So the solar system is really quite enormous. By the time we reach Pluto, we have come so far that the Sun—our dear, warm, skin-tanning, life-giving Sun—has shrunk to the size of a pinhead. It is little more than a bright star.

In such a lonely void you can begin to understand how even the most significant objects—Pluto's moon, for example—have escaped attention. In this respect, Pluto has hardly been alone. Until the Voyager expeditions, Neptune was thought to have two moons; Voyager found six more. When I was a boy, the solar system was thought to contain thirty moons. The total now is "at least ninety,"about a third of which have been found in just the last ten years.

The point to remember, of course, is that when considering the universe at large we don't actually know what is in our own solar system.

Now the other thing you will notice as we speed past Pluto is that we are speeding past Pluto. If you check your itinerary, you will see that this is a trip to the edge of our solar system, and I'm afraid we're not there yet. Pluto may be the last object marked on schoolroom charts, but the system doesn't end there. In fact, it isn't even close to ending there.

We won't get to the solar system's edge until we have passed through the Oort cloud, a vast celestial realm of drifting comets, and we won't reach the Oort cloud for another—I'm so sorry about this—ten thousand years. Far from marking the outer edge of the solar system, as those schoolroom maps so cavalierly imply, Pluto is barely one-fifty-thousandth of the way.