Oort cloud

The Oort cloud is the hypothetical region, said to be 50,000 to 100,000 AU in radius, that contains many billions of comets in orbit around the Sun. It is the supposed source of all observed long-period comets in the solar system. In addition to comets, the cloud is believed to be a reservoir of gas and dust.

Background
Comets are divided into two groups, the long-period and the short-. A comet having a period longer than 200 years is considered long-period. The two groups have the following characteristics, with occasional exceptions:


 * 1) Short-period comets orbit prograde around the Sun and with little or no inclination with respect to the ecliptic.
 * 2) Long-period comets are as likely to be in retrograde as in prograde orbits, and have been observed to have any inclination with respect to the ecliptic.

Weisstein observes that the aphelia of long-period comets are distributed about two modes: 10,000 AU and 50,000 AU.

Faulkner observes that Comet Halley, a short-period comet, is highly inclined and in a retrograde orbit. Comet Halley is, therefore, an important exception to the distinction. Faulkner also observes that short-period comets do not differ from long-period comets in composition.

Cometary orbits are highly elliptical, and some of these orbits are almost parabolic. Hyperbolic cometary orbits have never been observed. This last observation is credited to Jan Oort himself.

The more important point is that comets have a limited life span. After a few hundred perihelion passes at the Sun, a comet loses all of its tail-forming substance. In addition, comets often destroy themselves in collision with planets and other bodies, or are ejected from the solar system entirely after making close flybys of Jupiter and other gas giants.

For these reasons, astronomers realized that the supply of comets would require replenishment over the supposed great age of the solar system. Comets cannot be interstellar, else they would have hyperbolic orbits. Some astronomers have proposed cryovolcanism as the source of comets, but comets are not sufficiently dissimilar in composition to support this.

In 1950, astronomer Jan Oort first proposed the existence of a "cloud of comets" in the outer fringes of the solar system. Oort calculated that this cloud might contain a trillion comets. Short-period comets are supposed to come from the Kuiper belt. Oort originally speculated that the total mass of the Oort cloud would be as much as the mass of Jupiter. More recent studies have suggested that the total mass might be forty times the mass of the earth.

Problems with the Oort Cloud Hypothesis

 * 1) The Oort cloud has never been directly observed. Most astronomers believe that individual comets would be unobservable so far away.
 * 2) Collisions between comets, over the course of billions of years, would have destroyed most of them by now, leaving only a combined mass for the entire Oort cloud of 1 to 3.5 times the total mass of the earth. Such a low mass is far less likely to have supplied the inner solar system with comets over billions of years. For that reason, astronomers are now speculating that the Oort cloud must have its own even more-distant supply.
 * 3) Comets have twenty times the concentration of deuterium (heavy hydrogen) found in the inner solar system. If, according to current models, the material in the Oort cloud came from inside the solar system, this would not be the case.
 * 4) The suggested formation mechanism for comets is the accretion of water vapor from the solar nebula, according to the nebula hypothesis. Such accretion would likely be stopped by simple Brownian motion.

An alternative hypothesis
Young earth creationists generally propose that the solar system is much younger than billions of years. In the six thousand years usually proposed, attrition of comets would not have destroyed many of them. Therefore, no source of replacement comets would be necessary.