Astronomy is arguably the oldest science. Observations of the motion of heavenly bodies date back more than 2,500 years. Nevertheless, as recently as 1992, immediately prior to the detection of the first Kuiper belt objects (KBOs) by Jewitt and Luu,1 little was known about the contents of the solar system beyond 30 AU.2 Distant bodies are dim because they reflect little sunlight back to Earth and our most powerful telescopes can only image small angular regions at a single pointing. Thus searches for widely-spaced, faint images are tedious and resource intensive. Before Jewitt and Luu’s discovery, Pluto and its large satellite Charon were the only directly detected bodies orbiting beyond Neptune.
Comets provide indirect information about reservoirs of bodies beyond 30 AU. Although comets are directly detected only when they come within a few AU, their orbits can be traced back to show where they came from. By 1950 it had been established that most long-period comets were visitors from distances in excess of 10,000 AU and that their orbits were randomly oriented with respect to the mean orbit plane of the solar system. These facts led Jan Oort to hypothesize that comets are stored in an enormous spherical cloud beyond 10,000 AU and that gravitational deflections by nearby stars are responsible for injecting those we detect into the inner solar system. Oort’s model is widely accepted and the hypothetical comet cloud carries his name. However, his assumption that short-period comets are descendants of long-period ones did not fare as well.
Short-period comets are rare in comparison to long-period comets. Their orbits are mostly prograde and of low-inclination. Oort proposed that gravitational scattering by planets transforms a small fraction of long-period comets into short-period comets. However, during the 1980s, dynamical studies aided by numerical simulations demonstrated that Oort’s proposal for the origin of short-period comets was untenable. Instead, they showed that an origin in a disk just outside the orbit of Neptune provides an excellent fit to the orbital characteristics of short-period comets. Independently, Jewitt and Luu began to search for KBOs during the time when these theoretical advances were being made. Success did not come quickly. It took them five years to find their first KBO.
After their initial discovery, Jewitt and Luu continue to lead efforts to characterize the spatial distribution and size spectrum of KBOs for more than a decade. As other astronomers joined the hunt, progress accelerated. During the past 20 years, more than 1,400 KBOs have been discovered. It has been established that the region between 30 and 50 AU is populated by a vast array of icy bodies with tens of thousands having diameters in excess of 100 kilometers. There are even a few whose diameters exceed 1,000 kilometers and one of comparable size to Pluto (diameter 2,000 kilometers) discovered by a team working with Mike Brown. This finding led to Pluto’s demotion from its high status as the ninth planet to merely a charter member of a new class of dwarf planets.
A significant fraction of KBOs are trapped in orbital resonances with Neptune. That is, their orbit periods are related to Neptune’s by the ratio of two small integers. Pluto is in a 3:2 resonance so Neptune circles the Sun three times while Pluto goes around it twice. As a result of this resonance, Pluto is protected from close approaches to Neptune even though its eccentric orbit crosses Neptune’s circular one. By sheer coincidence, a paper by Renu Malhotra proposing that Neptune’s outward migration was responsible for the resonant trapping of Pluto’s orbit was published in 1993 in Nature, the same year and the same journal in which Jewitt and Luu’s seminal article appeared. Similar dynamics apply to all orbital resonances, so Malhotra’s proposal has been widely embraced as the explanation for the large fraction of resonant KBOs. Other resonances known to have substantial populations include the 5:2, 7:4, and 5:1. Trapping of KBOs in these resonances implies that Neptune’s currently circular orbit maintained a substantial eccentricity during the time the planet was migrating.
Jewitt and Luu discovered a new component of the solar system and the source of the short-period comets. KBOs grew by the accretion of solids during the early stages of planet formation until some undetermined process, probably involving perturbations by Neptune while its orbit evolved, increased their relative velocities to the extent that further accretion could not take place. Owing to their great separations, large KBOs have suffered little collisional evolution since. Thus they offer us a frozen record of the early stages of planet formation. As impressive as progress has been, much unexplored territory remains. Almost nothing is known about the content of the solar system from 100 to 10,000 AU. These and other challenges remain for the future.
Astronomy Selection Committee
The Shaw Prize
17 September 2012, Hong Kong
1 Gerard Kuiper was a Dutch-American astronomer.
2 Distances quoted here are relative to the Sun. An astronomical unit (AU) is the mean radius of the Earth’s orbit about the Sun. Neptune’s orbital radius is 30 AU and the nearest stars are at a distance of about 150,000 AU.