I watched Hawking's Discovery Channel show on aliens, and I have to compliment him on being the first person I've heard seriously suggest that whatever aliens there may be may be a kind of life that is completely beyond our concept of life.
Even the image of a little green man is based upon our own earthly evolution.
Like Hawking said, since there are so many gas planets, why can't there be lifeforms made out of gas? Is life really dependent on oxygen or water? What is life anyway?
All good questions. Some pretty deep philosophical questions.
Hawking is a physicist , he is not a chemist. Chemistry tells us that there cannot be any life forming out of gas. The prerequisite to form life is the ability to form long chain molecules, like carbon atom is capable of. Proteins, carbohydrates, fats etc, are long chain molecules, which contain chains of carbon atoms, anywhere from 10 or 20 carbons to several thousand carbons.
No other element is capable of forming such long chains, with the exception of silicon. There may possibly be silicon based life, though silicon does not form long chain molecules to the same extent as carbon.
So there is no way that life form can be made out of gas. Carbon cannot form long chain molecules at high temperatures, in gaseous state. It will have to be really weird, totally incomprehensible life form, something we cannot even imagine. The world famous astronomer, Fred Hoyle postulated just such a life form in his classic story ‘The Black Cloud’.