The vast number of small solar system bodies that lie beyond Neptune and include Arrokoth and the dwarf planets such as Pluto are arranged in groupings that have various names, but the one broadly-defining designation that is most frequently used is the Kuiper Belt. Here we look at the life and work of astronomer Gerard P. Kuiper (originally Gerrit Pieter Kuiper) and see how his name came to be used to denote a distant region populated by a vast number of planetary bodies and termed the "third zone" of the solar system.

Stellar and Planetary System Formation

Black and White Picture of G. P. Kuiper Lecturing in Class

G. P. Kuiper lecturing on the structure of the Solar System and his views on the origin of Pluto. Circa 1955. Dale Cruikshank collection.

Kuiper was born in 1905 in the Netherlands and educated at Leiden University by Ejnar Hertzsprung, Willem de Sitter and other important scientists. Upon completion of his studies, in 1933, Kuiper came to the United States on a Kellogg Fellowship at Lick Observatory near San Jose, California. At Lick Observatory he continued to observe double stars and conducted searches for white dwarf stars.

Kuiper was driven by some of the compelling and overarching questions in astronomy of the day: How did the Sun and planets form in the cloud of dust and gas that accumulated in a region of interstellar space? How did this genesis relate to the formation of other star systems, some surely possessing planets of their own? And in our own solar system, what can we learn about the atmospheres and surfaces of the planets and their moons from observations with telescopes?

With these questions foremost in his mind, Kuiper devoted his intellectual life to observational studies of the evolution and characteristics of star systems, and to the properties of the Sun's family of planets.

Kuiper developed a theory of the origin of the solar system based on theoretical ideas about the motions of a rotating cloud of dust and a mix of hydrogen and helium gas. In this cloud, the Sun formed as gravity drew in gas from its surroundings, transforming the cloud into a disk with large scale turbulent eddies at different distances. As the fledgling Sun grew in the center of disk, its temperature and the density of the gas rose to the point where nuclear reactions began in its interior. Meanwhile, the major planets were taking form from the remaining dust and gas in the turbulent eddies. Kuiper's contribution to this picture was related to details in the eddies that were stable against disruption by the growing young Sun.

Kuiper had come to his theory of the origin of the solar system as an outgrowth of the observational studies of double stars that he began as a student in Leiden. He had concluded that many stars form in pairs, and that in instances where the second star in a pair failed to reach a minimum size required for nuclear reactions to begin, its material would circle the successful star and proceed to form planets. From this reasoning, Kuiper concluded that a very large fraction of the stars in the galaxy must have planetary systems. His prediction has been largely substantiated in recent years by discoveries of a great number of stars with planetary systems in our local part of the Milky Way galaxy.

Kuiper remained in the United States for his entire career, first at Lick Observatory and then briefly at Harvard College Observatory. In 1937 he received an academic appointment at the University of Chicago's Yerkes Observatory, joining a group of distinguished scientists that included Otto Struve, William W. Morgan, Bengt Strömgren and S. Chandrasekhar. These astrophysicists and their students established Yerkes Observatory as one of the most important centers for astronomy and astrophysics in the world.

The War Years

During World War II (1943-1945), Kuiper joined Harvard's Radio Research Laboratory, where he worked on radar countermeasures in the United States and England. In January 1945, Kuiper went to Europe with the Alsos Mission of the U.S. War Department to interview German scientists and engineers to assess the state of German science at the war's end. Fluent in German, Dutch and French, through his interviews he provided intelligence from parts of occupied Europe where communication among scientists had been impossible during the war.

During a brief break from war work, in 1943-1944, Kuiper used the newly completed 82-inch telescope at McDonald Observatory in Texas to make spectroscopic observations of white dwarf stars and the planets. In the course of this work, he discovered the atmosphere of Saturn's largest moon, Titan, by detecting the characteristic spectral fingerprint of methane gas. This was the first detection of an atmosphere on a planetary satellite.

Infrared Astronomy

Black and white photo of kuiper looking into spectrometer

Kuiper observing with an infrared spectrometer at the Catalina Observatory, 1969. Photo: D. P. Cruikshank

A paradigm-shifting consequence of Kuiper's war work was the discovery that German scientists had developed a light sensor that worked in the infrared part of the electromagnetic spectrum that is invisible to the eye and that cannot be detected with photographic techniques. Upon his return to research after the war, Kuiper and colleague Robert Cashman, who had also worked on developing infrared sensors, collaborated to apply the new technology to astronomy. This successful work that began in 1946 immediately yielded new perspectives in both stellar and planetary astronomy, notably with the discovery of carbon dioxide gas in the atmosphere of Venus.

The Father of Modern Planetary Astronomy

In addition to this and his other discoveries in the emerging discipline of infrared astronomy, Kuiper used the McDonald Observatory telescope to photograph the regions around the planets to search for satellites. In 1949, he discovered Miranda, the fifth moon of Uranus, and Nereid, the second known moon of Neptune.

Kuiper's discoveries in planetary astronomy were key in recovering the moribund study of solar system bodies from a long period of profound indifference on the part of most astronomers. The subject had been tainted in the early 20th century by claims of canals on Mars constructed by inhabitants of that planet to carry water from the icy polar regions to temperate zones for the purpose of agriculture. For his discoveries in the study of planetary atmospheres, for finding the moons of Uranus and Neptune, and for his theory of the origin of the solar system, Kuiper is appropriately considered the father of modern planetary astronomy.

Lunar Studies

Kuiper remained on the faculty at Yerkes Observatory until 1960, when he left the University of Chicago and took a position at the University of Arizona to establish the Lunar and Planetary Laboratory (LPL). Recognizing the importance of integrating geology, atmospheric science and chemistry with astronomy to study the planets, he brought together researchers in all these disciplines and began to educate students with a specialty in planetary science. One of the major initial activities at LPL was to gather the best high-resolution photographs of the Moon to assist in guiding the human exploration of the lunar surface, ultimately with landing humans and returning them safely to Earth. This work resulted in the publication of several atlases of lunar photographs.

In an extension of the lunar photographs taken at observatories, the United States was engaged in three large-scale projects to find landing sites on the Moon for astronauts. Kuiper was the principal investigator of the Ranger program, which had its first success in 1964. He was also a key scientist in the Surveyor program, which landed five remotely operated spacecraft softly on the Moon in 1966-1968. Simultaneously, five spacecraft called Lunar Orbiter sent high-quality photos of the surface from orbit around the Moon. These programs were essential to the successful landing of two Apollo 11 astronauts on the Moon in July 1969.

Airborne Observatories

In the 13 years from the founding of LPL to his death in 1973, Kuiper relentlessly and energetically pursued infrared studies from the ground and with a telescope on a jet airplane flying above most of the Earth's atmospheric water vapor, which is an impediment to infrared studies. He was a key figure in establishing airborne astronomy, and a NASA aircraft with a large telescope was put into service in 1974 and named the Kuiper Airborne Observatory (KAO). After 21 years of major discoveries, including finding the atmosphere of Pluto, the KAO was retired in favor of an even larger airborne telescope on NASA's Stratospheric Observatory For Infrared Astronomy (SOFIA). Kuiper was also instrumental in finding good sites for telescopes on mountains, playing an essential role in defining Mauna Kea in Hawaii and Cerro Tololo in Chile as superlative sites for astronomy. At the time of his death, he was in Mexico in search of suitable observatory sites.

Clouds the day before the SOFIA flight

Credit: Geoff Haines-Stiles

Kuiper Airborne Observatory

Kuiper Airborne Observatory
Credit: Geoff Haines-Stiles

Beyond the Edge of the Solar System

In the course of his studies of the origin of the solar system, Kuiper speculated on the vast region beyond Pluto and the other known planets. He convincingly demonstrated that short-period comets, that is, comets with orbits lying close to the plane of the orbits of the planets and with orbital periods shorter than about 200 years, must come from a region not too much more distant than Pluto. As for Pluto itself, Kuiper reasoned that it was an escaped satellite of Neptune. He did not explicitly predict that Pluto is a member of a population of large icy bodies at that distance from the Sun.

A contemporary of Kuiper, Kenneth Edgeworth, a retired, decorated English soldier in World War I, wrote about the structure and origin of the solar system, and speculated that in the region beyond Pluto there must have been clumps condensed matter forming a vast reservoir of potential comets. In his papers in 1943 and 1949, he predicted that some of these clumps are occasionally displaced and enter the inner solar system as comets. Edgeworth's work was not mathematically or physically rigorous, but he seems to have had the basic correct idea of a population of icy bodies in the solar system beyond Neptune, although like others, he thought that Pluto was an escaped satellite of Neptune.

Some scientists describe the region of the solar system, the "third zone," where Pluto, Arrokoth, and thousands of other small bodies lie, as the Edgeworth-Kuiper Belt, but it most often referenced as the Kuiper Belt. Kuiper never cited Edgeworth in his publications, and may have been unaware of his work. Kuiper was also a very prominent scientist, where and Edgeworth was more of an amateur and not widely known. Names aside, the third zone of the solar system is the most populous, and the story of the origin and evolution of the Sun and planets told by the bodies comprising it has only begun to be revealed.