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Oort Cloud: Past the Kuiper Belt and the Solar System

Short-period comets reside in the "scattered disk" (not the Kuiper belt as once thought). The scattered disk is a sub-region of a larger body of objects know as the Trans-Neptunian objects, or TNOs. Scattered disk objects, SDOs, have highly eccentric and elliptical orbits extending from 30-100+ AU from the Sun; indeed, their orbits have been "scattered" by the gravitational perturbations of the gas giants, particularly by Neptune. "Long period" comets that take over 200 years to circle the Sun, however, must come from beyond the Kuiper belt and even from beyond the scattered disk. The true origin of these objects hasnít been well known or hypothesizedóuntil recently.

In 1950 the Dutch Astronomer Jan Oort noticed three things that all comets had in common:

1. Comets seem to come from no preferred direction;

2. No comet has been observed with an orbit indicating it came from "interstellar" space:

         1000 times further from the Sun than Pluto;

3. Thereís a strong tendency for the furthest points of long-period comets from the Sun

         (their "aphelia") to group at about 4.6 trillion miles (50,000 AU, or 0.8 light-years) from the Sun.


From these observations Oort proposed that long-period comets reside in an immense spherical cloud (see the above diagram or the one on the Kuiper belt page) surrounding the planets and extending from about 200-400 billion miles from the Sun (50-100 times Pluto's distance) out to about 1.5-3 light years (10-20 trillion miles; the nearest star is 4ľ light years distant). This vast distance is thought to be the boundary of the Sunís influence. Known as the "Oort Cloud," statistics imply it contains as many as six trillion comets! Since the individual comets are so small and at such great distances, there is no direct evidence of this hypothetical cometary cloud, but the indirect evidence grows yearly.

Within the Oort Cloud comets are thought to be typically tens of millions of miles apart. Only weakly bound to the Sun, they are easily disturbed when the Oort Clouds of passing stars pass through or close to the Sunís Oort Cloud, hurling them into the inner solar system or out into interstellar space. Shock waves from the greatest explosions in the universe, supernovae, also alter the positions of these comets. And they are pulled on with an even greater force by stars in the Milky Wayís galactic disk and core. Comets beyond about 4.5 trillion miles from the Sun are thus easily lost to interstellar space.

The Oort Cloudís inhabitants are most strongly perturbed, however, by the tidal forces of giant molecular clouds. Far more massive than the Sun, these vast, interstellar accumulations of cold hydrogen serve as stellar nurseries and the birthplace of stars (pages 216-217). These are thought to be encountered every three hundred to five hundred million years, at which time the comets within the Oort cloud are violently redistributed.

The Oort Cloud may account for a significant fraction of the solar systemís mass, perhaps as much or even more than Jupiter.

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