Is Earth Special? Reconsidering the uniqueness of life on our planet

Earth is almost the perfect place for life as we know it to begin, prosper, and diversify. That makes it a very odd world. Most planets are too hot or too cold; too wet or too dry; too small or too big; or just plain wrong for life in any one of a hundred other ways.

One life-friendly property of Earth particularly intrigues me and compelled me to write my latest book, Lucky Planet—it has had a remarkably stable climatic history. Global average temperature is controlled by just three things: the brightness of its sun; the fraction of sunlight a planet reflects rather than absorbs; and the concentration of greenhouse gases in its atmosphere. This has been known for more than a century, but we have only recently realized that astronomical, geological, and biological

processes have massively altered all three factors over the 4 billion years that life has existed on Earth. In that time our Sun has warmed 40 percent as it has aged; the Earth’s reflectivity has altered as clouds, ice caps, and continents have changed. And thanks largely to biology, our atmosphere’s composition is now utterly different from that of 2 billion years ago. These changes could have produced surface temperatures varying by hundreds of degrees Centigrade—making complex life impossible—but, somehow, these multifarious influences counteracted each other to produce a climate that has generally only fluctuated by tens of degrees Centigrade.

There are three scientifically respectable explanations for our 4 billion years of life-friendly weather. Firstly, it could be a fundamental principle that biogeochemical processes on inhabited worlds tend to stabilize climates. Thus, we shouldn’t be surprised by Earth’s suitability for life because the physical, chemical, and biological laws of the Universe guarantee the existence of many such worlds. Alternatively, it may be that life is extraordinarily adaptable and will thrive under a wide range of conditions. Again, we shouldn’t be surprised that Earth fits life because, in fact, life has adapted to fit Earth. Finally, perhaps planets suitable for complex organisms occur only rarely and purely by chance. But even then we shouldn’t be surprised that we inhabit one of the few lucky worlds. Intelligent observers can only arise on planets where conditions allow complex life, even if such worlds are so unusual that we’d need to search a billion galaxies to find another. The tautology that we must inhabit a habitable world, even if such planets are extraordinarily peculiar, is an extreme example of the scientifically common problem of observational bias. This anthropic selection effect, as it is known, is a central theme in Lucky Planet.

In making the case that our climate is anthropically selected, Lucky Planet takes the reader on a journey through the 4.5-billion-year history of our planet, global warming, life on Mars, and the surprisingly biology-friendly nature of our Universe. In addition, it shows how my own research on the climatic influences that drive ice ages has led me to view good fortune as an important component of Earth’s biological story. The book also discusses how recent discoveries of planets orbiting other stars will allow my ideas to be tested in the near future. At the end of this tour of cosmology, geology, climatology, and biology, the book comes to a simple conclusion: Earth is a rare, beautiful, and very special place—one of the luckiest planets in the Universe.

David Waltham is an astrobiologist, geophysicist, and former head of the department of Earth Sciences at Royal Holloway College, University of London. You can read an excerpt of Lucky Planet.

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