Published online by Cambridge University Press: 22 May 2020
The habitable zone is the circumstellar region in which a terrestrial-mass planet with an atmosphere can sustain liquid water on its surface. However, despite the usefulness of this concept, it is being found to be increasingly limited in a number of ways. The following is known: (i) Liquid water can exist on worlds for reasons unrelated to its specific distance from a star. (ii) Energy sources can exist for reasons unrelated to the distance to a star. Furthermore, the habitable zone is based on both astronomy – the distance and stellar energy – and chemistry – liquid water and the right temperature. However, these factors are only part of the consideration. Thus, discussions of habitability and the possibility for the emergence of life on a world must consider the evolutionary principles that govern life as well as the laws that govern stellar and planetary science. This is important because the following is also known: (iii) The temporal window for the emergence of life is within 600 million years. (iv) The Earth was an extreme environment overall in the period when this window existed. (v) The first life was necessarily fragile. Therefore, chemical evolution must have taken place in a relatively protected and restricted environment. Thus, rather than as in the Goldilocks zone, which focuses too narrowly on the world as a whole, this paper suggests that it is better to focus on a particular region and time period on a world, in which conditions fit for habitability exist. Thus, the following is suggested: ‘The Goldilocks Edge is a spatial and temporal window on an astronomical body or planemo, where liquid solvents, SPONCH elements and energy sources exist’. Furthermore, since the mere presence of these does not necessarily lead to the emergence of life, this possibility only arises when these interact. Thus, it will also be suggested that the following can exist within the Goldilocks Edge: ‘The great prebiotic spot is an environmentally relaxed and semi-shielded area on an otherwise extreme world, wherein conditions advantageous for chemical evolution exist’. Such Goldilocks Edges and the great prebiotic spots within them can collectively represent the full distribution of possibilities for life in a solar system. Thus, the concept of a ‘limbus mundi’ or ‘world's edge’ is introduced.