Comet Hale-Bopp, photographed over Boulder, Colorado, during its appearance in 1997.

After Pluto's discovery, many people continued searching the skies in hopes of discovering a tenth planet, sometimes called "Planet X." Given the sensitivity with which our telescopes now scan the skies, it is unlikely that another large planet orbits our Sun (though other Pluto-size objects are possible). Thus, Pluto's orbit marks the end of the realm of the planets. However, the most numerous objects in the solar system still lie ahead: comets.

You are probably familiar with the occasional appearance of a comet in the inner solar system, where the heat of the Sun evaporates some of its ice and it grows a long, beautiful tail. But with very few exceptions, nearly all comets spend nearly their entire lifetime in the extreme outer reaches of the solar system. Our present technology does not allow us to detect comets when they are very far from the Sun. However, to account for the frequency with which we see comets in the inner solar system, astronomers have calculated that there must be something like a trillion (10¹²) comets inhabiting the outskirts of our solar system.

As we will discuss in Chapter 12, the realm of the comets probably consists of two vast regions of space. The region containing the Pluto-like "giant comets"--called the Kuiper belt--begins near the orbit of Neptune and probably continues out to several times this distance. The comets in this region have orbits that lie close to the plane of planetary orbits and go around the Sun in the same direction as the planets. The second and much larger region--called the Oort cloud--may extend more than one-fourth of the way to the nearest stars. Comets in this region have orbits that are inclined at all angles to the plane of planetary orbits. Thus, the Oort cloud would look roughly spherical in shape if we could see it. But it is so vast that, even if it has a trillion comets, each comet would typically be separated from the next by more than a billion kilometers.

Comets and asteroids are discussed together in the Voyage scale model solar system on the National Mall in Washington, DC. Most comets actually reside well beyond the orbit of Pluto and are too small to see on this scale.