Numerical simulations have revealed a region of space between the orbits of Uranus and Neptune where it is stable enough for a belt of small objects residing there to survive over the age of the Solar System without being ejected by gravitational interactions with the giant planets - Jupiter, Saturn, Uranus and Neptune. In the study, test particles were used to identify possible regions of stability. The results show that in the region between 24 to 27 AU from the Sun, about 0.3 percent of an initial population of test particles moving on low-eccentricity, low-inclination orbits could survive over the 4.5 billion year age of the Solar System.
Figure 1: A size comparison of Neptune and Earth.
Figure 3: Simulation results of test particles in the region between Uranus and Neptune. The test particles were initially on low-eccentricity, low-inclination orbits. The red points indicate test particles that survived the full 4.5 billion year integration, while the green points indicate test particles that did not survive. (Holman M.J., 1997)
Although members of this hypothetical belt of objects between Uranus and Neptune have yet to be detected, observations that have been carried out so far do not preclude its existence. Instead, the non-detections help constrain the size distribution of objects within this belt. At a very high level of confidence, there can no more than 7 objects larger than 640 km and 2,900 objects larger than 50 km. The total mass of this hypothetical belt is estimated to be comparable to the mass of the asteroid belt between Mars and Jupiter. Survey telescopes such as the proposed Large Synoptic Survey Telescope (LSST) may be able to detect member objects of this belt.
The discovery of objects in the 24 to 27 AU region between Uranus and Neptune would imply that processes happening during planet formation left material in this region undisturbed or that some subsequent process repopulated the region. Nevertheless, an absence of detectable objects would place more significant limits on the population of objects in this region and would suggest that processes such as planet migration have disrupted this fragile population.
Holman M.J., “A possible long-lived belt of objects between Uranus and Neptune”, Nature, 387, 785-788 (19 June 1997)