Not even Tombaugh and his mentors could have forecast how fascinating their new planet would turn out to be.

Why would you listen to an astronomer about a planet?

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

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.

No one predicted Mercury would be a planetary core with the mantle stripped off. No one predicted volcanoes on the Jovian moons, or oceans on the inside of them. I can tell you, for every single planet, huge 'we never guessed that' things.

Pluto is still active four and a half billion years into its history. It was expected that small planets like Pluto would cool off long ago and not still be showing geological activity. Pluto is, in fact, showing numerous examples of geological activity on a massive scale across the planet.