The Science of Discworld IV

nuclear reactions. His calculationsreveal no sign of fine-tuning. Instead, stars exist for a huge range of constants. Choosing these ‘at random’, in the sense usually employed in fine-tuning arguments, the probability of getting a universe that can make stars is about 25%. It seems reasonable to allow more exotic objects to be treated as ‘stars’ too, such as black holes generating energy by quantum processes, and dark matter stars that get their energy by annihilating matter. The figure then increases to around 50%.
    As far as stars go, our universe is not improbably balanced on an incredibly fine knife edge, battling odds of billions to one against. It just called ‘heads’, and the cosmic coin happened to land that way up.
    Stars are only part of the process that equips a universe with intelligent life forms, and Adams intends to look at other aspects, notably planet formation. It seems likely that the results will be similar, debunking the almost infinitesimal chances alleged by advocates of fine-tuning, and replacing them by something that might actually
happen
.
    What, then, went wrong with the fine-tuning arguments? Failures of imagination and blinkered interpretations. For the sake of argument, let us accept that most values of the constants make atoms unstable. Does this prove that ‘matter’ cannot exist? No, it just proves that matter identical to that in our universe can’t exist. What counts is
what would happen instead
, but advocates of fine-tuning ignore this vital question.
    We can ask the same question for the belief that the only viable aliens will be just like us, as many astrobiologists still maintain – though fewer of them than there used to be. The word ‘astrobiology’ is a compound of astronomy and biology, and what it mostly does is put the two sciences together and see how they affect each other. To analyse the possibility of alien life, especially intelligent alien life, conventional astrobiology starts with the existence of humans, as the pinnacle of life on Earth. Then it places them in the context of the rest of biology: genes, DNA, carbon. It then examines ourevolutionary history, and that of our planet, to find environmental features that helped bring life, and us, into existence.
    The upshot is an ever-growing catalogue of special features of our, and Earth’s, history, alleged to be necessary for alien life to exist. We mentioned some of these features earlier; now we’ll discuss some of them in more detail. They include the following conditions. Life needs an oxygen atmosphere. It needs water in liquid form. That implies being at a suitable distance from the Sun – the much-emphasised habitable or Goldilocks zone, where temperatures are ‘just right’. Our unusually large Moon stabilises the Earth’s axis, which would otherwise change its tilt chaotically. Jupiter helps protect us from comet impacts – remember how it sucked up Shoemaker-Levy 9? The Sun is neither too big nor too small, both of which make terrestrial planets less likely. Its rather dull and boring position in the galaxy – not at its centre, but out in the boondocks – is actually the best place to be. And so on and so on and so on. As the list grows ever longer, it is hard not to conclude that life is extraordinarily unlikely.
    An alternative approach, which we like to call xenoscience, reverses the direction of thought. What are the possible types of habitat? We now know, as we did not until recently, that there is no shortage of planets. Astronomers have found over 850 exoplanets – planets outside our solar system – enough to provide a statistical sample that suggests that there are at least as many planets in the galaxy as stars. The physical conditions on those planets vary enormously, but that provides new opportunities for new kinds of life. So instead of asking, ‘Is it like Earth?’ we should ask, ‘Could some form of life evolve here?’
    We’re not even restricted to planets: subsurface oceans on moons whose surfaces are thick layers of ice would be a good place for life, even for Earthlike life. We should take into account local conditions, but we should not assume that features that appear favourable in our solar system necessarily apply elsewhere. Without a large moon, aplanet’s axis may indeed tilt chaotically, but it could do so on a scale of tens of millions of years. Evolution can cope with that; it might even be enhanced by that. Life in a big enough ocean