This is where we switch from the geography of alien life to its biology – in other words from its distribution across the observable universe to its ‘nature’ in many senses of that word, including its chemical composition, its physical form, its means of acquiring energy, and, in some cases, its intelligence. For me, the nature of life beyond Earth is even more interesting than exactly which planetary bodies it inhabits, and many other scientists feel likewise. However, in moving from geography to biology things also become more controversial, because the so-called ‘sample size of one’ problem comes into sharp focus.

While we wait for our first conclusive evidence of life beyond Earth, we can contemplate its possible nature. In particular, we can ask the following question. To what extent should we expect evolution elsewhere to take a similar course to the one it has taken on Earth? That could be described as the key question about the biology, as opposed to the geography, of extraterrestrial life. But the way I’ve just put it isn’t ideal – it’s too centred on our home planet as a reference point. Let’s try to rephrase it in a Copernican manner, so that Earth doesn’t occupy a special place. Here’s one such rephrased version. To what extent does evolution follow similar courses on different inhabited planets? Earth is implicit here, but just as one of many inhabited planets, and almost certainly not the first one.

To look for life in the universe beyond Earth, we need to understand what is meant by ‘universe’, just as we need to understand what is meant by ‘life’. In the end, we can probably ignore most of the universe and focus our search in some very specific places. However, those places are best understood against a backcloth of what can be called – it’s an understatement really – the big picture.

The thought of there being millions of planets with life in the observable universe is inspirational. But it’s only that – a thought. Or perhaps a bit more than that – a thought with probability on its side. But the gap between probability and certainty is a huge one. We won’t really feel the presence of extraterrestrial life until we know for sure that it’s there. So we need evidence. I started the book with a look at a paper that focused on the need for a cool assessment of evidence and the importance of not jumping to conclusions. In this chapter we’ll return to that issue.

With the exception of planets orbiting the most massive and luminous stars, planetary lifespans are measured in billions of years. Evolution on Earth has taken about four billion years so far, and probably has about another two or three billion to run, depending on when our ever-brightening Sun eventually boils away all our surface water. In the absence of evidence to the contrary, it’s probably a good idea to assume that evolution elsewhere takes billions of years too. It’s hard to imagine an evolutionary process in which intelligence is an early result rather than a late one. So, to look for intelligent alien life, we need to concentrate on planets that aren’t too young. Earlier, I suggested that good yardsticks for planetary age when looking for photosynthetic or intelligent life were at least two and four billion years, respectively. In general, we can imagine at least four stages in the life of a planet – no life at all, chemosynthetic life only, a stage characterized by a mixture of forms of energy acquisition including photosynthetic life, and a final stage that also includes intelligent life. In the present chapter, we’re concerned with the final one.

For many millennia, humans have gazed up in wonder at the night-time sky. The full panoply of the Milky Way is an awesome sight. The scale of space is immense. Is there life out there somewhere? If so, where, and what form does it take? In the space of a couple of sentences, we’ve already gone from generalized wonder to specific questions. The next step is from questions to hypotheses, or, in other words, proposed answers. Here are two such hypotheses that I’ll flesh out as the book progresses: first, life exists on trillions of planets in the universe; second, it usually follows evolutionary pathways that are broadly similar to – though different in detail from – those taken on Earth.

I started the book by considering the possibility that our generation may be the one to discover the first persuasive evidence of extraterrestrial life. Suppose this turns out to be true, and humanity’s first such evidence arrives in a decade or so. What would the possible impacts be? We can deal with them under four headings: scientific, social, religious, and philosophical, with all of these being defined very broadly.

People involved in the modern era of the search for life – from the early days of SETI in the 1960s to the present – have tended to think of some planets as being potentially ‘habitable’ (or ‘inhabitable’, though that synonym is rarely used) and others not. For example, in our own system, Mars might once have been habitable, but Jupiter never so. Why this apparent certainty about Jupiter’s unsuitability for life? There are two main reasons.

Vesta’s surface is dominated by two overlapping impact basins: the older ~400 km Veneneia basin and the younger ~500 km diameter Rheasilvia basin.Their age and nature, along with the ejecta they produced in the form of V-type asteroids, can help us probe Vesta’s evolution.By modeling the production of craters superposed on these basins or on features created by their formation, we predict Veneneia and Rheasilvia basins are 3.2–3.5 Ga and ~1 Ga, respectively. Numerical models indicate they were created by the impact of ~60–70 km projectiles. These impacts likely dredged up material formed at >50 km depths within Vesta. The evidence for the formation time of Veneneia and Rheasilvia in the eucrite and howardite meteorite record exists but is limited. The absence of an obvious spike of 40Ar/39Ar shock degassing ages may be a consequence of low Main Belt impact velocities (< 5 km/s). Most V-type asteroids in the inner main belt are ejecta from one of these two basins. The scattered and limited population of V-types in the central and outer main belt have no clear source. We postulate they are fragments from Vesta-like bodies that originally formed in the terrestrial planet region.

This book depicts a vivid and vibrant image of modern Main Belt asteroid science. In the last decade, thanks to the exploration by the NASA Dawn mission and the advent of high-resolution Earth-bound observations, we have entered a renaissance of Main Belt asteroid science. Formation theories, dynamical models, meteorite geochemical data, remote and in-situ observations synergistically show asteroids are leftover building blocks of planetary formation and tracers of important evolutionary processes (e.g., collisions, orbital migration) that have shaped the evolution of the early Solar System. Planned missions such as NASA’s Lucy and Psyche (scheduled to launch in 2021 and 2022) will surely provide additional colorful strokes to our ever-evolving portrait of the Main Belt.

A search for volcanic and plutonic features on Vesta was an important driver for a geomorphological examination of the asteroid. Another goal was to determine if the asteroid was a protoplanet, one of the remnants of the material that formed the Solar System. Therefore, NASA’s Dawn spacecraft collected imaging, spectroscopic, and elemental abundance data, which were utilized to examine the asteroid’s surface. A digital terrain model was created and the asteroid’s various geomorphic features were analyzed. Large scale features include the Rheasilvia and Veneneia impact basins, the Divalia Fossae and Saturnalia Fossae trough sets, and the Vestalia Terra plateau. Small scale features include deposits of dark material, pitted terrain, pit crater chains, mass-wasting deposits, and impact craters. While these geomorphic analyses revealed no evidence of volcanism, evidence of magmatic activity on Vesta was identified. In addition, analysis of Vesta’s geomorphology suggests that it is not only a protoplanet, but also an intermediate body between asteroids and planets.

Nucleosynthetic and radiogenic isotope data from meteorites have significantly advanced the understanding of how the protoplanetary disk was structured during the accretion of planetary precursors. Meteorites exhibit an isotopic dichotomy between carbonaceous (CC) and non-carbonaceous (NC) meteorites. This NC–CC dichotomy, combined with the chronology of meteorite parent body accretion, implies a potentially strict spatial divide between the inner (NC) and outer (CC) protoplanetary disk which lasted several million years. This divide may have been facilitated by early formation of the gas giant planets, which acted as a barrier, thereby significantly influencing the chemical evolution of the disk and thus the planet building process. These meteorite-derived findings and their implications for planet evolution are discussed here, with an emphasis on the role that Vesta and Ceres play in piecing together the history of the Solar System, as these bodies may be considered as samples of the inner and outer protoplanetary disk, respectively.