This chapter explores a core question in astrobiology: what is the future of life on Earth and beyond? The first part describes the cessation of habitable conditions in Earth’s distant future (about a billion years hereafter), and the myriad risks that apparently confront humanity on shorter timescales, ranging from wars and artificial intelligence to asteroid impacts and massive volcanoes. The second segment outlines the possibility of humans migrating to other worlds in the solar system, and the numerous technological and logistical challenges expected to arise during this endeavour. The even more daunting notion of interstellar travel is also touched upon, and the propulsion systems and spacecraft advanced in this regard are sketched. The textbook comes to a close by taking stock of the fates that might await humankind.

Mars has always been one of the most promising targets in the search for current or extinct extraterrestrial life. The chapter commences with a brief summary of Mars’ basic characteristics, before describing its potential for instantaneous habitability (e.g., energy sources, bioessential elements), with an emphasis on the availability of water. This is followed by an exposition of how several aspects of Martian habitability have diminished over time, ranging from extensive atmospheric loss to the shutdown of its dynamo, both of which might have contributed to the emergence of its cold and arid climate today. Nevertheless, some specialised abodes where life may have persisted are touched upon (e.g., deep subsurface). In the last part of the chapter, the contentious history of life detection on Mars – the Viking mission experiments in the 1970s and the meteorite ALH84001 – is reviewed, and forthcoming missions to Mars are surveyed.

Icy worlds with subsurface oceans are potentially among the most common repositories of liquid water in the Universe. Moreover, the solar system is confirmed to host a number of such worlds, notably: Europa, Enceladus, and Titan. Motivated by these considerations, this chapter examines the habitability of icy worlds from a general standpoint. The oceanic properties of Europa, Enceladus, and Titan are reviewed, followed by a simple analysis of the physical conditions in which subsurface oceans may be supported. The pathways for the formation of the building blocks of life, their assembly into polymers, and subsequent delivery to the subsurface ocean are elucidated. The possible constraints on the availability of energy sources and bioessential elements are delineated, as well as the types of organisms and ecosystems that could exist. The chapter concludes by briefly speculating about the trajectories of biological evolution conceivable on icy worlds.

The conditions on early Earth prior to four billion years ago (Hadean Earth), which shaped the origin(s) and early evolution of life, are discussed in this chapter. It begins with a summary of the various sources of internal heat on terrestrial planets and the types of heat transport (e.g., conduction), as these factors influenced the habitability of early Earth and its temporal evolution. This is followed by an exposition of the characteristics of Hadean Earth: the Moon-forming impact, oceans, landmasses, and atmosphere, including the faint young Sun paradox – how did Earth stay unfrozen despite the Sun’s lower luminosity? The chapter concludes with sketching the putative Late Heavy Bombardment (a potential spike in the impactor rate) about four billion years ago, and a general treatment of the positives and downsides of large impacts.

This chapter is devoted to a foundational question in astrobiology: how and where did life originate? The narrative commences with a brief description of the four major categories of biomolecules (proteins, nucleic acids, carbohydrates, and lipids) on Earth and their associated functions. Partly based on this knowledge, biophysical and biochemical constraints on the minimum size of a viable cell are derived. The various origin(s)-of-life hypotheses are discussed next – like the replication-first (e.g., RNA world) and metabolism-first paradigms – along with their attendant strengths and weaknesses. The pathways by which the building blocks of life (e.g., amino acids) could be synthesised through non-biological avenues, such as the famous Miller experiments, are elucidated. Subsequently, the abiotic channels that may facilitate the polymerisation of these molecules to yield biomolecules are delineated. The focus of the chapter is then shifted to the specialised environments that might have enabled the origin(s) of life to readily occur. Two candidates are reviewed in detail (submarine hydrothermal vents and hydrothermal fields), with others mentioned in passing. Finally, the concept of entropy and its subtle connections with living systems are sketched.

The search for extraterrestrial intelligence (SETI) represents a well-known area of astrobiology. This chapter is dedicated to technosignatures, that is, markers produced by extraterrestrial intelligences (ETIs). The famous Drake equation for roughly estimating the number of communicative ETIs is introduced, its various factors are defined, and some of its shortcomings and implications for detecting technosignatures are discussed. Next, the Fermi paradox is delineated: if ETIs are widespread, where are they? Three major classes of solutions to this classic paradox (e.g., we are effectively alone) are considered, along with their accompanying ramifications. After a brief segue into the Kardashev scale for grouping ETIs, the final segment of the chapter categorises the diverse landscape of technosignatures – ranging from artificial radio and optical signals to atmospheric pollutants and waste heat arising from energy harvesting and dissipation – and outlines the current limits derived for the frequency of technosignatures, as well as the anticipated future constraints in this context.

The manifold requirements for a world to sustain habitability on long timescales (continuous habitability) are delineated in this chapter. The first part offers a brief introduction to climate physics (e.g., greenhouse effect), and thereupon formulates the notion of the habitable zone, that is, the region where liquid water could exist on rocky planets orbiting stars; the boundaries of the habitable zone as a function of the stellar temperature are also presented. In the second part, the various stellar factors potentially involved in regulating planetary habitability are sketched: winds, flares and space weather, and electromagnetic radiation. The third part chronicles some planetary variables that may affect habitability: mass, plate tectonics, magnetic field, tidal locking, and atmospheric composition. The last part is devoted to examining the high-energy astrophysical processes that might impact habitability on galactic scales: candidates in this regard include supernovae, gamma-ray bursts, and active supermassive black holes.

To address the fundamental question of ‘Are we alone?’, a cornerstone of astrobiology, it is necessary to search for signatures of extraterrestrial life (biosignatures). This chapter is thus divided into two parts: in situ biosignatures and remote-sensing biosignatures. In the first, a variety of potential biomarkers are described, such as isotope ratios, individual and collective microfossils, homochirality (i.e., presence of molecules of the same handedness), distributions of biomolecular building blocks, and agnostic methods. In the second, the categories include gases (e.g., molecular oxygen and methane), surface components (e.g., pigments like chlorophylls), and temporal variations of certain features. This chapter concludes by delineating emerging criteria and techniques for evaluating the credibility of putative life detection.

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.