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The surest sign that intelligent life exists elsewhere in the universe is that it has never tried to contact us.

Perhaps, as some wit remarked, the best proof that there is Intelligent Life in Outer Space is the fact it hasn't come here. Well, it can't hide forever - one day we will overhear it.

Perhaps we've never been visited by aliens because they have looked upon Earth and decided there's no sign of intelligent life.

If we ever established contact with intelligent life on another world, there would be barriers to communication. First, they would be many light years away, so signals would take many years to reach them: there would be no scope for quick repartee. There might be an IQ gap.

I believe that there may be intelligent life on other planets.

If we find life out there, and it's not us, we will deem it not intelligent. But what may be equally as likely is that we find life that's vastly more intelligent than we are. If that's the case, we are putty in their hands.

We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet.

I believe alien life is quite common in the universe, although intelligent life is less so. Some say it has yet to appear on planet Earth.

Should we find a second form of life right here on our doorstep, we could be confident that life is a truly cosmic phenomenon. If so, there may well be sentient beings somewhere in the galaxy wondering, as do we, if they are not alone in the universe.

If this is the only planet on which not only life, but intelligent life, has arisen, that would be very unusual.