One in 200 stars has habitable Earth-like planets surrounding it - in the galaxy, half a billion stars have Earth-like planets going around them - that's huge, half a billion. So when we look at the night sky, it makes sense that someone is looking back at us.

I'm not a great one for looking back.

As chief scientist, it's sort of my job to look at bridges between what we do and to see the connections. But when we try to understand how are planets around other stars habitable... to looking back at the Earth - how are the changes that are taking place, how are they going to affect humanity?

You can't look back; you have to keep looking forward.

Everybody wants to look in the mirror and see Cary Grant looking back at them, but that's just not the case.

Our astronauts, when they go orbiting around the earth, they actually come back slightly younger than a twin that they would have on the planet Earth who was stationary. This is called the twin paradox.