I introduced exoplanets – planets beyond our solar system – in Chapter 1, discussed them in the context of planetary systems in Chapter 2, and considered them in relation to the concept of habitability from a general perspective in Chapter 4. But so far I’ve given little detail about them. How many exoplanets, or exoplanetary systems, have I mentioned so far? Very few. That’s about to change, but not in the sense of replacing a dearth of detail with a wealth of it. Rather, I’ll be very selective about the particular exoplanets I discuss. This is essential, given that the number now known is huge, and most of them are irrelevant to the search for alien life.