A few astronomers continue to think there may be a Planet X out there-a real whopper, perhaps as much as ten times the size of Jupiter, but so far out as to be invisible to us. (It would receive so little sunlight that it would have almost none to reflect.) The idea is that it wouldn’t be a conventional planet like Jupiter or Saturn-it’s much too far away for that; we’re talking perhaps 4.5 trillion miles-but more like a sun that never quite made it. Most star systems in the cosmos are binary (double-starred), which makes our solitary sun a slight oddity.

As for Pluto itself, nobody is quite sure how big it is, or what it is made of, what kind of atmosphere it has, or even what it really is. A lot of astronomers believe it isn’t a planet at all, but merely the largest object so far found in a zone of galactic debris known as the Kuiper belt. The Kuiper belt was actually theorized by an astronomer named F. C. Leonard in 1930, but the name honors Gerard Kuiper, a Dutch native working in America, who expanded the idea. The Kuiper belt is the source of what are known as short-period comets-those that come past pretty regularly-of which the most famous is Halley’s comet. The more reclusive long-period comets (among them the recent visitors Hale-Bopp and Hyakutake) come from the much more distant Oort cloud, about which more presently.

It is certainly true that Pluto doesn’t act much like the other planets. Not only is it runty and obscure, but it is so variable in its motions that no one can tell you exactly where Pluto will be a century hence. Whereas the other planets orbit on more or less the same plane, Pluto’s orbital path is tipped (as it were) out of alignment at an angle of seventeen degrees, like the brim of a hat tilted rakishly on someone’s head.