100 billion planets sounds like a lot, but if the probability of intelligent life evolving is very small, then it might not be enough planets to make likely the existence of another technological civilisation even in our vast galactic neighbourhood.
The take-away message of this extremely interesting book is that there are a lot of things that have to go right for intelligent life to emerge, and while no one factor is enough to rule out the hope of finding an interstellar interlocutor, when all the factors are multiplied together intelligent life might be so unlikely as to render us almost certainly alone.
The author discusses other solutions to the Fermi Paradox, classed as "They are here"—which is not in evidence, so I'll pass over them—and "They exist but have not communicated"—potentially feasible, it seems to me, especially if super-high-tech interstellar civilisations communicate in ways we cannot detect, or even if they prefer simulated life to the real universe. But it is the final set of solutions, "They do not exist", which is the most problematic and, for me, potentially saddening.
Among the conditions which Webb discusses, and which the course of cosmology, geology, chemistry and biology may be required to satisfy in order to produce intelligent life, are:
- a G-type star like the Sun;
- outside the centre of a galaxy for the avoidance of a concentration of lethal stellar radiation events;
- a galaxy and solar system with enough heavy elements to construct planets and biomolecules;
- a rocky planet or moon (or two!) within a continuously habitable zone, meaning a liveable orbit during billions of years of evolution;
- having tectonic activity for the regulation of carbon dioxide levels and therefore temperature;
- having experienced a collision with a smaller planet so as to give it a moon of the right mass to stabilise its axial tilt;
- and possessing an ozone layer to protect against ultraviolet radiation;
- the formation of water-bearing asteroids;
- which are driven out of orbit by the gravity of a well-placed gas-giant so as to smash into a planet in the habitable zone and thus hydrate it;
- the abiogenetic emergence of self-replicating, protein-encoding genes;
- the emergence of cells;
- endosymbiotic events to endow single cells with organelles and nuclei, thus producing eukaryotes;
- the avoidance of overly catastrophic mass-extinction events (from asteroid impacts to climate change to super-volcanic eruptions);
- an oxygenation event;
- a Cambrian-like explosion of multi-cellular life;
- the evolution of sufficient technical intelligence to create a radio transmitter;
- without such a civilisation destroying itself by nuclear weapons or climate change.
Let's run some numbers of our own, then, however inexpertly (which is very inexpertly);
- 1.2% of stars that host habitable planets makes 1.2 billion interesting stars in the Milky Way, averaging 1 habitable planet each;
- since only Earth of the rocky planets in the Solar System has plate tectonics, let's imagine a 25% probability for a given habitable planet, accepting for sake of argument that plate tectonics are necessary for bringing carbon dioxide and thus temperature to equilibrium, making 300 million interesting planets;
- a large moon might not be needed for axial stability, so let's ignore that;
- let's give all the remaining planets water, oxygen and ozone;
- since all life on Earth has a common ancestor, we may have to rate abiogenesis unlikely, although on the other hand life emerged fairly quickly in Earth's history, after only 1 billion years of its likely 12 billion-year existence until the Sun engulfs it, and scientists have recently found lifelike metabolism of chemicals occuring naturally, so let's go for 50%, leaving 150 million life-bearing planets;
- let's assume cells are a fairly easy evolutionary improvement once metabolic replicators exist;
- endosymbiosis has provided eukaryotic cell lineages with organelles several times, so let's rate the probability of eukaryogenesis at 50%, leaving 75 million planets with eukaryotic cells;
- let's allow catastrophes to destroy life on half the planets that have it, leaving 37.5 million eukaryotic ecologies;
- let's allow life to oxygenate the planet on its own;
- let's give the probability of eukaryotic life leading to a species with the technical intelligence to build a radio transmitter as a challenging 1 in 10 million, leaving 3 or 4 advanced extra-terrestrial civilisations to exist at some point in our galaxy.
After reading this book, I will struggle to believe any more that the vast number of stars makes it likely, but I'm not so sure that it's totally implausible. Then there's the whole rest of the universe with its maybe 200 billion galaxies, where surely, surely, the vast numbers add up to many civilisations we will never know.