have included some further reading. This was, perhaps, the most difficult task. It is impossible to do justice to all the literature that exists in every field that comprises astrobiology, let alone list all the main contributions. Instead, I've suggested two or three popular and/or technical books that relate to each chapter that might provide some enjoyable additional reading. I have also listed a set of papers. They are papers I think will give a representative sprinkling of just some of the possible avenues an interested reader might pursue and that relate to the main themes and subjects covered in each chapter. Again, no significance should be placed on the omission of many important papers or the inclusion of the ones listed.
At the end of each chapter, I have provided some questions for review and reflection. The answers to these questions are within the chapters themselves and do not require new information. Their purpose is to offer a means for some structured review of the chapter content and to help consolidate your knowledge. Even if you choose not to produce written answers, you might like to read them and ponder the answers as a way to ensure you have grasped the main ideas in each chapter.
There is yet another dimension of astrobiology that I have touched on in this book. I often get asked by students, “What degree do I need for astrobiology? What field should I take an interest in to become an astrobiologist?” Any degree allows a person to explore aspects of astrobiology from different angles. Science should never be closed to inquisitive minds on account of narrow human discipline definitions. Throughout the book, I've included sections that contain some personal information about a selection of astrobiologists, particularly colleagues early in their careers. I've shown their original degree areas, what motivates them, and how they got into astrobiology. Their career interests are not necessarily directly aligned to the subject of the chapter in which they appear, but their stories are distributed throughout the book. There are many fine people in astrobiology, and I'd like to emphasize that there is no significance to my selection or omission of these astrobiologists. I chose a selection of outstanding colleagues whom I thought would exemplify the variety of disciplines from which astrobiologists are currently launching their careers.
1.4 A Brief History of Astrobiology
Having explored the main questions in astrobiology and summarized them, we might ask ourselves when all this scientific interest began. Sometimes a historical perspective on any science can be useful to understand how we got here and why certain questions have become prominent lines of enquiry. A thorough review of the history of space sciences and where astrobiology fits within it could (and indeed does) consume entire books. This textbook is more about the science than the history of astrobiology, but at this point it is worth making some observations about its historical origins to provide some introductory context.
The questions that astrobiology asks are, from a philosophical standpoint, ancient. This is particularly the case for the question of whether we are alone in the Universe. Although this is just one part of astrobiology, let us briefly explore this history. Greek philosopher, Metrodorus of Chios (fourth century BCE; Figure 1.8), a student of Democritus (c. 460–370 BCE) (Democritus proposed an early atomic theory of matter) stated: “It would be strange if a single ear of wheat grew in a large plain, or there were only one world in the infinite.” In other words, if you walk into a field, you rarely see one ear of wheat in a large field that is otherwise completely dead. Where there is one ear of wheat, there are usually lots. Metrodorus was essentially saying: Surely the existence of planet Earth implies many planet Earth's in the Universe? The Greeks had a very different view of the Universe than the one we have today, so we shouldn't draw too many conclusions about what was going through his mind. The Greeks had no real concept of the planets as rocky bodies or the vast distances to the stars. They thought that all the stars were held on the surface of a huge sphere. Metrodorus's statement was, nevertheless, a remarkable line of thought, because even today we still ask the question: Does life on Earth imply life elsewhere? We ask this with the benefit of modern technology and astronomical observations, but the basic conceptual question remains the same: Do the origin and evolution of life on our planet allow us to say anything about the universality of biology?
Figure 1.8 Metrodorus of Chios, ancient Greek philosopher. He wondered about the existence of other worlds like our own.
Source: Reproduced with permission of Keith Schengili-Roberts, https://commons.wikimedia.org/wiki/File:Metrodorus-PergamonMuseum.png.
Metrodorus's view of the world was very different from Aristotle's (384–322 BCE), who asserted the uniqueness of Earth in the cosmos. The idea that Earth was the center of the Universe was based on the observation that the stars never moved with respect to one another, which the ancients interpreted to be a result of the fixed position of Earth rather than the great distances to the stars. Aristotle's view would dominate for many centuries. Until the Enlightenment (during the seventeenth and eighteen centuries), the idea that Earth was the sole inhabited world in the cosmos held its grip on the public view, bolstered by religious doctrine.
In the sixteenth century, the geocentric view of the Universe, which firmly placed Earth as the center of the action, was overturned by Nicolaus Copernicus (1473–1543) (Figure 1.9). He was not the first person to consider that the Sun was at the center of the Solar System and Earth orbited around it. Greek philosopher Aristarchus proposed a heliocentric model, but Copernicus's treatise, De revolutionibus orbium coelestium (On the Revolutions of the Heavenly Spheres), caused a landmark shift away from the geocentric view. The Copernican outlook paved the way for a view of the cosmos that would allow for the idea that stars may be other suns.
Figure 1.9 Nicolaus Copernicus, renaissance mathematician and astronomer. He advanced a heliocentric model of the Solar System.
Source: Reproduced from the “Torun portrait,” anonymous, c. 1580.
In the sixteenth century, more enquiring and inquisitive minds appeared and, with them, new speculations about Earth's place in the larger order of things. One of the most astonishing speculations about worlds beyond Earth was made by the Italian Dominican friar, mathematician, and philosopher Giordano Bruno (1548–1600; Figure 1.10), who stated in his book On the Infinite Universe and Worlds: “In space there are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours. For no reasonable mind can assume that heavenly bodies that may be far more magnificent than ours would not bear upon them creatures similar or even superior to those upon our human earth.”
Figure 1.10 Giordano Bruno, Dominican friar, mathematician, and philosopher. He speculated about other Earth-like worlds (A bronze statue by Ettore Ferrari [1845–1929], Campo de' Fiori, Rome).
Source: Reproduced with permission of Jastrow, https://commons.wikimedia.org/wiki/File:Giordano_Bruno_Campo_dei_Fiori.jpg.
This was a prescient statement about the possibility of extrasolar planets, and a person couldn't do much better today in writing a clear summary of why Earth-like exoplanets are hard to find – because they are small and dark. Bruno was eventually burned at the stake for a variety of charges, most of which related to him holding beliefs contrary to the Catholic Church
