A speculative but intriguing discussion that sometimes crops up when talking to people engaged in exoplanetary science goes like this; let’s suppose that we find an unmistakably terrestrial style planet around a relatively nearby star (less than about 30 light years away), perhaps even around one of the Alpha Centauri members, a touch over four light-years distant. Let’s further suppose that – possibly with the James Webb Space Telescope, or a next-generation ground-based super ‘scope – we gather evidence for an atmosphere and even find big chemical clues that there could plausibly be a biosphere on this world. What do we do next?

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