These questions have been with us since our earliest forays into thinking about the sky, the stars and the wandering planets. The atomists of ancient Greece were convinced that the Universe was infinite and must therefore have an infinity of infinite worlds. In 1277 Bishop Tempier of Paris claimed that God could have created other worlds. Many scholars were unconvinced and argued that even if He could, He would choose not too. In 1600 Giordano Bruno was burnt at the stake for various heresies and it is likely that his advocacy of a plurality of inhabited worlds did not win...