In the outer reaches of the Solar System, beyond the orbit of Neptune, lies a region permeated by celestial objects and minor planets. This region is known as the “Kuiper Belt“, and is named in honor of the 20th century astronomer who speculated about the existence of such a disc decades before it was observed. This disc, he reasoned, was the source of the Solar Systems many comets, and the reason there were no large planets beyond Neptune.
Gerard Kuiper is also regarded by many as being the “father of planetary science”. During the 1960s and 70s, he played a crucial role in the development of infrared airborne astronomy, a technology which led to many pivotal discoveries that would have been impossible using ground-based observatories. At the same time, he helped catalog asteroids, surveyed the Moon, Mars and the outer Solar System, and discovered new moons.
Gerard Kuiper, nee Gerrit Kuiper, was born on December 7th, 1905, in the village of Harenkarspel in northern Holland. As a child, he had extraordinarily sharp eyesight, and was able to see magnitude 7.5 stars with the naked eye (which are about four times fainter than most stars visible to the naked eye). His keen eyesight spurred his interest in astronomy, which was evident from an early age.
In 1924, Kuiper began studying at Leiden University, where the famed 17th century Dutch astronomer Christiaan Huygens also studied. At the time, a very large number of astronomers had congregated at the university, and Kuiper went on to befriend many of them. Among his teachers were fellow Dutch astronomer Jan Oort (for whom the Oort Cloud is named) and Paul Ehrenfest, the Austrian-Dutch physicist who developed the phase transition theory of quantum mechanics.
In 1927, he received his B.Sc. in Astronomy and went straight into his graduate studies. In 1933, he finished his doctoral thesis on binary stars and then traveled to California to become a fellow at the Lick Observatory. In 1935, he left to work at the Harvard College Observatory, where he met his future wife, Sarah Parker Fuller. The two became married on June 20th, 1936.
Achievements in Astronomy:
In 1937, Kuiper took a position at the Yerkes Observatory at the University of Chicago and became an American citizen. Over the course of the next few decades, he participated in many astronomical surveys and made many discoveries that advanced the field of planetary science. The first came in between 1944 and 1947, while making observations of Mars and the outer Solar System.
Using ground-based telescopes, Kuiper confirmed the existence of a methane-rich atmosphere above Titan (Saturn’s largest moon). In 1947, he used similar methods to discover that carbon dioxide was a major component of Mars’ atmosphere. That same year, he predicted that the rings of Saturn were primarily composed of particles of ice, and discovered Miranda, the fifth moon of Uranus.
In 1949, Kuiper initiated the Yerkes-McDonald asteroid survey, a photometric study of asteroids conducted by the University of Chicago and the University of Texas at Austin, which ran from 1950 to 1952. At the time, the survey was limited to magnitude 16 asteroids, but also paved the way for the Palomar-Leiden survey, which Kuiper also initiated, in 1961.
This collaborative effort involved the Lunar and Planetary Laboratory (LPL) in Arizona, the Palomar Observatory in San Diego, and the Leiden Observatory in the Netherlands (Kuiper’s Alma Mater). This survey used photographic plates taken by the LPL with the 48-inch Schmidt camera at the Palomar Observatory.
Once minor planets (and asteroids with magnitudes greater than 20) were discovered, their orbital elements were computed at the Cincinnati Observatory, with all other aspects of the program – including analysis of the photographs – conducted at the Leiden Observatory. This survey resulted in the discovery of a large number of asteroids, with roughly 200-400 asteroids being discovered per plate, and a total of 130 plates being used.
In 1956, Kuiper proved that Mars’ polar icecaps were not composed of carbon dioxide, as had been previously thought, and were instead composed of water ice. In the 1960s, Kuiper also helped identify landing sites on the Moon for the Apollo program, and even predicted what the surface of the Moon would be like to walk on. His claims that the lunar surface would be “like crunchy snow” were confirmed in 1969 by astronaut Neil Armstrong.
It was also in the 1960s that Kuiper made his seminal contributions to the development of infrared airborne astronomy. In 1967, NASA’s four engine jet Convair 990 aircraft became available with an onboard telescope, which was used to conduct infrared studies at an altitude of 12,192 meters (40,000 feet). Kuiper used it extensively to make spectroscopic studies of the Sun, the stars, and the Solar planets.
Kuiper spent most of his career at the University of Chicago, but moved to Tucson, Arizona, in 1960 to found the Lunar and Planetary Laboratory at the University of Arizona. To his colleagues, Gerard was known for being a demanding boss, whose routine included hard work and long hours. Dale Cruikshank, a fellow scientist who worked at the LPL under Kuiper, claimed that:
“He worked extremely hard himself, and he demanded the same dedication, devotion and seriousness from everybody around him. If they didn’t give that, or if they didn’t perform, they ran afoul of him. That applied to students. It also applied to fellow faculty, technical associates and engineers — anybody around him. But at the same time, he had a humorous side, a warm side, a personal side that was in some ways appealing.”
But while difficult to work with, Kuiper was also known for having a warm side and a sense of humor. He also prided himself on being knowledgeable and surrounding himself with people who knew things that he did not. Kuiper was the laboratory’s director until his death in 1973.
The Kuiper Belt:
The possible existence of a Trans-Neptunian population of objects had been speculated about since shortly after the discovery of Pluto in 1930. One of the first was astronomer Armin O. Leuschner, who in 1930 suggested that Pluto “may be one of many long-period planetary objects yet to be discovered.”
In 1943, in the Journal of the British Astronomical Association, Kenneth Edgeworth further expounded on the subject, claiming that the material within the primordial solar nebula beyond Neptune was too widely spaced to condense into planets, and so rather condensed into a myriad of smaller bodies.
In 1951, in an article for the journal Astrophysics, Gerard Kuiper speculated how a similar disc could have formed early in the Solar System’s evolution. Occasionally one of the objects from this disc would wander into the inner Solar System and become a comet, he claimed, thus explaining the origins of comets while also offering an explanation of why there were no large planets beyond Neptune.
However, it would be many decades before the existence of this disc was proven and a name given to it. The first step came in 1980, when Uruguayan astronomer Julio Fernández submitted a paper to the Monthly Notices of the Royal Astronomical Society in which he speculated that a comet belt that lay between 35 and 50 AU would be required to account for the observed number of comets. It was this paper that later astronomers would draw upon when it came time to name the belt.
In 1987, astronomer David Jewitt of MIT and graduate student Jane Luu began using the telescopes at the Kitt Peak National Observatory in Arizona and the Cerro Tololo Inter-American Observatory in Chile to search the outer Solar System. After five years of searching, on August 30th, 1992, Jewitt and Luu announced the “Discovery of the candidate Kuiper belt object” (15760) 1992 QB1. Six months later, they discovered a second object in the region, (181708) 1993 FW, and many more followed.
Similarly, in 1988, a Canadian team of astronomers (team of Martin Duncan, Tom Quinn and Scott Tremaine) began running computer simulations which determined that the Oort cloud could not account for all short-period comets. With a “belt”, as Fernández described it, added to the formulations, the simulations matched observations.
In their 1988 paper, Tremaine and his colleagues referred to the hypothetical region beyond Neptune as the “Kuiper Belt”, apparently due to the fact that Fernández used the words “Kuiper” and “comet belt” in the opening sentence of his paper. While this has remained the official name, astronomers sometimes use the alternative name “Edgeworth-Kuiper Belt” to credit Edgeworth for his earlier theoretical work.
Death and Legacy:
Gerard Kuiper died in 1973 while on a vacation with his wife in Mexico, where he suffered a fatal heart attack. Because of his many achievements and long history of work in the field of astronomy, he has received many accolades over the years. These include naming the Kuiper Belt in his honor, as well naming the asteroid belt object 2520 P–L after him (aka. 1776 Kuiper).
Three craters have also been named in his honor – the Kuiper crater on the Moon, the Kuiper crater on Mars, and the Kuiper crater on Mercury. Owing to his work in airborne astronomy, NASA’s now-decommissioned Kuiper Airborne Observatory (KAO) – a highly modified Lockheed C-141A Starlifter which carried a 91.5 cm (36 inch) telescope – was also named after him.
The Kuiper Prize is also named after him, and is the most distinguished award given by the American Astronomical Society‘s Division for Planetary Sciences. The prize is awarded annually to scientists whose lifetime achievements have advanced our understanding of the planetary sciences.
Winners of this award include Carl Sagan, James Van Allen (discover of the Van Allen Radiation Belt around Earth), and Eugene Shoemaker (who co-discovered Comet Shoemaker–Levy 9 with his wife Carolyn S. Shoemaker and David H. Levy).
Because of his dedicated leadership at the Lunar and Planetary Laboratory, one of the three buildings that make up the facility (the Kuiper Space Sciences Building, shown above) was named in his honor. And one hundred years after Gerard’s birth, NASA’s New Horizons mission was well on its way to the Kuiper Belt region of our Solar System, as part of its mission to study Pluto and its moon Charon.
Dr. Richard Binzel, the New Horizons co-investigator and professor at the Massachusetts Institute of Technology (MIT), acknowledged his team’s dept to the departed scientist. “Kuiper was one of the first scientists to focus almost exclusively on exploring the properties of planets,” he said. “His work laid the foundation for the spacecraft missions of the late 20th and early 21st centuries.”
During his lifetime, Kuiper also received many rewards in his recognition for his work. In 1947, he was awarded the Jules Janssen Award by the French Astronomical Society, which is their highest honor. In 1959, the American Astronomical Society awarded him the Henry Norris Russell Lectureship, in recognition of his many years of astronomical research. And in 1971, Kuiper received the Kepler Gold Medal from the American Association for the Advancement of Science and the Franklin Institute.
As we move forward in our exploration of the Solar System, we cannot deny the great debt we owe to Gerard Kuiper. What we know about Mars and Titan, and their potential habitability, rests upon Kuiper’s work with infrared and spectroscopic astronomy. Without him, the Apollo missions might not have happened, and our knowledge of asteroids and the outer Solar System would be greatly diminished.
One can imagine that when we begin to study the Kuiper Belt in more detail, and begin cataloging the many, many objects within, that many will bear names that call the late great Kuiper to mind.
We’ve also recorded an entire episode of Astronomy Cast all about Dwarf Planets. Listen here, Episode 194: Dwarf Planets.