We call it seeing in the dark.

As a planetary scientist, I don't know what else to call Pluto: It's big and round and thousands of miles wide.

A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to.

If you put Earth out beyond Neptune, you wouldn't be able to call it a planet because it couldn't clear its zone.

The Kuiper belt region, which I call the third zone because it lies beyond the rocky terrestrial planets and beyond the giant planets, is a bizarre frontier.

I tend to think of Pluto and its moons as presents sitting under a Christmas tree. They're wrapped, and from Earth all we can do is look at the boxes to see whether they're light or heavy, to see if something maybe jiggles a bit inside. We're seeing intriguing things, but we really don't know what's in there.