The impact of a wide range of post-Keplerian perturbing accelerations, of whatever physical origin, on different types of observation-related quantities (Keplerian orbital elements, anomalistic, draconitic, and sidereal orbital periods, two-body range and range rate, radial velocity curve and radial velocity semiamplitude of spectroscopic binaries, astrometric angles RA and dec., times of arrival of binary pulsars, characteristic timescales of transiting exoplanets along with their sky-projected spin-orbit angle) is analytically calculated with standard perturbative techniques in a unified and consistent framework. Both instantaneous and averaged orbital shifts are worked out to the first and second order in the perturbing acceleration. Also, mixed effects, due to the simultaneous action of at least two perturbing accelerations, are treated.

The precessions of the Keplerian orbital elements induced by several modified models of gravity are calculated. The latter ones are Yukawa, power-law, logarithmic, dark matter density profiles (exponential and power-law), once per revolution accelerations, constant accelerations, and Lorentz-violating symmetry.

The orbital precessions of the Keplerian orbital elements induced by the 1pN gravitomagnetic spin octupole moment of a rigidly rotating oblate spheroid are calculated in their full generality for an arbitrary orientation of the primary’s spin axis and a general orbital configuration of the test particle.

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 impact of the 1pN gravitoelectric mass monopole acceleration, both in the test particle and in the two-body system of finite, comparable masses cases, is calculated for different types of observation-related quantities (Keplerian orbital elements, anomalistic, draconitic, and sidereal orbital periods, two-body range and range rate, radial velocity curve and radial velocity semiamplitude of spectroscopic binaries, astrometric angles RA and dec., times of arrival of binary pulsars, characteristic timescales of transiting exoplanets). The results are applied to a test particle orbiting a primary, a Sun–Jupiter exoplanet system, and to a S star in Sgr A*.

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.

An overview of General Relativity is provided to a basic level. Its different nature with respect to the Newtonian Universal Gravitation is outlined. A cursory resume of the post-Newtonian approximation and its importance in testing Einstein’s theory is offered. A brief overview on the modified models of gravity that appeared in the last decades is outlined. A plan of the book is provided.

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.