One dark night 20 years ago, a small boy we knew was taken outside to view Halley's Comet as it flashed fuzzily by on its once-in-a-lifetime visit to the inner solar system. He gazed skyward through his grandfather's binoculars for a long time, then lowered them solemnly and pronounced in tones of awe: "I didn't see any Halley's, but I saw Pluto."

Ah, Pluto. Whether he saw it or not -- and it is safe to say not -- the then-ninth planet from the sun was the one celestial body that gripped his 4-year-old imagination. And why shouldn't it? Pluto was far out. Literally. So far out, in fact, that it put a whole new spin on the newly chic word edgy. Small, cold, dark and lonely, graced with a funny name, it radiated an appeal that none of its more robust fellow planets did. It was the shy little kid of the planet family.

But it also had the status of being the solar system's official unmanned checkpoint. Inside Pluto's orbit, you were mentally at home in the universe. Here, you knew what was what and pretty much where. Pass beyond it, and, well, who knew what black hole or red giant might grab you? Pluto marked an existential boundary.