Three of the solar system’s “big guys” are in great view this evening. The brilliant planets Venus and Jupiter are low in the west at nightfall, with Jupiter to the right of brighter Venus. And the planet Saturn is about a third of the way up the southern sky. Jupiter and Saturn are the largest planets in the solar system. Combined, they account for about 90 percent of all the mass in the solar system other than the Sun. Yet they’re far outnumbered by the little guys — smaller chunks of rock and ice left over from the solar system’s birth. Billions of these objects reside in the Kuiper Belt — a wide band beyond the orbit of Neptune, the most-distant major planet. They were hurled into this cold, dark realm by the gravity of the giant planets as the solar system was taking form. Most members of the Kuiper Belt are no more than a few miles across. But a few are hundreds of miles across or bigger. Some, in fact, are big enough to have thin atmospheres and their own moons. Only one of thes
