Figure 1.3 The future of the Earth is a topic in astrobiology. When the Sun turns into a Red Giant star in several billion years from now, the Earth will be a dead planet. However, the increasing luminosity of the Sun over the next one to two billion years will ensure that life on Earth will already have long since been extinguished and the oceans boiled away when the planet enters a greenhouse state. Therefore, to understand the future of life on our planet, we need to know about the evolution of stars. Biology and astronomy are inextricably linked in astrobiology.
Source: Reproduced with permission of Fsgregs, https://upload.wikimedia.org/wikipedia/commons/6/60/Red_Giant_Earth.jpg.
In summary, investigating the past and future of life on Earth means that we need to look beyond Earth to get answers. Astrobiology is about taking that journey and linking the diverse scientific fields needed to understand life on our own planet and, potentially, life beyond.
Let me address an interesting question that I often get asked: What's the use of astrobiology and how is it different from biology? Can it really claim to be a field in its own right? I hope that you've already got some understanding of why it is a useful area of science. Without a long digression into the philosophy of science, it is worth remembering that scientific fields are human constructions. It is true to say that the existing field of “biology” covers investigations into all the living things that we know, which might make you wonder why we need astrobiology at all. However, astrobiology is merely a way to think about living things in their astronomical or cosmic context, to address questions such as the origin of life or the existence of life elsewhere that requires linking biology with other fields such as planetary sciences or astronomy. Does that make astrobiology a subfield of biology or something wider? You decide! I don't think it really matters. What is important is that it is a useful term to encompass a set of scientific questions. It brings together a wide range of scientists, in textbooks like this or at scientific conferences, and of course in scientific papers. If a scientific word achieves nothing more than the advance of collaboration between thinking human beings, then it is an outstanding thing.
1.2 The Major Questions of Astrobiology and the Content of the Textbook
Astrobiology is a very broad field, and it can be difficult to grasp all the topics and fascinating questions that it addresses. This textbook is focused on providing a comprehensive overview of some of the major strands of science that underpin the study of life in its cosmic context, while hopefully achieving an appropriate depth of understanding in key subjects such as physics, biology, chemistry, and geosciences. The textbook is based on six years of teaching an astrobiology course at undergraduate level at the University of Edinburgh. During that time, I have been able to observe what undergraduates find interesting and the things for which they have less enthusiasm. The content of the chapters is based around these experiences. In particular, concepts that are important and sometimes more tricky to grasp have more pages devoted on them. The textbook should by no means be regarded as exhaustive, and there are many other texts that can provide ancillary information and deeper discussion on particular subjects.
There is a point I would like to make right away. If you bought this textbook to read all about aliens, you are going to be disappointed. The matter of whether life exists elsewhere in the Universe is certainly one of astrobiology's major questions. “Does life exist beyond Earth?” sometimes expressed as “Are we alone in the Universe?” or similar formulations of the question of whether we are the only type of life in the Universe grips the public as well as the scientific imagination. It justifiably finds itself center stage whenever the word “astrobiology” comes into view. In fact, one of the challenges of being an astrobiologist is to explain to people that the subject is not just about searching for alien life! Perhaps one reason why the question of whether alien life exists is so pervasive is that it is a statistically reasonable question to ask. Earth is one planet in a galaxy which might have about 200 billion stars (the exact number is not known) in a Universe with possibly something like 150 billion galaxies (the exact number is also not known). These numbers tell us that there are about 1022 stars in the Universe (give or take orders of magnitude), which we could explore for the presence of habitable worlds. There is no shortage of planetary bodies for the attention of astrobiologists. It's quite reasonable that anyone who looks into the night sky should wonder whether we are alone. As soon as you start to talk about life in its cosmic context, the question of alien life therefore comes into full view.
However, astrobiology is concerned with many other fascinating questions that relate to life on our own planet and its future. In Table 1.1, you can see a summary of the textbook, its chapters, their content, and the fields they cover. This will give you an instant glimpse of what those topics are.
Table 1.1 The content of the textbook.
| Chapter number | Subjects covered | Chapter title | Summary of content |
| 1 | Introduction | Astrobiology | Summary of the content of the textbook, the field of astrobiology and its history |
| 2 | B, H | What Is Life? | Discussion on what life is and the history of attempts to define life |
| 3 | B, C | Matter and Life | The basic structure of matter, major bonding types, and relevance to life |
| 4 | B, C | The Molecular Structure of Life | How molecules are assembled into the major molecules of life |
| 5 | B | The Cellular Structure of Life | The cellular structure of life and the major components and characteristics of cells |
| 6 | B | Energy for Life | How life gathers energy from the environment and the major types of metabolism |
| 7 | B | The Limits of Life | Life in extremes and its astrobiological significance |
| 8 | B, G | The Tree of Life | How phylogenetic trees are used to order biological information and test scientific hypotheses |
| 9 | A, B | The Universe, the Solar System, and the Elements of Life | The origin of the Universe, stellar evolution, the formation of planets, and the origin of the elements important for life |
| 10 | A, C | Astrochemistry: Carbon in Space | Introduction to chemistry in space and how complex carbon compounds can be formed in space |
| 11 | B, G | The Early Earth: The First Billion Years | The environment of the early Earth and the important characteristics for life |
| 12 |
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