Early on in his book Lucky Planet, David Waltham writes about the planet-detecting capabilities of the Kepler space mission. When discussing the chances of Kepler finding a truly Earth-like planet – a body that’s the same size as our Earth and orbiting in the habitable zone of its stellar system – he states that the “announcement of such a world could well come between me finishing this book and its publication (an occupational hazard of discussing topical subjects)”. He was spot on. At about the same time Lucky Planet hit the bookshops, NASA announced the discovery of Kepler-186f, a planet whose radius is about 10% greater than Earth’s and that sits towards the outer edge of the habitable zone of its dwarf star.
Let’s face it. It’s likely that there are billions, perhaps trillions, of Earth-like stars in the cosmos. Can we plausibly argue that our Earth is somehow special, that it alone of all similar worlds possesses complex (which is to say multicellular) life?
Well, yes we can. With the limited data available to scientists we can’t say with any degree of certainty that Earth is the only planet that hosts complex life. But we can certainly make the argument that Earth is exceptional – and Waltham does so beautifully.
Lucky Planet gives the most accessible treatment I’ve yet read of the anthropic selection effect. Our planet has enjoyed four billion years of clement weather, and this climate stability has surely been key to life’s continued existence. Earth’s climate could easily have followed a trajectory towards ice or fire, turning the planet into a snowball or a boiling hell; the climate could have done rapid flip-flops between periods of frigidity and periods of heat. Instead, the average surface temperature of our planet has seen a gentle cooling trend on top of which are relatively minor fluctuations, measured in tens of degrees. With this climate life has been able to thrive.
We can explain life’s continuing existence by saying that life is robust, resilient, capable of withstanding any of the shocks that Nature throws its way. Or we can go even further and argue that various feedback mechanisms allow life itself to create, maintain and develop the conditions needed for life to survive and thrive. This is the Gaia hypothesis. Combine this approach with the vast number of Earth-like planets that exist and one surely must conclude that the Galaxy is teeming with life.
But we can just as well explain life’s continuing existence by attributing it to luck. As Waltham points out, the anthropic selection effect means that intelligent observers must find themselves on planets on which the past climate was such that life could evolve. They can hardly find themselves on a planet on which the past climate was such that their ancestors died. A planet capable of hosting complex life might be a one-in-a-trillion fluke; but if there are trillions of planets then that fluke is going to happen. Goldilocks not Gaia; luck not life.
In Lucky Planet Waltham discusses a number of ways in which Earth might be special, but the most interesting discussion (to me, at least) was the influence of the Moon. Like most people with an astronomy background I believed that the Moon played a large role in stabilising the Earth’s spin axis. Waltham shows that the true story is more complicated than that. It turns out that … but, no, you should get the book and find out for yourself.
The only criticism I have of the book is that, at two pages, the Further Reading section is rather sparse. The book is aimed at a popular audience, so I can understand the reasons for having only a short section bibliography, but I would have appreciated more detailed references. (The author does link to his website, however, and you can find more technical references there.) That minor quibble aside – thoroughly recommended!