Wednesday, December 21, 2011

Earth, life's only home

In Alone in the Universe: Why our planet is unique, cosmologist John Gribbin makes a compelling case that no other planet could sustain life
AS BIG questions go, it's hard to get much bigger: Are we alone in the universe? Instinctively, it feels that there must be another intelligent civilisation out there peering through their telescopes at our pale blue dot. Modern astronomy reinforces this hunch. The Milky Way is just one of a trillion galaxies in the observable universe and contains just as many stars. As the haul of planets we discover around those stars grows, so does the feeling that it's only a matter of time before we find someone else sharing our cosmic patch.
In Alone in the Universe, John Gribbin dares to shatter that myth. Chapter by chapter, he describes how we are anything but an ordinary intelligence, living on an ordinary planet around an ordinary star in an ordinary galaxy. Instead, our existence relies on a series of remarkable cosmic coincidences.
Our solar system, for example, formed at a very fortuitous time and place within the Milky Way. Had it formed earlier or farther away from the galactic centre, it would not have had the supply of elements needed to build our rocky planets, or the reserves of metals our technological civilisation needs. Had it formed closer to the galactic centre, where the density of stars is greatest, our planet would likely have been sterilised - by a gamma ray burst, perhaps, or a star exploding as a supernova, or even a blast from the supermassive black hole that lies at the heart of the galaxy.
Then there's the arrangement of our solar system and its debris. Earth has the rare privilege of being in a family of planets with more or less stable orbits. Our home is just the right distance from the sun for liquid water, while asteroids of the kind that killed off the dinosaurs are today mostly sequestered safely away in the asteroid and Kuiper belt. Without the right sort of collisions at the right time, Earth would have been a waterless world with a toxic atmosphere, constantly bombarded by cosmic rays.
Gribbin's argument proves compelling as he ranges over issues of astrophysics, geology, atmospheric chemistry and evolution. Although he cannot quantify exactly how the potential for life around those trillion trillion other suns whittles down to zero, you can feel any optimism that ET is out there ebb away with each turn of the page.
Alone in the Universe is thought-provoking and sobering. Gribbin makes any ideas of decamping to another Earth-like planet in the future seem absurd. Our telescopes might find seemingly similar planets around other stars, but these worlds will inevitably be nothing like ours. He concludes that we must look after the extraordinary planet we have. It has survived far worse catastrophe than climate change and will go on, but we won't.

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