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Here, I inspect the layout of our own solar system, and consider the extent to which other planetary systems are similar or different. Discoveries so far show that there are many possible layouts, and suggest that quasi-replicates of our system, with four inner rocky planets and four outer giant planets (gaseous or icy) are rare. Every system is different from every other one. This is a consequence of the chaotic process of collisions that leads from a protoplanetary disc to planets. Planets can be found orbiting large, medium, and small stars – with consequences for their maximum lifespans. Some planets are found in binary systems and even in systems with more than two stars. The nearest system to ours – Alpha Centauri – has three stars. This system has at least one planet, which orbits the smallest of the three stars. Planetary systems are also thought to typically contain smaller bodies than planets, as seen in the solar system – moons, minor planets, asteroids, and comets. Life is most likely to occur on planets, but life on moons is also possible. Finally, there are some lone planets that do not orbit a star at all. These are the least probable homes for life.
Here, I start with a key paper on searching for broadcasting civilizations written in 1959, which can be regarded as the theoretical beginning of the age of SETI. We move on from there to the first practical SETI endeavour – Project Ozma. I distinguish between incoming and outgoing signals; the latter are sometimes characterized as ‘active SETI’. Regarding incoming signals, nothing has yet been interpreted as having come from alien life. The LGM signal (little green men) of the 1960s was a pulsar; the ‘Wow’ signal of the 1970s remains enigmatic, but no repeats of it have been observed; the FRBs (fast radio bursts, discovered in 2007) are almost certainly not from biological sources. Regarding outgoing signals, we look at the pioneering Morse and Arecibo messages and the various later ones up to the time of writing (2019). So far, no outgoing signal has been replied to, but this is not surprising since most have not reached their targets yet. Next, I turn to the Breakthough Initiatives, launched in 2015. These include an ambitious plan – Breakthrough Starshot – to send micro-spacecraft to the Alpha Centauri system. I end by briefly looking at ethical issues involved in SETI.
Here, I examine whether some of the exoplanets that we have already discovered might be inhabited. However, I start by cautioning against an overly optimistic stance. Although we now know of thousands of exoplanets, and although the Drake equation estimates of Chapter 12 suggest that millions of planets are inhabited, a quick calculation suggests that of the exoplanets discovered so far, only a few are likely to have microbial life and none to have animal life. Against that background, we look at four planetary systems that are reasonably promising. One of these is the Kepler-186 system, where planet f may be habitable. Another is the Alpha Centauri system, where Proxima b may be habitable. A third is TRAPPIST-1, where there are three potentially habitable planets. The final one is Kepler-452, where planet b may be habitable. Whether any of these planets are actually inhabited will only be answered by particular kinds of observation – most likely spectroscopic studies of their atmospheres. How realistic such studies are depends on the distance to the system concerned. The four systems used as examples here range from 4 to nearly 2000 light years – from doable to quasi-impossible.
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