Book review: Destiny or Chance Revisited

What have the recent discoveries of thousands of exoplanets told us about how we got here, and whether we are alone? In Destiny or Chance Revisited, Stuart Ross Taylor attempts to answer those two questions. The book provides an engaging summary of the modern understanding of how our solar system and other stellar systems formed, in a style accessible to people lacking a technical background. Taylor's style is conversational and I often found myself smiling even though the occasional off-color joke made me wince. The book is not long, though it could've been made shorter by editing out some of its repetition. It's quite up-to-date, including recent results from Kepler, COROT, and HARPS.

Destiny or Chance Revisited argues strongly in favor of the "chance" side of the argument: "planets are different and owe their particular composition to the operation of random accumulation processes." Composition, position, order, inclination, the presence of moons -- all of these result from stochastic events, a point that Taylor makes over and over. Taylor says that the primacy of randomness is evident from the variety of the physical properties of planets and moons in our solar system, with the conclusion being reinforced by the unanticipated variety of exoplanetary systems. The apparent order of our solar system, with four terrestrial planets and four giant planets evenly spaced in circular orbits, fooled us into thinking that there were general rules that applied to the formation of planetary systems, but we've found few exoplanetary systems that look anything like ours. We are an accident.

The other major thesis of Destiny or Chance Revisited is that observations will always triumph over models. And it is certainly generally true that new kinds of observations tend to overturn explanations that we develop based on the data we already have. I myself am often skeptical about the conclusions of modelers, preferring to wait and see what observations bring. So it's really saying something that I consider Taylor to be excessively gloomy about modeling when he says (regarding attempts to predict whether terrestrial exoplanets have moons): "While one must admire the prodigious intellectual and computing power employed in such calculations, possibly, it will be better to wait for observations in view of the huge uncertainties involved in trying to make predictions about an essentially random process."

I don't think it's quite so worthless to model the exoplanets and their likely moons; neither do I agree, as Taylor writes on the same page, that "none of our 160 or so [natural] satellites resemble one another." But I don't have to agree with such assertions to find the book a thought-provoking, valuable, and enjoyable summary of what we think we know about how planets come to be. Reading it has already helped me to understand the context of new discoveries of exoplanetary systems, and of the implications of new models for the formation of the myriad worlds within our own solar system.