Letter from the Editor

For many years, the study of planetary bodies in the Solar System remained as a rather minor branch of astronomy, largely confined to the tasks of determining the orbits and sizes of various objects and mapping and documenting features visible on their surfaces. With the advent of more sophisticated techniques—such as ultraviolet, infrared and radio astronomy and spectroscopy—it became possible to measure more subtle properties of planetary bodies and their space environments; facilitating our capacity to characterize other planets in much more detail (their composition and chemistry, atmospheric structure and meteorology, magnetic fields and charged particle environment). The ability to carry out sophisticated analyses of meteorites, recovered from various places on Earth, has also provided important insights into the composition and early history of primitive material from which our Solar System was constructed.

But it has only been with the advent of space exploration in the 20th century that the interdisciplinary subject of Planetary Science has really come of age to stand in its own right as a major discipline, distinct from astronomy. By being able to visit the planets and minor bodies of our Solar System directly with spacecraft in close fly-bys, orbiters and even landers, a range of sub-disciplines has emerged that directly parallel many of the disciplines commonly associated with the Earth sciences; interior geophysics, geomagnetism, geochemistry, geology and geomorphology, atmospheric science, space weather and aeronomy to name just a few. After more than 40 years of planetary exploration from space, it is now possible to make detailed quantitative comparisons between different planets and with the Earth, opening up new possibilities, from discovering previously unknown facets of these objects to addressing such basic questions as: “where did Earth come from?”; “why did Earth evolve in the way it did?”; “why was this different from the way Mars or Venus evolved?” and so on… and subjecting theories to quantitative evaluation.

With the discovery of the first extra-solar planets just 20 years ago, we are also beginning to see the emergence of a parallel sub-discipline of exoplanetary science. Until recently, this has mainly focused on the discovery and cataloguing of new planetary bodies around other stars and measuring their sizes and orbits. Such discoveries have already told us that extra-solar planetary systems can be very different from our own Solar System, although detailed comparisons between individual exoplanets and our near neighbours have been hampered by lack of suitable measurements. Nevertheless, recent advances in observational techniques are already beginning to clarify the physical characterization of these planetary bodies. Before long, it will be possible to make meaningful comparisons between Solar System planets and those elsewhere in our Galaxy. In this sense, we are moving rapidly from what has been largely the realm of science fantasy towards science fact!

Why should we be concerned with investigating these remote worlds in such astonishing detail? Understanding the origins of life on Earth and the search for life elsewhere in the Universe is one commonly cited and compelling motivation. It is also a major driver behind numerous missions of space exploration mounted by national and international space agencies across the world. Many of the lessons of Planetary Science have shown us that much more can be learned about aspects of the Earth itself from the systematic study of the physical attributes of other planets; not just about the question of life. In reality, the reasons for turning our eyes toward other planets are many and complex, ranging from pure curiosity to the desire to develop new aerospace and robotic technologies or to assert national prestige.

This research encyclopaedia aims to tackle all of these issues across the spectrum of the Earth and planetary sciences. It starts with the re-orientation of disciplines traditionally associated with the study of Earth toward other planetary bodies, both within the Solar System and beyond: (a) to satisfy our natural curiosity and advance fundamental scientific understanding but also (b) to make insightful comparisons with the Earth itself and thereby learn more about our own planet’s origin, evolution, and place in the Universe. The technologies that enable planetary exploration are also distinctive and necessary for understanding how advances in the science are made. But the history and sociology of planetary science is fascinating in its own right and strongly tied up with many other aspects of human society, playing a role in literature, politics and the social sciences. We aim also, therefore, to look beyond the science itself toward evaluations and commentaries on how planetary exploration impinges upon some of the most profound questions concerning the origin and ultimate fate of life on Earth and humankind in particular.

Peter Read
Editor in Chief