Seth Shostak is the senior astronomer at Search for Extra Terrestrial Intelligence (SETI) Institute in California. Besides a PhD in astrophysics, and experience at working in various astronomical observation institutions in the Netherlands and US, Seth is a great science populariser having written over 400 articles, made films and TV shows, and written scripts. He has also co-authored a text book on astrobiology.

What is the possibility of life existing anywhere in the Universe, besides the Earth?

Obviously, we don't know, as we haven't found any yet. But what we do know is that the majority of stars have planets, which means that there are approximately a trillion planets orbiting stars in our galaxy. If even only one in a million of these planets has life, that's still a million inhabited worlds in the Milky Way.

And, what about the solar system?

There are at least a halfdozen other worlds in our solar system that might have liquids either on or beneath their surface, and therefore could have spawned some biology. These include three moons of Jupiter (Europa, Ganymede and Callisto), two moons of Saturn (Titan and Enceladus ), and of course Mars. Finding life on any of these worlds, assuming it exists, is something we could do with robotic landers -but such missions are expensive.

How do scientists search for life - what exactly are they looking for?

This is not an easy thing to do, as microbial life - particularly if it's no longer extant - can be hard to find. But the best tests are chemical - looking for the compounds that only life makes, such as nucleic acids (think DNA), proteins, or lipids. Lipids are fatty compounds that microbes use to make cell walls. If alien life is built anything like life on Earth, it might use lipids as well.

Isn't it possible that life may exist in a completely different form based on a totally different chemistry?

We tend to look for analogs to terrestrial life because - at least in that case - we know what to look for. Of course it's possible that there are entirely different chemical schemes for life (although not many have been thought of), but we can't easily search for phenomena that we don't understand.

Suppose we find some form of life on, say, Enceladus. What will it mean in terms of our understanding of the solar system, the Universe, life and its evolution on Earth, the future of space exploration, etc. ?

The implications of finding life on Enceladus would be different from finding it on Mars. The latter might be life that was related to our own, earthly biology, thanks to the exchange of rocks early in the history of the solar system. Any life found on Enceladus would surely be part of an independent evolution of life. That would tell us that life is not a miracle, but a common feature of the cosmos.

Is there any danger of hostile contamination from extra terrestrial life after such missions as the Curiosity Lander?

There's always some danger, and NASA does have an office of planetary protection to deal with this. However, most biologists agree that the danger of a hostile life form from Mars, for instance, is very remote. It would not have spent four billion years adapting to life on Earth and the organisms that exist here.

Some people say that Nasa is facing a huge budget crunch and so it is overhyping all this business of finding life within the solar system. Is that view correct?

I don't agree with this. The public is interested in life more than just about any other solar system question. The emphasis on biology elsewhere is largely the consequence of finding lakes on Titan, geysers on Enceladus, and evidence for past water on Mars.

Finally, what is your personal prediction - when will we find life elsewhere, and most likely where?

Within twenty years, I think we'll find life on some other body of our solar system - either Mars, or a moon of Jupiter.