LET'S BURN HIM AT THE STAKE ALL OVER AGAIN:

With his usual mix of a poet's ear and scientist's acumen, Gleiser gives us a grand tour of astronomy, physics, and astrobiology to set the stage for asking the question: How can we matter in such a vast Universe? Detailed descriptions of the evolution of planets and life lead him to a startling conclusion: There can only be one Earth. It is from that vantage point that a new perspective on ourselves and our future emerges.

That ten billion trillion number certainly tells us that the cosmos has had a lot of planets on which to run experiments with life and civilization. But what it does not reveal is how specific the outcome of those experiments will be. The details of evolution on each world will be extraordinarily contingent on so many accidents that no two worlds will have the same history. That might seem like a small point, but when we add the evolution of life into the mix, those accidents start to matter.

Take the balance of land and water. Earth is approximately two-thirds covered by oceans. Why only two-thirds and not more or less? It turns out the delivery of water to the planet came through its early bombardment by comets and asteroids. The exact number of those planetary interlopers was a complete accident. In fact, we should expect that most planets will lie on the extremes of water delivery. Either they got so much water through comet impacts that all basins are entirely filled, and the water rises above whatever continents exist, or they got almost no water at all. This means most planets will either be water worlds or desert worlds. The almost half and half mix we ended up with may be very improbable. This has huge implications for the specifics of life's evolutionary trajectory on each world. On Earth, tidal regions at the intersection between ocean and land played an important role in our world's biological linages.

What all this means is we will not find another Earth. Our planet's history is unique, and as a result, so is its life. There may be other planets with life, but they will have their own trajectories -- including the possible development of minds. The origin of self-awareness on Earth is likely to have attributes that reflect our planet's specific history. That means we are likely to be utterly and particularly unique in all the Universe.

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