Abstract
The fact that Earth is teeming with life makes it appear odd to ask whether there could be other planets in our galaxy that may be even more suitable for life. Neglecting this possible class of “superhabitable” planets, however, could be considered anthropocentric and geocentric biases. Most important from the perspective of an observer searching for extrasolar life is that such a search might be executed most effectively with a focus on superhabitable planets instead of Earth-like planets. We argue that there could be regions of astrophysical parameter space of star-planet systems that could allow for planets to be even better for life than our Earth. We aim to identify those parameters and their optimal ranges, some of which are astrophysically motivated, whereas others are based on the varying habitability of the natural history of our planet. Some of these conditions are far from being observationally testable on planets outside the solar system. Still, we can distill a short list of 24 top contenders among the >4000 exoplanets known today that could be candidates for a superhabitable planet. In fact, we argue that, with regard to the search for extrasolar life, potentially superhabitable planets may deserve higher priority for follow-up observations than most Earth-like planets.
Highlights
Earth is our home planet and our only reference scale in regard to life in all aspects
Large differences in the habitability of our planet were present before and after the Great Oxygenation Event about 2.4 billion years ago, whereby the higher oxygenation content played a significant role in the development of aerobic metabolism and with it the advent of complex life (Bains and Schulze-Makuch, 2016)
In regard to suitable stellar properties, we need to consider that different types of stars have different life spans and energy output, and dwarf stars of spectral type G similar to our Sun, referred to as dwarf G (dG) stars, may not be the most suitable host stars for life on their planets because of their relatively short life spans (Kasting et al, 1993)
Summary
Earth is our home planet and our only reference scale in regard to life in all aspects. The natural history of Earth shows that habitability fluctuated quite significantly over geological eons This does refer to major asteroid impacts or other calamities that wiped out large parts of the biosphere, and longlasting Snowball Earth Events, which resulted in subdued biospheres for millions of years (Ward and Brownlee, 2000). We understand multicellular macroscopic life that has similar function to animals, plants, and fungi on Earth With these caveats in mind, in comparing the potential habitability of extrasolar worlds to that of Earth, we use Earth’s habitability as it is today—with all the biomass and biodiversity familiar to us
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