The Oort Cloud; September 1998; Scientific American Magazine; by Weissman; 6 Page(s)

It is common to think of the solar system as ending at the orbit of the most distant known planet, Pluto. But the sun¿s gravitational influence extends more than 3,000 times farther, halfway to the nearest stars. And that space is not empty--it is filled with a giant reservoir of comets, leftover material from the formation of the solar system. That reservoir is called the Oort cloud.

The Oort cloud is the Siberia of the solar system, a vast, cold frontier filled with exiles of the sun¿s inner empire and only barely under the sway of the central authority. Typical noontime temperatures are a frigid four degrees Celsius above absolute zero, and neighboring comets are typically tens of millions of kilometers apart. The sun, while still the brightest star in the sky, is only about as bright as Venus in the evening sky on Earth.