The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,129

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, a planet’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 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 surveyed the 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.