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S.C. Tegler (N. Arizona U.), W. Romanishin (U. Oklahoma)
The Kuiper belt represents an exciting, new frontier in solar system research. About 200 Kuiper belt objects (KBOs) with diameters larger than 100 km are known to exist between 30 and 50 AU from the Sun. Surveys indicate that there may be as many as 100,000 KBOs larger than 100 km and perhaps billions of KBOs larger than 1 km between 30 and 50 AU. Although the total mass in these bodies is perhaps a few tenths of an Earth mass, accretion calculations indicate that the primordial Kuiper belt must have contained 10 to 30 Earth masses of material between 30 and 50 AU in order to explain the growth of large KBOs and the Pluto and Charon system in the 100 million years before the onset of the disruptive influence of Neptune.
Once Neptune reached a fraction of its current mass, dynamical studies indicate that a combination of erosional collisions and mean motion and secular resonances sculpted the belt into its present day mass and structure. The influence of the resonances can be seen in the belt today as about one-third of the known KBOs are in a stable 2:3 mean motion resonance with Neptune, i.e. eccentric and inclined orbits, that approach or cross the orbit of Neptune, and semi-major axes, a, about 39.4 AU. Many KBOs with a > 42 AU are sufficiently far from Neptune that they are on stable, low inclination, low eccentricity, non-resonant orbits. A combination of resonances and disruptive collisions continue to deplete the Kuiper belt today as they inject KBOs or collision fragments inward into the solar system as Centaur objects and Jupiter family comets.
Physical studies of KBOs are in their infancy. Perhaps one of the most surprising results is the observation that KBO colors and hence their surface compositions divide neatly into a grey and an extraordinarily red population. The red population suggests some surfaces are rich in complex carbon-bearing molecules. The colors exhibit no trend with resonant or non-resonant orbits or object size and suggest that something unexpected and important happened in the outer solar system. Visible and infrared spectra are available for only a few of the brightest KBOs and indicate the presence of frozen methane and water. Optical light curve studies suggest that the smallest KBOs are irregular in shape. Albedo measurements and searches for comae around KBOs are just beginning.
Many fundamental questions remain unanswered about the Kuiper belt. For example, no KBO has been found beyond 50 AU. Does 50 AU represent an edge to the belt? On the other hand, is the region beyond 50 AU sufficiently far from the disruptive influence of Neptune that Pluto-sized or larger bodies await discovery?