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Personally, I do not know whether humankind is alone in this vast universe. But I do know that we should cherish our existence on this precious speck of matter... the greatest gift that could be bestowed upon us. For all practical purposes, there is only one planet Earth.

Sometimes I think we're alone in the universe, and sometimes I think we're not. In either case the idea is quite staggering.

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.

I believe that we're not alone. How can we be alone in an infinite universe? I'm using the word 'alien' with a little trepidation because I know that sparks so many different versions of that word, and there are so many different images that come into one person's head when someone says it.

The universe is almost 14 billion years old, and, wow! Life had no problem starting here on Earth! I think it would be inexcusably egocentric of us to suggest that we're alone in the universe.

It's just too egotistical to think that we are the only lifeform in the universe.

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.

We're the first technology-creating species. We use technology to extend our reach. We didn't stay in the caves, and we haven't stayed on the planet. To play jazz with our genomes and the universe might ultimately be what we're all about.

There's life all over this universe, but the only life in the solar system is on earth, and in the whole universe we are the only men.

It might be arrogant to think that we're the only living creations in all of the solar systems that there are. Space is so vast.