In a sonnet of 1816 refl ecting upon his experience of reading a new translation of the works of Homer, the young English poet John Keats wrote of ‘breathing the pure serene’ in ‘realms of gold’, and continued:

　　Then felt I like some watcher of the skies

　　When a new planet swims into his ken;

　　Or like stout Cortez when with eagle eyes

　　He star’d at the Pacifi c-and all his men

　　Look’d at each other with a wild surmiseSilent, upon a peak in Darien.

　　Keats’s new planet metaphor was either inspired by Sir William Herschel’s sighting of Uranus in 1781, or by discoveries of the fi rst four asteroids (1801–7). Being more recent, the latter would have been fresher in people’s memories. A layman such as Keats would have thought of them as new ‘planets’, although today they are regarded as too small to qualify.

　　I still travel in ‘realms of gold’ when I see Saturn with my own eyes through even a small telescope, though the novelty has somewhat faded when it comes to seeing a newly discovered remote ice-ball as a single pixel in a digital image, or a hint of a Jupiter-sized companion to another star expressed as a minute wobble in the star’s position.

　　However, for me, the true ‘Cortez experience’ recurs whenever I see a new planetary landscape (in some cases, a cloudscape)unfold before me on images sent back from a visiting spacecraft.Exploration of our Solar System has reached a stage that allows us to appreciate other planets and their large satellites as worlds,endowed with geographies, geologies, and meteorologies as complex and fascinating as those of our own planet, Earth. Many of them are places that you and I could, in principle, visit. They are not generally suitable for a picnic, but we could at least jump up and down, scoop up handfuls of dirt, climb a hill, or slither down into a valley. Some are even places where life might be found.

　　In this book, I will share with you what is known of the origin,evolution, and especially the present-day condition of the planets in our Solar System. Here, astronomers now offi cially recognize only eight planets (Pluto has been demoted, as I will describe), but there are plenty of other bodies big enough to behave like planets so far as geologists like myself are concerned. These are fascinating, so I will not ignore them, though they are too numerous to treat individually.

　　Finally, I will turn to ‘exoplanets’, which are planets orbiting other stars. The fi rst was discovered as recently as 1995, and by now several hundred have been documented. We cannot see them in any detail, but we do have enough information to make some comparisons between the layouts of those exoplanetary systems and the Sun’s family.