moon Titan. In 1948 he predicted (correctly) that carbon dioxide is a major component of the atmosphere of Mars, and he also correctly predicted that the rings of Saturn are composed of particles of ice. That same year he discovered the fifth moon of Uranus (Miranda), and in 1949 he discovered the second moon of Neptune (Nereid). In 1950 he obtained the first reliable measurement of the visual diameter of Pluto. In 1956 he proved that Mars's polar icecaps are composed of frozen water, not of carbon dioxide as had been previously assumed. Kuiper's 1964 prediction of what the surface of the Moon would be like to walk on (“it would be like crunchy snow”) was verified by the astronaut Neil Armstrong in 1969.