Cosmogony , unlike its bigger brother Cosmology , concerns the origin of planets, and not the Universe as a whole. As such, it is a much more relevant subject to the student of physics and astronomy. Living as we do on a fairly typical terrestrial planet, we, as inquiring scientists, must often wonder where our home came from. It is a short step from this general interest to questions concerning the similarities and differences between Earth and nearby Moon, Venus, Mercury and Mars, and the differences between our Sun's solar system and the planets that have recently been found around other stars. Few topics in astronomy lend themselves better to the detailed analysis and interpretation of physical observations, in the hope of answering the two big questions: 'how common are Earth-like planets, and do they have university physics departments?' and 'are other planetary systems similar to ours and, if not, why not?'