This is a generalized version of the Terraspermia hypothesis for all inhabited planets. Wherever life has evolved, its origin was on the same planet as its evolution. If successful space travel and planetary invasion by dormant spores occurs anywhere, it is only within very small-scale planetary systems, and even this seems rather unlikely.

Here, I consider other factors than distance from a star that may affect a planet’s habitability. These include its atmosphere, its magnetic field, and whether it has any moons. However, I emphasize that it is important not to draw up a list of all the Earth’s specific features, for example its unusually large Moon (which helps to stabilize its axial tilt) and make the assumption that all of these are necessary for another planet to support life. Making such an assumption leads to the Rare Earth hypothesis, which I regard as flawed. For life to originate on a planet, there must be places where conditions favour the biochemical evolution that leads to proto-cells and hence to life. For life to continue and diversify, there must be places where organisms can survive. Even if conditions are normally benign, all planets are subject to occasional major threats, such as impacts and glaciations. I examine the mass extinctions on our own planet, some of which were caused by asteroids impacting our surface. Finally, I examine a problem that Earth is not subject to – tidal locking. This may be a major problem for planets in the habitable zones of red dwarfs.

Here, I examine whether some of the exoplanets that we have already discovered might be inhabited. However, I start by cautioning against an overly optimistic stance. Although we now know of thousands of exoplanets, and although the Drake equation estimates of Chapter 12 suggest that millions of planets are inhabited, a quick calculation suggests that of the exoplanets discovered so far, only a few are likely to have microbial life and none to have animal life. Against that background, we look at four planetary systems that are reasonably promising. One of these is the Kepler-186 system, where planet f may be habitable. Another is the Alpha Centauri system, where Proxima b may be habitable. A third is TRAPPIST-1, where there are three potentially habitable planets. The final one is Kepler-452, where planet b may be habitable. Whether any of these planets are actually inhabited will only be answered by particular kinds of observation – most likely spectroscopic studies of their atmospheres. How realistic such studies are depends on the distance to the system concerned. The four systems used as examples here range from 4 to nearly 2000 light years – from doable to quasi-impossible.

The ubiquity of gravity and topography, coupled with the presence of surface water, means that the broad range of habitat types on most inhabited worlds will parallel that of the Earth.

Here, I consider our current view of the universe. I start with the Hubble Ultra-Deep Field, which shows about 10,000 galaxies in a tiny field of view. The whole of the observable universe contains over two trillion galaxies. I discuss two important principles regarding the nature of the universe and our place within it. The cosmological principle holds that the universe is homogeneous provided that we make comparisons at a high enough level of spatial scale. The Copernican principle maintains that our position within the universe is not central. We are certainly not central in the solar system or the galaxy; whether we are central in the universe is a tougher question to grapple with. We are at the centre of our own observable universe, but by definition any other observer is at the centre of theirs. We then turn from seeing galaxies in general to seeing individual events. These include long-known phenomena such as the Crab Nebula, which was produced by a supernova explosion. They also include more recently observed events such as collisions of neutron stars. We end by looking at the relative power of radio signals produced by biological and non-biological sources.

Carbon-based life is the most probable, and hence the most common, form of life in the Milky Way, and indeed in the universe. However, it may not be the only form of life. We should keep an open mind on this; the King Carbon hypothesis should not give way to carbon chauvinism.

The mass–radius relation for polytropes is introduced and analyzed. The Newtonian Lane–Emden equation is derived analytically. Known analytic solutions are discussed. The famous Chandrasekhar mass is obtained as a solution of the Lane–Emden equation. Corrections to the equation of state for white dwarfs are pointed out by estimating the Coulomb corrections for a lattice of nuclei immersed in a sea of electrons. The different layers of a typical white dwarf and the typical sizes are worked out in detail. Thermal effects for the mass relation are examined. Finally, astrophysical observations of white dwarfs are shown and discussed in terms of the overall composition of white dwarfs and the resulting mass–radius relation.

The detection of gravitational waves from the merger of black holes and the merger of two neutron stars are discussed. The linearized theory of general relativity is introduced. The concept of the gauge invariance is put forward and the transverse-traceless gauge for gravitational waves is presented. Einstein‘s famous quadrupole formula for gravitational waves is developed. The principle of detecting gravitational waves is outlined. As applications, the emission of gravitational waves from a nonvanishing ellipticity of rotating neutron stars is derived. The chirp mass is introduced and the emission of gravitational waves from compact binary systems is obtained. The formula for the tidal deformability and the Love number is put forward and discussed with regard to the recent measurement of a neutron star merger by the LIGO–Virgo scientific collaboration.

The rich history of neutron stars, observationally as well as theoretically, is sketched. The different layers of a neutron star are discussed step-by-step. The outer crust is described as a lattice of nuclei with a sea of electrons. The nuclear shell model and the phenomenon of magic numbers are introduced. The sequence of nuclei in the outer crust of a neutron star is shown explicitly. Features of the inner crust, as nuclear superfluidity and superconductivity, and the connection to pulsar glitches and cooling of neutron stars are described. The possible appearance of geometric structures in the inner crust, the pasta phases, are discussed. The concept of nuclear matter is established. The present-day knowledge of the neutron matter equation of state is put in place. The mass–radius of neutron stars are conceptionally investigated with a detailed discussion of the stability of neutron star configurations. The possible presence of exotic matter in the inner core of neutron stars is considered. The experimental data on hypernuclei is summarized and the paradigm of the so-called hyperon puzzle for neutron stars is analyzed.