People involved in the modern era of the search for life – from the early days of SETI in the 1960s to the present – have tended to think of some planets as being potentially ‘habitable’ (or ‘inhabitable’, though that synonym is rarely used) and others not. For example, in our own system, Mars might once have been habitable, but Jupiter never so. Why this apparent certainty about Jupiter’s unsuitability for life? There are two main reasons.