A double review originally written for the Literary Review
Ed Jim Al-Khalili
All these worlds are yours
In 1995, for the first time a planet was discovered orbiting a bright star other than the Sun. Now, several thousand “extrasolar” planets are known, some of them rocky worlds not much bigger than the Earth, orbiting their parent stars at distances which mean that liquid water could possibly exist on their surfaces. The fact that they are in the “habitable zone” does not necessarily mean that they are habitable. Venus, for example, is a roughly Earth-sized rocky planet orbiting in the habitable zone of our Sun, but that has not stopped it developing a runaway greenhouse effect, losing all its water into space, and becoming a searing hot desert. Nevertheless, the discovery of potential “other Earths” has made the subject of astrobiology respectable, even though astrobiologists do not actually have any astrobiology to study as yet. This provides fertile ground for speculation about the nature of the kind of life that might exist on other worlds, some of it more sensible than others.
Two very different books about the possibility of life elsewhere in the Universe, and how and where we might find it, cover a large part of this spectrum of ideas. Aliens is a collection of 19 short essays (plus an Introduction) squeezed into a mere 219 pages; a mixture of the good, the bad and the indifferent, but all pretty superficial. All These Worlds Are Yours is unashamedly “pop”, but tells a coherent story drawing on a lecture course given by the author, a Professor of Astronomy at the University of Victoria, British Columbia. The former includes a chapter on aliens in science fiction (good) and one on alien abductions (indifferent), which gives you some idea where it is coming from. The latter has an impressively concise and reasonably accurate account of the place of the Earth and Solar System in the Universe, then concentrates on the chances of finding extraterrestrial life among the Sin’s family of planets.
Both books address the question of how we might detect signs of life on distant planets, and point out that we are just developing the technology to identify gases such as oxygen in the atmospheres of such planets, using spectroscopy. Oxygen would be an obvious sign of life, because it is highly reactive and only persists in the atmosphere on Earth because it is constantly being manufactured by living things; if all life on Earth died tomorrow, all the oxygen would be gone in a couple of million years. But neither of them gives due emphasis (although Aliens does at least mention it in passing) to the underlying point, developed by James Lovelock, which is that life involves non-equilibrium processes, in chemical terms, graphically described as operating “on the edge of chaos”. It isn’t the presence or absence of oxygen, per se, that matters, but the presence or absence of stable chemical equilibrium. The atmosphere of Mars, for example, is a stable equilibrium (almost entirely carbon dioxide) which is a clear indication that searching for life there is a waste of time.
The worst of the bad in Aliens picks up on a silly blunder made by Fred Hoyle, who once suggested that the universe is not big enough nor old enough for complex cellular life to have been produced by chance alone. This unfortunate comment has given succour to creationists who, like Hoyle, miss the point that evolution by natural selection does not occur by chance alone (the clue is in the word “selection”), and that this operates from the moment a single self-replicating molecule appears. Many biologists have pointed this out, and Jim Al-Khalili should be ashamed to have allowed the howler to pass his editorial inspection.
On a happier note, Jon Willis gives due emphasis to the discovery that many complex molecules, the precursors of life, exist in clouds of gas and dust in space, and in comets. Indeed, one of the last observations made by the comet probe Rosetta revealed the presence in a comet of amino acids, which are the building blocks of proteins. Such molecules must have rained down on the early Earth, and it is also now clear that bubbles analogous to the membrane walls of living cells form spontaneously from solutions containing certain simple chemicals. A proto “cell” containing proto life molecules could (and probably did) emerge almost as soon as the Earth cooled. The rest is simply a matter of reproduction and natural selection. But as Matthew Cobb points out in Aliens, while it is highly likely that life exists elsewhere in the universe, it is highly unlikely that there is our kind of intelligence “out there”. There are two many so-called bottlenecks (such as the kind of impact that did for the dinosaurs) on the road from single cells to people like us.
But even if we restrict our ambitions to the search for life, discarding our ambitions of making contact with alien civilizations, if the cosmic rain led to life on Earth, why not on the other planets and moons of the Solar System? It turns out that moons are the best bet, not least because a few of them have abundant water, and the meat of Willis’ book deals with the prospects of finding life on the moons Europa, Enceladus and Titan, which orbit giant planets in the outer reaches of the Solar System. If he had a budget of $4 billion, Willis says, he would spend most of it on a probe to bring back samples of water from the moon Enceladus, where jets of water are “constantly vented into space, and the technology exists to sample it and return [it] to Earth”. Where does the figure of $4 billion come from? It is, says Willis, the amount the world spends on so-called defence each day. “Defense from what? One another”. If our priorities were right, we could tackle the question of life elsewhere with ease. Which makes it a pity that he used a quote from Arthur C. Clarke for his title. David Bowie would be more apposite. We could be heroes — just for one day.
Dr John Gribbin is a Visiting Fellow in Astronomy at the University of Sussex, and author of The Reason Why: The miracle of life on Earth (Penguin)