Caption: "An artist's conception the Kuiper belt (in the inset) and Oort cloud. Sizes of individual astronomical objects have been exaggerated for visibility." (Slightly edited.)

Features:

1. The Kuiper belt and the Oort cloud were both proposed as hypothetical reservoirs (or population) of rocky-icy bodies (i.e., icy planetesimals, protoplanets) beyond the Neptune orbit mean orbital radius 30.11 AU.

2. The Kuiper belt and Oort cloud were hypothesized in order resupply, respectively, short-period comets and long-period comets. As we discuss below, comets of either sort can only exist for a short period of time compared to the Solar-System age = 4.6 Gyr.

   The Oort cloud (which more of a swarm than a cloud) was first proposed by Jan Oort (1900--1992) in 1950 (see Wikipedia: Jan Oort: A few of Oort's discoveries). By the by, my colleague UNLV astronomy professor George Rhee was a graduate student of Oort in the 1980s when Oort was still very research active.

3. Short-period comets have orbital periods less than 200 years and have orbital planes somewhat concentrated near the ecliptic plane.

4. Long-period comets have orbital periods ranging from 200 years to thousands of years and their orbital planes have a spherically symmetric distribution.

5. Both types of comets originate when astronomical perturbations perturb members of the appropriate reservoir into highly eccentric elliptical orbits.

   The astronomical perturbations can be gravity assists by other reservoir members, collisions (with probably fragmentation) with other reservoir members, and gravitational perturbations by a passing star on a near flyby.

6. Comets, as aforesaid, can only exist for a small fraction of Solar-System age = 4.6 Gyr. They lose volatiles with every trip through the inner Solar System (which includes a perihelion necessarily) and eventually become extinct comets. So comets must be resupplied from reservoirs: i.e., the Kuiper belt and the Oort cloud.

7. What happens to extinct comets?

   Extinct comets are just Near-Earth asteroids (NEAs) (see Wikipedia: Extinct comet: Distinction between comets and asteroids). So their fate is the same as for other NEAs.

   A NEA eventually impacts some other Solar System astro-body (Sun, planet, moon, etc.) or, via a gravity assist, is launched back into a long-lasting orbit probably entirely beyond the Neptune orbit or ejected from the Solar System on an escape trajectory (AKA escape orbit): i.e., a parabolic trajectory or hyperbolic trajectory.

8. The Kuiper belt has been confirmed since 1992 by the discovery over a 1000 of Kuiper Belt objects (KBOs) (see Wikipedia: Kuiper Belt). Though Pluto (discovery 1930) and its moon Charon (discovery 1978) are KBOs) their discoveries are NOT considered the discovery of the Kuiper belt.

   How many KBOs are there? In fact, it's hard to do an exact count of known KBOs since there is tendency in references to lump all trans-Neptunian objects (TNOs) together in lists (see Wikipedia: List of trans-Neptunian objects). It is estimated that there over 10**5 KBOs with diameter over ∼ 100 km (see Wikipedia: Kuiper Belt).

   We now know that the Kuiper belt extends approximately from 30 AU to 50 AU from the Sun and has approximately a torus shape (but all KBOs orbit the Sun, of course). In some respects it is similar to the Asteroid Belt, but it is much larger: ∼ 20 times as wide and with 20 to 200 times more mass. Also, its members probably have a much higher fraction of ices than asteroids which are more carbonaceous and/or rocky and/or metallic at least on the surface.

9. The Oort cloud, on the other hand, has NOT been confirmed to exist observationally since NO Oort cloud object (OCO) has ever been discovered. OCOs are very hard to identify: they are very dim, tiny, slow-moving far-off astronomical objects and they exist against a background of jillions of dim stars which to 1st order look exactly the same as OCOs. Note that KBOs, which are similar astronomical objects to OCOs, are only 30 to 50 AU away and they are hard to discover.

   The upshot is that the Oort cloud is likely to remain unobserved system for a long time to come, but it certainly exists.

10. However, the distribution of long-period comets allows some inferences about the Oort cloud. It is an approximately spherically-symmetric reservoir of rocky-icy bodies (i.e., icy planetesimals, protoplanets, and fragments there of) extending approximately 50,000 to 200,000 astronomical units (AU) = 1.49597870700*10**11 m (0.8 to 3.2 light-years (ly) = 0.94607304725808*10**16 m = 63241.0770 ... AU = 0.306601393 ... pc ≅ 0.3066 pc; 0.24 to 0.97 parsecs (pc) = 3.08567758 ... *10**16 m = 206264.806 ... AU = 3.26156377 ... ly ≅ 3.26 ly) from the Sun. The image shows the Oort cloud with some structure: probably this is based on some analysis.

    It is estimated that the Oort cloud may contain 10**11 to 10**12 OCOs of at least a few kilometers in size scale that can become long-period comets (Se-569; HI-248).

    The OCOs probably did NOT form in situ. It is thought that the concentration of dust and gas was probably too low in situ in the primordial solar nebula to have brought about coalescence into largish bodies like the OCOs. Instead it is thought that the rocky-icy bodies that formed among the gas giant planets were kicked via gravity assists by the gas giant planets far out to the Oort cloud region (Se-573).

11. Because the Oort cloud is approximately spherically symmetric long-period comets can come into the inner Solar System from any direction. They are NOT confined to the vicinity of the ecliptic plane as are most short-period comets that originate in the Kuiper belt.

12. Keywords: asteroids, astronomical objects, astronomical unit (AU) = 1.49597870700*10**11 m, Charon, comets (extinct comets, long-period comets, short-period comets), eccentricity, elliptical orbits, Kuiper belt, mean orbital radius, Near-Earth asteroids (NEAs), orbital inclination (to the ecliptic plane), Oort cloud, orbital period, planet, planetesimal, Pluto, protoplanet, rocky-icy bodies, Solar System (inner Solar System, outer Solar System), Sun.