The Reason Why: The Miracle of Life on Earth

Life on our planet is certainly miraculous - not in a theological sense, but in the extraordinary sequence of events and processes that seem to have been crucial in creating an Earth suitable for life. In this latest book, John Gribbin explains just how our home world appears to be special, and exactly what we have to be thankful for.

For me, the keystone of the book is Gribbin's argument that it is the Moon that is the single most important factor in keeping our planet suitable for the development and sustenance of complex life such as us humans. Our planet did not originally form with a moon (neither did Venus or Mars), but was endowed with a large satellite by a chance occurrence at the very dawn of the solar system.

The young Earth formed in an orbit around the Sun perilously close to another newly minted planet, roughly the size of Mars, and inevitably the two came together in a world-shattering collision. The violence of this encounter melted the entire surface of the Earth into an ocean of magma, and the other, smaller planet was obliterated and splashed back into space, along with a large volume of molten rock from Earth's surface. This swirling disc of ejected rock rapidly coalesced into the Moon that is now familiar in our skies.

The Moon seems to be important for the life-bearing potential of our world in several ways. For example, the presence of such a large orbiting body acts to stabilise the spin of our world, holding it upright and preventing wild swings in the axis that would throw our stable climate into total disarray. Other crucial factors stem not from the influence of the Moon itself, but are consequences of the violent impact that created it.

The process of plate tectonics, and the constant creation and re-melting of our planet's crust, is thought to be crucial for the recycling of vital elements around the planet, and also for helping regulate the temperature of the Earth to one suitable for liquid water and life. Earth has a very dynamic surface through plate tectonics, whereas Venus, which is practically our identical twin in terms of size, appears to have a single solid plate. The reason for this is thought to be the Earth's thin crust, a direct result of the ancient big-splat event that created our Moon. So but for these key factors, and a handful of others that Gribbin describes, intelligent life on Earth might not have existed, and those factors, in turn, are due to the grace of our Moon.

The problem, as Gribbin points out, is that although Earth-like planets could well be commonplace in the galaxy, Moon-like moons are likely to be very rare. Colossal impacts on to planets are thought to be frequent in the early years of any solar system, but it takes a particularly violent encounter, and, like a snooker shot, the two globes need to strike at just the right angle, to tear off enough material to form a large moon.

Gribbin explains that telescope surveys of young planetary systems have found that only about 4 per cent of them contain large amounts of dust, as would be expected from such a moon-forming impact. Perhaps, then, Earth-like planets are rife throughout the galaxy, but few of them have the large moon that is thought necessary to allow evolution to progress beyond primitive microbial life to the lumbering complex species capable of developing technology and building civilisations.

Many of these arguments concerning the importance of the Moon, as well as other quirky features of our planet or the solar system we reside in, have already been covered (see, for example, Peter Ward and Donald Brownlee's 2000 book Rare Earth: Why Complex Life is Uncommon in the Universe), but Gribbin does an admirable job of bringing the discussion up to date with recent scientific discoveries.

A veteran science writer, Gribbin has published more than 100 books over the past four decades. Indeed, it was reading some of his books as a young teenager that helped fire my own passion for science and sent me down my chosen career track. The Reason Why exhibits the comprehensive research and lyrical writing that admirers of Gribbin have come to expect. For example, when describing the impact into the ocean of an asteroid 10km in diameter that served to finish off the dinosaurs 65 million years ago, Gribbin points out that "the appropriate image is not tossing a pebble into the sea, but dropping a brick into a puddle".

The Reason Why is not without its faults, however. One of Gribbin's pet theories is that three major features of the history of the solar system may all be linked. Venus rotates on its axis exceedingly slowly, and in the opposite direction to the other planets. A second curious feature of this world is that its entire face seems to have been smothered in thick magma flows from global volcanic activity an estimated 700 million years ago.

Another major event is thought to have occurred around this time on our own planet, when an extreme cold spell seems to have frozen over the whole surface of the world, making it "Snowball Earth". Gribbin is of the opinion that these are all too much of a coincidence not to be causally linked. His theory is that a very large object fell into the inner solar system and broke up about 700 million years ago. The largest fragment struck Venus, reversing its spin and triggering worldwide volcanic eruptions, and dust from this fragmentation settled down into the Earth's atmosphere to block out sunlight and force a global ice age.

While such a theory conveniently solves three puzzles of the solar system, there is no real evidence to support it. In fact, there is no reason to suspect that the reverse spin of Venus doesn't date back to the beginning of the solar system, while the volcanic resurfacing of the planet is difficult to date exactly and also could just as likely have been spread over 100 million years rather than occurring in one simultaneous event, as Gribbin supposes. For such a provocative hypothesis, he would have done well to have presented the current knowledge more evenly and with acknowledgement of the uncertainties involved.

A final point may illustrate just how fast-paced the field of exoplanet research - the search for planets orbiting other stars in our galaxy - really is. In his introduction, Gribbin describes the excitement of the discovery of new worlds, and that at the time of writing nearly 400 extrasolar planets were known. This tally dates his writing quite precisely to mid-2009. But we are able to accomplish so much more than simply detect the presence of exoplanets; follow-up telescopic observations can characterise what their environments are like.

Gribbin describes two proposed missions, Darwin and the Terrestrial Planet Finder, which have been designed to analyse the infrared light coming from nearby Earth-like worlds and so read the chemistry of their atmospheres from across the gulf of space. These missions are still on the drawing board, but since Gribbin submitted The Reason Why to his publisher, an exciting development has been announced.

The Exoplanet Characterisation Observatory (EChO) has been shortlisted by the European Space Agency for launch as early as 2020. If it is selected, EChO will be the first dedicated space telescope for studying the atmospheres of worlds beyond our solar system and addressing their suitability for hosting extraterrestrial life.

Minor reservations aside, The Reasons Why is an enormously readable book, and it will leave you with a lot to chew over. I heartily recommend it.

THE AUTHOR

John Gribbin says his interest in astrophysics was sparked at the age of 8 by the science fiction of Astounding magazine and the authors Arthur C. Clarke and Isaac Asimov. He received a bachelor's degree in physics and a master's in astronomy from the University of Sussex, where he is now a visiting research Fellow in astronomy. After completing a PhD in astrophysics at the University of Cambridge in 1971, he went on to write for Nature and New Scientist.

The author of books ranging from In Search of Schrodinger's Cat (1984) to Not Fade Away: The Life and Music of Buddy Holly (2005), he is a keen chess player and an enthusiastic cook. As well as writing science fiction, he has composed songs for Three Bonzos and a Piano (the surviving members of the Bonzo Dog Doo-Dah Band) and follows Kent County Cricket Club. "I would have liked to have been a cricketer but lacked the talent," he says.

In 2009 he received a Lifetime Achievement award from the Association of British Science Writers, and in 2010, the G. B. Lacchini Prize from the Unione Astrofili Italiani.

Chloe Darracott-Cankovic