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Its limitations are those of the physical universe: it won't let you play with some really wild ideas that aren't possible, but are fun to speculate about.

It bears mentioning that the Milky Way is only one of 150 billion galaxies visible to our telescopes - and each of these will have its own complement of planets.

We are probably nearing the limit of all we can know about astronomy.

Of all celestial bodies within reach or view, as far as we can see, out to the edge, the most wonderful and marvellous and mysterious is turning out to be our own planet earth. There is nothing to match it anywhere, not yet anyway.

Everybody has busy lives, but you can tell people, 'Go outside and look at the night sky. We've been able to demonstrate that every star you see probably has a planet around it.'

There are too many accidents that can befall life on a single planet.

Planets are too dim to be detected with existing equipment, far away, except in these very special circumstances where they're seen by their gravitational effect.

I think it'd be pretty unrealistic to think we're the only planet in the world with thinking beings. It's kind of a strange conceit. Especially given how many universes there must be.