Non-Fiction Reviews

One of Ten Billion Earths
How we learn about our planet's past and future
from distant exoplanets

(2018) , Oxford University Press, £25 / US$34, hrdbk, vii + 460pp, ISBN 978-0-198-79989-4


The end of the 20th century (well, 1995) saw the detection of the first exoplanet: a planet outside of our own star system, the Solar system. Over a quarter of a century on and the number of exoplanets detected has boomed by so much that it is no longer news unless a planet is detected of a broadly similar size to the Earth and is habitable if not, preferably orbiting a mid-main sequence star.  In One of Ten Billion Earths Karel Schrijver effectively reviews the exoplanetary discoveries made in addition an examination of our Solar system.

Before going any further I should declare my expertise and lack of expertise on this topic. Obviously as an environmental scientist into SF, I am interested in the prospects for alien life. Indeed in recent, semi-retired years, I have devoted some time to this subject coming at it as I do from a bioscience and geoscience perspective.  Conversely, what Karel Schrijver has done is to come at it from an astronomical and astrophysics approach.  Schrijver's and my own science bases are distinct but they do overlap. As such am familiar with some but not all of the ground Schrijver covers and can use my knowledge of the former to judge, hopefully meaningfully, latter with which I am not familiar.

This book covers a lot of ground and is pitched at the proverbial New Scientist level but with chapter academic references so that someone with a knowledge of science can follow up the primary research for themselves.

This in turn means that this book would not only be of interest to school leavers thinking of undertaking an astronomy related degree, as well as non-exoplanetary astronomers seeking a easy-to-read review of exoplanetary science, but also those scientists who are also into science fiction (arguably this website's principal target following).  Indeed SF fans will be pleased that One of Ten Billion Earths does give mentions to: Douglas Adams, Poul Anderson, Ray Bradbury, Arthur C. Clarke, Robert Forward, Robert Heinlein, Ridley Scott and John Wyndham plus the now almost obligatory (sadly as they are not necessarily the best exemplars of SF no matter how much we love them) Star Trek and Star Wars fortunately diluted with a few other visual media SF references.  Karel Schrijver is clearly one of us. (One of us, one of us…)

Now, I am not going to go through the contents pages for you – you can look those up easily enough on the internet – suffice to say that much of the ground you would expect to be covered is covered. However, frequently Karel Schrijver does this with a slightly different, and invariably interesting take to that one usually finds in similar books.  For example, the Drake equation (the one whereby multiplying key probabilities – the rate of star formation in the galaxy, the proportion of stars with planets, the proportion of potentially habitable planets, the proportion actually sporting life, etc) he does mention but he does not go into the estimates commonly given, his own take, or even the N = L (number of technological civilisations in the galaxy = their mean longevity in years) Drake perspective. Most of us know that, or if not can find out easily enough. What he does do is come at it from a radio communication perspective (we use radio astronomy) and estimated we'd need to survey 74 million stars to get one technological civilisation signal which means we'd need to be able to radio detect out to 2,000 light years.  Could this explain the Fermi paradox?

And on the Fermi Paradox, Karel Schrijver notes that the literature on what exactly happened at that dinner party is inconclusive, but teases together the commonalities of the more authoritative accounts to give us an idea of some of what might have been said. Fascinating stuff.

Interestingly, he looks at how explanatory systems might form and as well as our own Solar system. Now, I am not familiar with the primary research literature on planetary system formation computer models, but by the time I got that far into the book I was sufficiently convinced from the science Karel Schrijver had covered that was within my sphere of knowledge, that I had confidence in what the man was saying. It would appear that though rocky planets in the habitable zone of a mid-main-sequence star (such as the Sun) form inside the snow line (beyond which water condenses out and gets attracted to planets), and so these inner planets like the Earth should be devoid of water, almost invariably outer planets' gravitation protuberances send water bearing comets inwards.  The chances, the models suggest, of water being carried to the arid inner planets is very high if not universal!

I could go on at length about the various takes on exoplanetary and life-possibility questions Karel Schrijver addresses, just take it from me that it is all interesting stuff.

The book is well illustrated and there is a full colour centre section where some of the diagrams that could benefit from colour get it.

Downsides, very few. Indeed the one slightly irritating thing (which was so occasional that it is almost an injustice to mention it) is that while for the most part it was not to difficult to tie a paragraph to a chapter academic reference, there were just a couple of instances where it was hard. This is because Schrijver's style is so accessible, almost conversational, that there is no breaking up of the flow of text with an insert pointing to an academic reference. In short, this downside can be considered a plus. But for very much the most part, Schrijver mentions the scientists whose work he references at the book's end and so often it is easy to find the academic reference to which he alludes.

Apparently Schrijver got the idea of this book from a class with which he has been involved for over a decade. And it seems that for much of this time he has been quietly working away on this book even though in the meantime he has had others published. That this book's gestation has been long shows, and we the reader reap the benefit.

Exobiology and related astronomy is such a big, as well as a developing, subject area that those into it really need to buy a decent new text every couple of years if not every year.  If you are into exoplanets and the possibility of life elsewhere from an astronomy perspective (as opposed to a bio- and geo-science viewpoint) then this could well be this year's choice.

Jonathan Cowie

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