Stephen Hawking is just the latest immensely respected scientist to posit life on other worlds: Back in 1912, Edmond Perrier, director of Paris' Museum of the Jardin des Plantes, did the same thing in the New York Times Magazine.
Here's an introduction to Perrier and his work that one could easily find applicable to a current scientist, with one notable exception — the license to fabricate, to speculate, to dream on the front page of a paper of record. From the Times piece, reproduced in full below:
"The great question, 'What is going on in other planets?' is answered by a man who at any rate knows as much about it as anybody else, however little or much that may be. He reasons everything and shows us what we might expect to see on the journey from Mercury to Neptune with a special chapter on the Martians. He is a naturalist 'more accustomed to turn his eyes toward the earth that toward the heavens' and he knows how scientists scorn speculation, but after all, why should he not allow himself his little dream? Dreams are not a crime, and in this case, contradiction is difficult."
(Via Ironic Sans)