The Oort Cloud

Beyond Pluto, we pass through the rest of the Kuiper Belt: billions of icy bodies left over from the early solar system which occasionally fall sunward to become short-period comets. At about 9 billion miles out, the Sun is merely the brightest star in the sky. Here we enter a realm that many are coming to believe holds the answer to the ancient question, "Where do comets come from?"

If you've already read the page on the Kuiper Belt, you might think that I am being very forgetful, that that question was just answered, and that the answer is, "The Kuiper Belt, you dummy. You just said so." But the answer also includes comets that we can’t track so easily, the so-called "long period" comets that take over 200 years to circle the Sun, and that must therefore come from beyond the Kuiper Belt. The true origin of these objects hasn’t been well known or hypothesized -- until recently.

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

1. No comet has been observed with an orbit indicating that it came from "interstellar" space (that is, the space between the stars: 1000 times further from the Sun than Pluto);

2. There is no preferred direction from which comets come;

3. There is a strong tendency for the furthest points from the Sun (aphelia) of long-period comets to group at about 4.6 trillion miles (50,000 AU or 8/10 of a light-year) from the Sun.

From these observations Oort proposed that (long-period) comets reside in an immense spherical cloud surrounding the planets and extending out about three light years from the Sun -- the boundary of the Sun’s influence. Known as the "Oort Cloud," statistics imply that it contains as many as six trillion comets. Since the individual comets are so small and at such great distances, however, there is no direct evidence of this hypothetical cometary cloud -- but the indirect evidence grows yearly.

Within the 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 Sun’s 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 Way’s galactic disk and core. Comets beyond about 4.5 trillion miles from the Sun are thus easily lost to interstellar space.

The Oort Cloud’s 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. Encountered every 300 to 500 million years, they violently redistribute the comets within the Oort cloud.