Content area
Full text
Author for correspondence: Samuel R. Levin, E-mail: [email protected]
Introduction
There are at least 100 billion planets in our Galaxy alone (Cassan et al. 2012), and at least 20% of them are likely to fall in the habitable zone (Petigura et al. 2013), the region of space capable of producing a biosphere. Even if 0.001% of those planets evolved life, that would mean 200 000 life-harbouring planets in our Galaxy; and it would only take one alien life form for our conception of the Universe to change dramatically. It is no wonder, then, that hundreds of millions of dollars have recently been invested in astrobiology research (Schneider 2016), the USA and Europe have rapidly growing astrobiology initiatives (Des Marais et al. 2008; Horneck et al. 2016), and myriad new work has been done to try and predict what aliens will be like (Benner 2003; Davies et al. 2009; Rothschild 2009; Rothschild 2010; Shostak 2015). The challenge, however, is that when trying to predict the nature of aliens, we have only one sample – Earth – from which to extrapolate. As a result, making these predictions is hard.
So far, the main approach to making predictions about extra-terrestrial life has been relatively mechanistic (Domagal-Goldman et al. 2016). We have used observations about how things have happened on the Earth to make statistical statements about how likely they are to have happened elsewhere. For example, certain traits have evolved many times on the Earth, and so we posit that extraterrestrial life forms will converge on the same earthly mechanisms. Because eye-like organs have evolved at least 40 times (von Salvini-Plawen & Mayr 1977), and are relatively ubiquitous, we predict that they would evolve on other planets, too (Conway Morris 2003; Flores Martinez 2014). Similarly, we have used a mechanistic understanding of chemistry and physics to make predictions about what is most probable on other planets. For example, carbon is abundant in the Universe, chemically versatile, and found in the interstellar medium, so alien life forms are likely to be carbon-based (Cohen & Stewart 2001). These kinds of predictions come from a mixture of mechanistic understanding and extrapolating from what has happened on the Earth. There is no theoretical reason why aliens could not be silicon-based...