In 1995, astronomers discovered a planet. Not a new body within our own solar system, but rather the first planet to be discovered orbiting a different star. Since then, many others have been discovered, and it is now accepted that the universe contains numerous planetary systems. Bearing in mind that there are more than 100 billion galaxies, each one containing typically 100 billion stars, perhaps we should not be too surprised. The only reason that we have not found more planets is that they are cloaked in the brilliance of their own suns, so the task of spotting planets is akin to finding a speck of dust on a 100-watt lightbulb on a mountain top several miles away.
The discovery of other solar systems has engendered a growing confidence among the public that life exists elsewhere in the universe. However, all the evidence gathered from the few planets that have been studied suggests that they are inhospitable places. The planets tend to be giant gassy bodies, similar to Jupiter, or they orbit so close to their stars that the surface temperature is searing. Hence, having established the existence of other solar systems, scientists are beginning to ask if it is possible that any of these other solar systems resemble ours and if they might be conducive to life. In Destiny or Chance , Stuart Ross Taylor explains our current understanding of how our solar system was formed in order to evaluate whether similarly fertile solar systems are inevitable or whether we inhabit an exceptional solar system, a fluke that was the result of a series of extraordinary coincidences.
Taylor starts with a historical perspective, giving credit to Pierre-Simon, Marquis de Laplace, who published System of the World in 1796, which marks the beginning of the science of solar systems. Having observed that all the planets in our system lie in roughly the same plane and all orbit in an anti-clockwise direction, he correctly concluded that the sun and planets had emerged from a rotating cloud, the so-called solar nebula.
Today, astronomers are confident that they have identified regions within galaxies where such solar nebulae are created. Vast gas clouds contain dense globules, which break off and condense under the force of gravity. The star is born at the centre of the collapsing cloud, giving rise to increasing heat and pressure, which in turn ignites the nuclear reactions that make the star shine. Although star formation is inevitable, the right type of star, namely a lone star, must form if it is to be the basis of a habitable solar system.
As the gas coalesces, it is highly likely that two stars may form. Such binary systems account for most stars, which is not good news for potential life-forms on orbiting planets. Planetary orbits are far more complex in binary star systems, resulting in seasons that are scorching or chilling, as the planets swing to and away from one or other of the two stars. In contrast, planets orbiting single stars follow a roughly circular path and enjoy a more consistent climate.
Once a single star has been formed, there are several other prerequisites. For example, as the star hots up, it drives off much of the gas in the solar nebula, which means that any large gas planets, such as Jupiter, must form before the gas is expelled or they will not form at all. Jupiter-like planets are themselves unlikely to support life, but they are essential if neighbouring planets are to harbour life. The gravitational force from these planetary giants sucks in asteroids and comets, objects that would otherwise batter smaller, rockier, habitable planets. In our solar system, even with our Jovian big brother to protect us, there is still a risk of asteroid impact, as shown by the demise of the dinosaurs. However, catastrophic impacts are rare, occurring only every few million years, whereas they would occur every few thousand years were it not for Jupiter.
Destiny or Chance is derived from a more scholarly text by Taylor, Solar System: A New Perspective , and he largely succeeds in reworking his ideas into an accessible, readable format. In particular, he includes many historical titbits and a multitude of marvellous facts and figures. For example, some meteorites contain diamond clusters of only 25 atoms, small enough to form the stones if bacteria wore engagement rings.
Taylor concludes by suggesting that our solar system is highly exceptional in having a planet with such a rich biosphere. In other words, despite the millions of other planets that probably exist throughout the universe, the earth may still be unique in having evolved intelligent life.
However, Taylor makes it clear that there are still many aspects of planetary science that are poorly understood, and so this conclusion is tentative. In particular, parts of our earth's history remain shrouded in mystery, and so we cannot accurately determine how easy or difficult it is to build an earth.
In 1492, a meteorite fell near the village of Ensisheim in Alsace; it was stored in the town hall and chained to stop it escaping. The meteorite is still there today, and so the chains were clearly a success. A notice next to the "stone" would be equally applicable to another stone, namely earth: "Many know much about this stone, everyone knows something, but no one knows quite enough."
Simon Singh holds a PhD in physics. His most recent book is The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography .
Destiny or Chance: Our Solar System and its Place in the Cosmos
Author - Stuart Ross Taylor
ISBN - 0 521 48178 3
Publisher - Cambridge University Press
Price - £17.95
Pages - 229