The Science of Discworld IV

wouldn’t even notice. A large gas giant may sweep up comets, but that could slow evolution down, because the occasional catastrophe adds variability. Jupiter may keep comets at bay, but it greatly increases the number of asteroid impacts on the Earth. The current best estimate suggests that Jupiter has done more harm than good, with regard to life. Some life forms such as tardigrades (commonly called waterbears or moss piglets) resist radiation better than most of those on our planet. The rest don’t need to, because the Van Allen belts, regions of electrically charged particles maintained by the Earth’s magnetic field, keep radiation away. In any case, if the belts hadn’t been there, life could have become more tardigrade-like.
    The so-called habitable zone is not the only region around a star where life might be possible. Some exotic chemical systems can make life-like complexity possible without water, and liquid water
can
exist outside the habitable zone. For example, if a world close to its star is tidally locked, so that one side perpetually faces the star and the other faces away, there will be a ring-shaped twilight zone on the boundary between the two faces, where liquid water might exist. Worlds far from the star can have liquid oceans underneath an outer coating of ice: Jupiter’s moon Europa is the best-known example in the solar system, and it is thought to have an underground ocean containing as much water as all of Earth’s oceans put together. The same goes for Ganymede, Callisto and Saturn’s moon Enceladus. Titan – another moon around Saturn – has liquid hydrocarbon lakes and an excess of methane, hinting at non-equilibrium chemistry, a possible sign of unorthodox life.
    The idea of a galactic habitable zone – the claim that alien life can exist only in the region of the galaxy with enough heavy elements but not too much radiation – is especially controversial. The Danish astronomer Lars Buchhave and his team have surveyedthe chemical composition of 150 stars, with 226 known planets smaller than Neptune. The results show that ‘small planets … form around stars with a wide range of heavy metal content, including stars with only 25 per cent of the sun’s metallicity’. So an excess of heavy elements is
not
required for Earthlike planets. NASA scientist Natalie Batalha remarked that ‘Nature is opportunistic and prolific, finding pathways we might otherwise have thought difficult’.
    And so on and so on and so on.
    Life adapts to its environment, rather than the other way round. Goldilocks doesn’t have the final word: Daddy Bear and Mummy Bear have valid opinions too. What is ‘just right’ for life depends on what kind of life. So-called extremophiles exist on Earth at temperatures below freezing and above boiling. It’s a silly name. To such creatures, their environment is entirely comfortable; it is
we
who are extreme. It’s even sillier to use the same name for creatures in two environments so different that each creature would consider the other to be even more extreme than us.
    The second approach is far more sensible: instead of successively cutting down the opportunities for life, it explores the full range of the possible. That vast and impressive shopping list of features ‘necessary’ for life, making humans seem extremely special, is poor logic. Life on Earth demonstrates that the list is
sufficient
– but that doesn’t make it
necessary
.
    These two ways of thinking about aliens are of course yet another example of Benford’s dichotomy. Astrobiology is human-centred, because it starts from us and narrows the universe down until it fits. Xenoscience is universe-centred: it keeps possibilities as broad as possible and sees where they lead. We are beautifully adapted for our environment because we evolved to be like that. This observation is much more reasonable than claiming that we humans are so special that the solar system, the galaxy, even the entire universe, was constructed in order to accommodate
us
.
    Cosmic balance …
    Is life really balanced on a knife edge, then? Or have we got it all wrong?
    Let us go back to our rod and sharp knife experiment. It seems undeniable. Try again to balance the rod on the cutting edge of the knife. However carefully you place it, it tips and slides to the floor. There is no question: the balance has to be extraordinarily precise.
    The mathematics is, if anything, even more compelling. The masses on each side,