Gerard Peter Kuiper

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(b. Harenkarspel, Netherlands, 7 December 1905; d. 24 December 1973, Mexico City, Mexico),

stellar astrophysics, binary stars, solar system astronomy, cosmogony, lunar and planetary studies, planetary probes.

One of the most influential astronomers of the mid-twentieth century, Kuiper made significant contributions to the study of binary stars before he turned to solar system research in the mid-1940s. He discovered the atmosphere of Saturn’s giant moon Titan, studied the characteristics of Mars and the outer planets, and worked out a cosmogonal model for the formation of the solar system that predicted the possibility of small bodies at the edge of the known solar system; although several of Kuiper’s assumptions were later proven wrong, trans Neptunian objects are commonly called Kuiper Belt objects in the early twenty-first century. The director of the Yerkes-McDonald Observatory, Kuiper founded the Lunar and Planetary Laboratory of the University of Arizona and served as a leading scientist on several National Aeronautics and Space Administration lunar projects in the 1960s.

Youth, Education, and Early Career . An ambitious, intellectually determined youth from a less-than-prosperous family, Kuiper (born Gerrit Pieter Kuiper) developed an early interest in astronomy. Successfully passing a particularly difficult entrance examination, Kuiper entered the University of Leiden, graduating with a BSc in science in 1927. He remained at Leiden for his graduate work, studying under the astrophysicists Ejnar Hertzsprung, Willem de Sitter, and Antonie Pannekoek as well as the theoretical physicist Paul Ehrenfest. In 1929 Kuiper spent eight months in Sumatra as part of the Dutch solar eclipse expedition.

On completing his doctoral thesis in 1933 on binary stars under Hertzsprung, and already fluent in English, he became a Kellogg Fellow at the Lick Observatory in California, then one of the largest observatories in the United States. A dedicated observational astronomer, Kuiper hoped to remain there but found a permanent appointment blocked by resentment against foreigners (exacerbated by his sharp, abrasive manners). He accepted a position at Harvard University in 1935 before becoming a permanent staff member the following year of the new McDonald Observatory in Texas, then operated by the Yerkes Observatory of the University of Chicago. Kuiper became a full professor in Chicago’s Department of Astronomy in 1943.

Early Stellar Research . Although Kuiper apparently began thinking about planetary research while still at Leiden, when asked to review a theoretical work on the solar system’s formation, his initial research involved stellar astrophysics. Beginning at Lick, Kuiper expanded his thesis research on physical double stars, discovering binary stars of extremely close periods (that is, in tight and rapid orbits). By 1942 Kuiper had discovered some twenty-one of the thirty-odd white dwarf stars then known. He also determined that some 50 percent of the stars closest to the Sun are binaries or members of multiple star systems. Writing up his work on the Beta Lyrae double-star system, Kuiper introduced the term contact binaries, and predicted that material drawn off from the larger star would create a ring about the smaller companion. Accretion disks caused by mass exchanges of closely orbiting stars would become a major focus of late-twentieth-century astrophysics.

Kuiper’s appointment at Yerkes—brought about by Otto Struve, the Russian-born director of Yerkes-McDonald—put him into close contact with other new hires, including the Danish astrophysicist Bengt Strömgren and the theoretical astrophysicist Subrahmanyan Chandrasekhar; together they stimulated a renaissance of stellar astrophysics at Chicago. Working closely with them, Kuiper began studying stellar motions within several star clusters in neighboring regions of the Milky Way; his work provided an effective foundation for calibrating the stellar temperature scale, then a major research concern of stellar astrophysics. Kuiper also studied low-mass stars such as white dwarfs and faint blue stars, his interest in them kindled by Chandrasekhar’s studies of degenerate matter. The aim of his research was to provide a basis for discriminating between theories of stellar energy production.

World War II and Solar System Research . Kuiper’s switch to solar system research occurred in 1944 when, on research leave from wartime research at Harvard’s Radio Research Laboratory, he used the 82-inch reflector telescope at the McDonald Observatory for scheduled observations. The McDonald Observatory was then the second-largest telescope in the United States, with greater sensitivity than the instruments at Yerkes, and Kuiper used the telescope both for stellar studies and to investigate the characteristics of planetary atmospheres. In the process he discovered that Titan, a large satellite of Saturn, possesses a methane-rich atmosphere. This was a surprising discovery, rich with implications for cosmogony. It suggested a fruitful avenue for research in solar system astronomy, but pursuing this would require him to abandon other promising avenues of stellar research in which he was deeply invested.

Kuiper considered these choices while he returned to wartime service. Because he was fluent in Dutch, German, and French, and had intimate knowledge of Western Europe, Kuiper was appointed a member of the secret Alsos mission in early 1945, which swept in behind advancing Allied troops to locate and interview Axis scientists about their wartime research, in particular seeking information about Germany’s atomic project. During his Alsos service (which reinforced his already strong antipathy toward Germany, and led him after the war to publish articles identifying Nazi sympathizers among German astronomers), Kuiper came into contact with researchers who had pioneered studies of planetary atmospheres, including the Parisian Bernard Lyot (polarization studies) and Erich Regener, who had planned to place scientific instruments on board a German V-2 to study Earth’s upper atmosphere.

On returning to the United States in late 1945, Kuiper still leaned toward returning to his stellar research. He soon decided against this for two reasons. His Alsos clearances led him to learn about wartime advances in lead sulfide cells that would enable astronomers to record infrared emissions far beyond the reach of infrared films, a great advantage for studying planetary bodies. He also became aware of new federal and military support for research involving planetary atmospheres. Kuiper decided to pursue solar system studies as his main research focus, a decision supported by Struve.

In the late 1940s, Kuiper made a number of significant planetary discoveries, including identifying carbon dioxide in Mars’s atmosphere and discovering that Saturn’s rings are composed of ice or covered with hoar frost, rather than the bare rock that he and other astronomers had expected. With visiting colleagues that Kuiper brought to Yerkes, he also studied the behavior of particles in planetary atmospheres and, with a graduate student, Daniel E. Harris III, made photometric measurements of planets, satellites, and asteroids. In 1948 Kuiper photographically discovered a fifth satellite of Uranus, later named Miranda, and one year later a second moon, Nereid, orbiting Neptune. He also initiated a survey of asteroids to obtain robust statistical data on all asteroids brighter than magnitude 16.5. By 1952, as a consequence of his emerging leadership in this field, Kuiper was elected president of the International Astronomical Union’s (IAU) Commission 16, dedicated to the study of planets and satellites.

Kuiper’s studies of planetary characteristics rekindled his interest in double stars, and he began to think of the solar system as an instance of an “unsuccessful” double-star system. Drawing on a recent nebular theory of cosmogony developed by German physicist Carl von Weizsäcker, Kuiper proposed, using insights from Struve and Chandrasekhar, that planets had formed in regions of gravitational instability within the solar nebula. Further drawing on his extensive studies of binary star systems, Kuiper introduced the then-startling notion that planetary systems were a fairly common consequence of star formation, not a rare event as many astronomers then believed. By the early 1950s Kuiper’s cosmogonic

theory emerged as a leading model among American astronomers. Contained in one published version of his theory in 1951 was a terse assertion that billions of comet-like bodies would be found at the periphery of the solar nebula at distances of 35 to 60 astronomical units (one astronomical unit being the mean distance of Earth from the Sun), on the assumption that Pluto was sufficiently massive to have diverted comets into a still more remote distribution of comets known as the Oort Cloud. While contemporary astronomers now accept a much smaller mass for Pluto, in 1992 the first object orbiting beyond Neptune was discovered (within a decade, more than a thousand additional bodies were identified). Many astronomers use the term trans-Neptunian objects to describe these remote bodies, but Kuiper Belt object has come into common use as well.

Already by the late 1940s Kuiper realized that the most significant advances in solar system astronomy would come through interdisciplinary cooperation with neighboring scientific disciplines, including chemistry, physics, and geology. By the early 1950s Kuiper began a close collaboration with a fellow Chicago scientist, the Nobel Prize–winning chemist Harold C. Urey, who had become interested in the geochemical evolution of Earth and the planets. Astronomers and geochemists in the 1950s both sought to determine the absolute abundances of the elements, since many scientists believed that concentrations of radioactive potassium, uranium, and thorium would indicate whether planetary interiors had become sufficiently hot to cause core formation and global melting. Initially Urey accepted Kuiper’s nebular cosmogony as the starting point for his own geochemical studies and advanced models of planetary geochemistry within the broad outlines of Kuiper’s failed-binary-star model. For his part, Kuiper accepted Urey’s geochemical arguments, which held that Earth and the Moon had formed and remained at relatively cool temperatures. But by 1954 Kuiper’s studies of the lunar surface convinced him that the Moon had been molten early in its history. One year later, in the pages of the Proceedings of the National Academy of Sciences, Urey strongly criticized Kuiper’s solar system research and privately sought to have him removed as chair of IAU’s Commission 16. Urey’s anger was not primarily over their scientific disagreement but instead over what he perceived (with justification) as Kuiper’s unwillingness to credit Urey’s priority in establishing a number of geochemical ideas.

The dispute left Kuiper convinced that astronomical rather than geochemical evidence was paramount in solving the puzzle of the solar system’s origin. But it also illuminated the frailty of interdisciplinary research programs that stretched across distinct disciplines and professional societies, where no means existed for resolving priority disputes, as well as the challenge of finding common frameworks for evaluating evidence from astrophysical and geochemical sources. Briefly Kuiper considered turning to a different field of research, and his then graduate student Carl Sagan recalled feeling during the controversy’s aftermath like “the child of divorced parents.” But by the mid-1950s Kuiper’s studies had helped to forge a new consensus among American astronomers about the formation and general characteristics of the solar system, and he remained firmly engaged in the field.

Planetary Research after Sputnik . The launch of Sputnik in October 1957 caused government officials and the general public to become much more interested in the Moon and the planets. Kuiper recognized the sea change and quickly began to take advantage of the situation to obtain more federal patronage for solar system studies.

Throughout the late 1950s Kuiper built up a large grant-supported group at Chicago’s Yerkes Observatory, where he had again become director. Using U.S. Air Force and National Science Foundation funds, he launched new programs to chart the lunar surface and to expand studies of the physical characteristics of planets and asteroids. While he also sought funding to aid stellar and galactic research (he was instrumental in securing a promise from the U.S. Air Force to build what became the first telescope placed at Cerro Tololo, Chile, later home to the Cerro Tololo Inter-American Observatory), Kuiper focused increasingly on developing knowledge about the Moon and planets, and sharing this information with the newly formed National Aeronautics and Space Administration (NASA) as well as an increasingly interested U.S. government.

Indeed, as the space program took shape amid the Cold War—and particularly as the space race became a means for the Soviet Union and the United States to demonstrate the technological superiority of their political systems—Kuiper found himself having to fulfill an additional role beyond active scientist, leader of solar system astronomy, and director of a major astronomical observatory: he was increasingly called on to interpret Soviet scientific advances to his government patrons. In 1959 Kuiper successfully secured a contract from the Central Intelligence Agency (CIA) to interpret Soviet research in astronomy, hiring a visiting Yugoslavian astronomer to do this work. He also struggled to interpret a controversial finding by the Soviet astronomer Nikolai Kozyrev suggesting that the Moon remained active volcanically, a critical issue for spacecraft engineers if accurate (Western scientists accepted that the Moon was seismically quiescent). Cold War limitations initially kept Western astronomers from evaluating Kozyrev’s evidence directly, and it was not until late 1960, when Kuiper could finally travel to Leningrad, that he resolved the issue (genuine observation, but misinterpreted). Convinced that science was more easily corrupted in authoritarian regimes, Kuiper embraced a public role as interpreter of the proper ethical boundaries of science.

The rapid growth of solar system astronomy at Chicago caused stellar and galactic astrophysicists at Yerkes-McDonald to grow anxious that their specialties would become overwhelmed by this increasingly popular field. Escalating tension between Kuiper and his colleagues climaxed in what contemporaries later called a “civil war,” a simmering professional dispute heightened by Kuiper’s haughty demeanor and intransigence in handling professional disputes. In 1960 Lawrence Kimpton, president of the University of Chicago, fired Kuiper as director of the Yerkes-McDonald observatories. As a tenured faculty member, Kuiper was entitled to remain at Chicago as a full professor. But several weeks later, Kuiper relocated his research associates, graduate students, and associated staff, ten people altogether, to the University of Arizona, with a new appointment split between Arizona’s Department of Astronomy and its Laboratory of Atmospheric Physics. His invitation had come from university president Richard Harvill, who perceived that federal funds for space exploration (and the new, nearby Kitt Peak National Observatory) would permit rapid expansion of lunar and planetary studies. Kuiper indeed created a major research center at the Laboratory of Atmospheric Physics (soon renamed the Lunar and Planetary Laboratory, and eventually the Institute of Atmospheric Physics), whose success became apparent during the 1960s. This was a significant moment for American astronomy, for it marked a bifurcation in the once-unified discipline of U.S. astronomy into increasingly specialized fields of planetary astronomy and stellar/galactic astrophysics. Thereafter the fields were supported and nurtured by distinct institutions, patrons, instruments, professional organizations, and graduate training programs.

At Arizona, Kuiper moved swiftly to develop new research programs and new instruments adapted for planetary research. Kuiper served as chief scientist for NASA’s lunar Ranger series, which radioed back photographs of the lunar surface until they crashed into the Moon. He and his colleagues used these photographs to determine potential landing sites for NASA’s unmanned lunar Surveyor program (he served as an experimenter) and for the later manned Apollo program. In the process Kuiper identified numerous ancient multiringed basins on the lunar surface, aiding interpretation of the Moon’s early history. While he remained committed to ground-based astronomy, promoting the development of an infrared telescope (funded by NASA) atop Mauna Kea in Hawaii, Kuiper also began using a telescope-equipped Convair 900 aircraft to make infrared observations of planets and stars from 40,000 feet (12,192 meters), above most of Earth’s atmosphere; in 1975 this facility was posthumously named the Kuiper Airborne Observatory.

Kuiper made numerous professional contributions as well. He edited two major multivolume text series (The Solar System and Stars and Stellar Systems), seeing them as vehicles for stimulating interdisciplinary research, and trained a handful of graduate students in planetary science between the 1950s and 1970s. Several of them (the most famous was Sagan) became leaders in the field in the first decades of the space age.

In 1950 Kuiper was elected a member of the National Academy of Sciences, three years after receiving the Janssen Medal of the Astronomical Society of France. He also received the Kepler Gold Medal at a joint meeting of the American Association for the Advancement of Science and the Franklin Institute in 1971. For his participation in Alsos, Kuiper received a high award from the Order of Orange and Nassau by the queen of the Netherlands.

Throughout his career Kuiper was viewed by colleagues and his graduate students as formal and distant, rarely away from work, prone to sudden coolness and acerbic comments when challenged. As a mentor, Kuiper let graduate students find their way to individual thesis questions, influencing them primarily through his memorable physical stamina and scientific style. On two-week observing runs, Kuiper slept just three to four hours per night and refreshed himself through brief catnaps; when confronting problems in planetary physics, Kuiper demonstrated a highly intuitive approach, making first-order computations on paper from physical principles.

Kuiper met the former Sarah Parker Fuller while in Cambridge (her family donated the land in Harvard, Massachusetts, where Harvard University placed its Oak Ridge Observatory). They married in 1936 and had two children, Paul Hayes and Sylvia Lucy. Kuiper died in Mexico City on Christmas Eve, 1973, just after his sixty-eighth birthday, while on a trip with his wife and Fred Whipple, a fellow astronomer and longtime friend.


Kuiper’s papers are housed in the University of Arizona Library Special Collections.


“The Empirical Mass-Luminosity Relation.” Astrophysical Journal 88 (1938): 472–507.

“On the Interpretation of βLyrae and Other Close Binaries.” Astrophysical Journal 93 (1941): 133–177.

“Titan, a Satellite with an Atmosphere.” Astrophysical Journal 100 (1944): 378–383.

Editor. Atmospheres of the Earth and Planets: Papers Presented at the Fiftieth Anniversary Symposium of the Yerkes Observatory, September, 1947. Chicago: University of Chicago Press, 1949.

“On the Origin of the Solar System.” In Astrophysics: A Topical Symposium Commemorating the Fiftieth Anniversary of the Yerkes Observatory and a Half Century of Progress in Astrophysics, edited by J. Allen Hynek, 357–424. New York: McGraw-Hill, 1951.

Editor. Nine volumes in the series Stars and Stellar Systems. Chicago: University of Chicago Press, 1960.

Editor. Four volumes in the series The Solar System. Vol. 1, The Sun; vol. 2, The Earth as a Planet; vol. 3 (with Barbara M. Middlehurst), Planets and Satellites; and vol. 4 (with Barbara M. Middlehurst), The Moon, Meteorites, and Comets. Chicago: University of Chicago Press, 1953–1963.


Cruikshank, Dale P. “20th-Century Astronomer.” Sky and Telescope 47 (March 1974): 159.

———. “Gerard Peter Kuiper, December 7, 1905–December 24, 1973.” Biographical Memoirs of the National Academy of Sciences 62 (1993): 258–295. This is the only comprehensive biography of Kuiper available.

Davidson, Keay. Carl Sagan: A Life. New York: Wiley, 1999.

Doel, Ronald E. “Evaluating Soviet Lunar Science in Cold War America.” Osiris, 2nd ser., 7 (1992): 238–264.

———. Solar System Astronomy in America: Communities, Patronage, and Interdisciplinary Science, 1920–1960. New York: Cambridge University Press, 1996.

Sagan, Carl. “Gerard Peter Kuiper (1905–1973),” Icarus 22 (1974): 117–118.

Tatarewicz, Joseph N. Space Technology and Planetary Astronomy. Bloomington: Indiana University Press, 1990.

Ronald E. Doel

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Kuiper, Gerard Peter

Dutch-American Astronomer 1905-1973

Gerard Peter Kuiper was the father of modern planetary astronomy. His work ran the gamut from star and planetary system formation to the study of the planets themselves. He used techniques ranging from visual observations to those requiring the latest technology, including infrared detectors, airborne observatories, and spacecraft.

Kuiper was born in Harenkarspel, the Netherlands. While in his native country, Kuiper made important contributions to the study of binary stars, which led to work on planetary system formation after he moved to the United States.

During the winter of 1943-1944, Kuiper made spectrographic studies of the major planets and satellites, leading to the discovery that Saturn's largest moon, Titan, had an atmosphere containing methane. Studies of the brightnesses of the moons of Uranus and Neptune led to the discovery of additional satellites: Miranda, orbiting Uranus, in 1948; and Nereid, orbiting Neptune, in 1949.

In 1951, he proposed that a disk of comet nuclei extends from the solar system's planetary zone out to as much as 1,000 times the Earth-Sun distance (the astronomical unit [AU]). This is now called the Kuiper Belt and is recognized to extend from Neptune's distance (about 30 AU) to perhaps 50 to 100 AU.

In 1960, Kuiper founded the Lunar and Planetary Laboratory at the University of Arizona. He remained active in his later years, traveling and conducting site surveys for new observatories. Kuiper died in 1973.*

see also Astronomy, History of (volume 2); Careers in Space Science (volume 2); Comets (volume 2); Kuiper Belt (volume 2); Neptune (volume 2); Saturn (volume 2); Uranus (volume 2).

Stephen J. Edberg


Cruikshank, Dale P. "Twentieth Century Astronomer." Sky and Telescope 47 (1974):159-164.

Pannekoek, Anton. A History of Astronomy. New York: Interscience Publishers, 1961.

Internet Resources

Jewitt, David. "Kuiper Belt." Institute for Astronomy. <>.

*A now-retired National Aeronautics and Space Administration airborne observatory that made groundbreaking infrared observations from the stratosphere was named after Kuiper.

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Kuiper, Gerard Peter (1905–73) US astronomer, b. Netherlands. He discovered the satellites Miranda (of Uranus) in 1948 and Nereid (of Neptune) in 1949. He found methane in the atmospheres of Uranus, Neptune and Titan, and carbon dioxide in the atmosphere of Mars.