There was a time when Pluto - which NASA's New Horizons spacecraft at last explored in 2015, a mission I led - was considered the last planet. We now know there are thousands of other - possibly inhabited - planets.

Human beings have long wondered whether they are alone in the universe.

People know a planet when they see one, and I think that's a pretty darn good test, in fact, for planethood.

I just think it's patently absurd for scientists to categorize objects on the basis of the numbers of objects that they can remember.

I think when people see Pluto revealed by New Horizons, its satellite system, its complex surface, its atmosphere, I think they'll have a hard time saying 'That's not a planet' because it obviously will be, and I think most people are already coming to that opinion anyway, but I think that's really going to drive it home viscerally.

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

A miniature poodle is not not a dog just because it's miniature.

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

How can an adjective in front of a noun not describe the noun? There are dwarf stars, but they're still considered stars.