The Comets of our solar system

Comets are dirty balls of ice and rock that orbit the Sun in highly elliptical orbits, taking them relatively close and then very far away from the Sun.  They are divided into two categories depending on their furthest distance from the Sun: "short-period" comets found in a disk-shaped volume of space known as the Kuiper Belt extending from about 3 to 9 billion miles out; and "long-period" comets residing in a spherical volume of space known as the Oort Cloud from about 3 to 18 trillion miles out. When comets approach close enough to the Sun their icy component becomes volatile and gaseous, contributing to their characteristic tail.  The Sun's solar wind causes comet's tails to always point away from the Sun, so that as comets move away from the Sun their tails precede them in their orbit.

The Kuiper Belt is the frozen remains of a disk of primordial material that condensed into our present solar system five billion years ago. As many as 10 billion icy bodies 10 miles or more across orbit the Sun in this region stretching from just beyond Neptune’s orbit to about 9 billion miles out. Many also believe it to be the source of "short-period" comets -- comets like Halley’s that take less than 200 years to orbit the Sun. The Kuiper Belt is estimated to contain more than 35,000 objects greater than 60 miles in diameter, several hundred times the number and mass of similar sized objects in the asteroid belt. And there may be 100 million comets 12 or so miles across in the Kuiper belt still detectable by Hubble.

In June 2002 a KBO (Kuiper Belt Object) some 800 miles across was discovered, the biggest solar system find since Pluto in 1930. Originally labeled 2002 LM60 and later called Quaoar, it orbits the Sun once every 288 years from four billion miles out - one billion miles further out than Pluto. Larger than all the asteroids combined, astronomers still do not consider Quaoar to be a planet, but rather the largest KBO discovered so far.

As larger and larger KBOs turn up, several of which are binary systems, the case for Pluto as a planet weakens (if a "planet" is a large body formed with the original solar system orbiting the Sun in a stable and unique orbit). Pluto, lying within the Kuiper Belt, may just be the largest KBO discovered so far.

In 1950 the Dutch Astronomer Jan Oort proposed that (long-period) comets reside in an immense spherical cloud surrounding the planets and extending out to about three light years -- 18 trillion miles – from the Sun. This vast distance is thought to be 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. These are thought to be encountered every 300 to 500 million years, at which time the comets within the Oort cloud are violently redistributed.