[ Also, in case you missed it: Part 1 ]
Asked about the likelihood of intelligent life elsewhere in the universe, or in the galaxy, or in the neighborhood (of a size for technologically-conceivable visits), most of us here would say it was possible. We wish it to be, want it to be, yearn for it to be. We might point to this or that argument from common sense, or common experience, or simply private hope, and proclaim even that it MUST be.
But in the same way we rein in our religious neighbors, drawing them away from all their wishes and wants and arguments from common sense, at some point we have to rein in ourselves, and look at, not just those factors that seem to make it likely, but those that seem to make it less than likely.
Gribben does that. But he does it carefully, almost reluctantly. The book is not a gleeful spoiler, a crowing of “Ha! You were wrong all along!” Instead, it is a clear-headed baring of facts and the probabilities that arise from them, touched with a hint of sadness at a conclusion that becomes more and more likely as the lengthy argument progresses.
Opening with a chapter on the Drake Equation and a couple of theoretical paradoxes bearing on the question, he follows with a mere seven chapters, each focusing on the question of life, and intelligent life, viewed at a particular scale.
What’s So Special About Our Place in the Milky Way? looks at the question from a galaxy-sized perspective. Stanton Freeman’s suggestion of super-advanced civilizations on ancient stars near the galactic core gives way to the realities of radiation, to the likelihood of cometary collisions, and even to stellar evolution and the elements necessary to life.
Because there is a close link between the metallicity of a star and the likelihood that it has planets, this had led to the idea of a ‘Galactic Habitable Zone’ (GHZ), the region around the thin disc where planetary systems like the Solar System and planets like the Earth are likely to be found. The crucial point, as far as the prospect of other intelligent beings existing in our Galaxy is concerned, is that this Galactic Habitable Zone slowly gets larger as time passes. According to some calculations, it also moves outwards through the disc as time passes; but the most up-to-date version of the idea, from Charles Lineweaver and his colleagues, suggests that it has always been centered on a ring about 26,000 light years from the galactic centre, began to emerge about 8 billion years ago, and at present extends from about 23,000 light years to about 29,000 light years. The sun is close to the centre of the GHZ, but not exactly at the centre. This is the region where the abundance of metals 5 billion years ago, when the Solar System formed, was sufficient to allow for the formation of planets like the Earth.
[...] The exact boundaries of the GHZ are not clear, but what is clear is that the inner regions of the Galaxy have plenty of metals but are hazardous for life, while the outer regions of the thin disc are safer, but metal-poor and unlikely to contain Earth-like planets. In between, there is a Goldilocks religion, the GHZ, that it just right for life. The Solar System sits near the centre of that zone.
What’s So Special About the Sun? looks at our own home star. We like to think of the Sun as average, and the formation of planets as a given, but Gribben lays out arguments why this is not so. Something like 95 percent of the stars in the Milky Way are less massive, and therefore less luminous, than the Sun, moving their individual Solar Habitable Zones – that area around a star between 0 degrees Celsius and 100 degree Celsius where liquid water can exist – so close-in that other factors such as solar radiation come into play to make life less likely. (A surprising percentage of stars are pairs, or even triplets, I was interested to discover, making planets much less likely.)
The Sun also appears to be unusually stable as stars go, a factor that plays into another concept in the subject, the CHZ, ‘Continuously Habitable Zone.’ This is the volume around a star not just where life might arise, but that volume through time, where conditions might be stable enough, and for long enough, for life to actually arise, and then evolve.
Even our Sun’s CHZ is smaller than might be expected, due to the fact that the life zone has moved outward as the Sun has brightened slowly over the past 4 billion years, making areas formerly habitable now deadly.
The [habitable] region that was on the outer edge of the zone (the cool edge) is now on the inner edge (the hot edge), and regions that used to be the hottest part of the zone, including the orbit of Venus, are now too hot for life.
What’s So Special About the Solar System? carries that same zone argument a step further, but adds in the hazardous mechanics of planetary formation, and the vast billiard game of solid bodies and gravitational break-shots that ensues.
Computer simulations show that by about a million years after the collapse of the cloud from which the Sun and planets formed, there would have been twenty or thirty objects in the region between the Sun and the present orbit of Mars, ranging from about the size of the Moon (roughly 27 percent of the diameter of the Earth today) to about the size of Mars, (roughly 53 percent of the diameter of the Earth today). They would have been accompanied by a huge number of smaller planetisimals, which would have been swept up by the larger objects in a series of collisions, while the larger objects themselves collided with one another and merged until eventually only four or five large objects were left – the objects that became Mercury, Venus, Earth and Mars, plus at least one other Mars-sized object.
[ Continued in Part 3 ]