This chapter discusses the requirements for a world to be deemed habitable at a given moment in time (instantaneous habitability), with an emphasis on the availability of energy sources and suitable physicochemical conditions. After a brief exposition of some concepts in thermodynamics, the significance of the molecule ATP (the ‘energy currency’ of the cell) and how it is synthesised in the cell by harnessing chemical gradients is described. The two major sources of energy used by life on Earth (chemical and light energy), and the various possible pathways for utilizing such forms of energy are sketched, most notably photosynthesis and methanogenesis. This is followed by delineating the diverse array of extremophiles that inhabit myriad niches on Earth that would be considered harsh for most life. The mechanisms that permit them to survive the likes of high/low temperatures, pressures, salinity, and radiation doses are reviewed.

Ever since the first exoplanets were discovered over 30 years ago, their detection has proceeded at a remarkable pace. This chapter describes the techniques for identifying these worlds, as well as characterising their atmospheres and surfaces to seek out possible signs of life. The most common methods for detecting exoplanets are reviewed: radial velocity measurements, transits, gravitational microlensing, astrometry, and direct imaging. This is followed by summarising avenues for characterising exoplanets through performing spectroscopy of three sources of radiation linked to them: (1) transmitted light passing through an exoplanetary atmosphere and reaching us; (2) thermal emission associated with the blackbody radiation of the planet; and (3) starlight reflected from that world. The chapter concludes by commenting on the bright future of exoplanetary science and future telescopes devoted to this area.

The physics and chemistry underpinning the origins of the Universe, stars, elements, and molecules is described in this chapter. It begins with outlining our understanding of the Big Bang, and how gravity subsequently facilitated the emergence of order and complexity in the Universe. This is followed by a brief exposition of star formation, stellar evolution of low- and high-mass stars, and the multiple pathways responsible for the production of elements in stars (i.e., stellar nucleosynthesis) such as the triple alpha process. The chapter concludes with an introduction to the broad subject of astrochemistry. The emphasis is on delineating the sites of molecule formation (e.g., molecular clouds), as well as the processes involved in gas-phase chemistry and grain-surface chemistry that drive the synthesis of molecules.

The theme of how life and its environment have coevolved together for about four billion years on Earth is explored in this chapter. The major evolutionary events that unfolded in the Archean eon (4 to 2.5 billion years ago), Proterozoic eon (2.5 to 0.539 billion years ago), and the Phanerozoic eon (0.539 billion years ago to present) are outlined, such as the origin(s) of multicellularity, eukaryotes, complex multicellular organisms, and humans. By drawing on this evolutionary timeline, theoretical paradigms for understanding and grouping the notable evolutionary events are sketched (e.g., major transitions in evolution). The next part of the chapter illustrates the intricate interplay between life and its environment by chronicling the rise in molecular oxygen levels, its possible causes and profound consequences, and its potential connections with key geological changes like the putative Snowball Earth episodes. Lastly, the ‘Big Five’ mass extinctions that transpired in the Phanerozoic, along with their triggers and ramifications, are described.

Life-as-we-know-it harnesses carbon for the scaffolding in biomolecules and liquid water as the solvent. This chapter delineates the beneficial properties of carbon and water, and then investigates whether viable alternatives to this duo exist (i.e., ‘exotic’ life). With regard to the latter, the likes of ammonia, sulfuric acid, and liquid hydrocarbons are expected to have some physical and/or chemical advantages relative to water, while also exhibiting certain downsides. In contrast, it is suggested that few options appear feasible aside from carbon, with silicon representing a partial exception. The chapter subsequently delves into the habitability of the clouds of Venus and the lakes of Titan, because the alternative solvents sulfuric acid and liquid hydrocarbons (methane and ethane) are, respectively, documented therein. Both these environments might be conducive to hosting exotic life, but it is cautioned that they are likely subjected to severe challenges.

The first part of this chapter introduces and defines key concepts that are commonly encountered in this subject: astrobiology, habitability, and life; in doing so, it also clarifies the ambiguities inherent in these terms. The second part briefly chronicles the lengthy and rich history of speculations about the plurality of worlds and extraterrestrial life in myriad societies across different epochs. It concludes with a summary of developments in astrobiology in the early- and mid-twentieth century, and describes how the future of this field looks optimistic.

This chapter elucidates the physical and chemical mechanisms involved in the formation of planets, the conventional abodes of life. The first part is devoted to protoplanetary discs, wherein planet formation unfolds. The topics covered include the minimum mass required for assembling the solar system (minimum mass solar nebula), the thermal and density structure of protoplanetary discs, and the rich chemistry that occurs in these settings. The second delves into the many stages of planet formation starting from the coagulation of dust to the hurdles encountered (e.g., metre barrier) in forming kilometre-sized planetesimals and subsequently to collisions between planetesimals engendering planetary cores and eventually terrestrial planets; a brief description of how giant planets are assembled is also delineated. The final part outlines how interactions between a given planet and its neighbouring gas or planetesimals can contribute to the migration of the former, as well as influence the delivery of water and other volatiles to the planet.

Given the current state of play in the search for life beyond the Earth, where as yet we have no conclusive evidence, it might seem inappropriate to discuss ‘common misunderstandings’. But it’s not. There can be misunderstandings about the way we search for extraterrestrial life, as well as in relation to the scientific basis for our search, and it is these that I focus on here, rather than misunderstandings about extraterrestrial life itself. I discuss them below in the order in which they’re first encountered in the book.

I introduced exoplanets – planets beyond our solar system – in Chapter 1, discussed them in the context of planetary systems in Chapter 2, and considered them in relation to the concept of habitability from a general perspective in Chapter 4. But so far I’ve given little detail about them. How many exoplanets, or exoplanetary systems, have I mentioned so far? Very few. That’s about to change, but not in the sense of replacing a dearth of detail with a wealth of it. Rather, I’ll be very selective about the particular exoplanets I discuss. This is essential, given that the number now known is huge, and most of them are irrelevant to the search for alien life.