I discuss how alternative and classical HZ definitions can complement one

I discuss how alternative and classical hz

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is unlike what is presently on this planet. I discuss how alternative and classical HZ definitions can complement one another and maximize our chances of successfully finding extraterrestrial life. 2. The Classical Habitable Zone This section summarizes the basic theory behind the classical definition. 2.1. Additional Assumptions The classical HZ definition of Kasting et al. [ 1 ] remains the most popular incarnation of this navigational tool, which includes additional assumptions that complement those discussed in the previous section. The classical HZ posits that CO 2 and H 2 O are the most important greenhouse gases for habitable planets throughout the universe. This is based on the idea that a long-term (i.e., over geological timescales) carbonate–silicate cycle maintains the habitability of the Earth, as well as other potentially habitable planets, by regulating the transfer of carbon between the atmosphere, surface, and interior [ 18 ]. The classical HZ is concerned with the detection of both simple and complex life. “Complex life” includes advanced life forms like animals, higher plants, and (possibly) even intelligence. Kasting et al. [ 1 ] explicitly mention that the HZ is designed to also find habitable planets that may be unsuitable for humans. In addition, Kasting et al. [ 1 ] speculate that intelligent life may take at least 1 or 2 billion years to develop (Kasting et al. [ 1 ]), arguing that advanced life forms may be found on habitable planets orbiting F–M stars (e.g., [ 1 ]). As I discuss in Section 6 , however, given the theoretical (e.g., atmospheric, geological, and biological) advances in recent years, an even wider spectral range should be employed if we wish to maximize our chances of finding life elsewhere. Likewise, the classical HZ definition is concerned with carbon-based life. For the liquid water HZ, this assumption is probably reasonable because silicon-based compounds, particularly silanes,
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Geosciences 2018 , 8 , 280 3 of 48 although soluble, can only exhibit the flexibility of carbon-based compounds in very cold environments, like those on Triton or Titan [ 19 ]. Indeed, silicon reactions can be catalyzed in such cold environments even at low sunlight levels, and its versatility has been demonstrated in engineering applications [ 20 ]. However, silicon-based compounds are likely to be less effective in the warmer environments characterized by surface liquid water (ibid). Additionally, all known life requires liquid water to survive. For these reasons, the focus on carbon-based life, which presumes liquid water, is probably reasonable for the near future. 2.2. Effective Stellar Flux The effective stellar flux ( S EFF ) is a key quantity used in HZ calculations and is defined as the normalized flux required to maintain a given surface temperature (e.g., [ 1 ]). It is an expression of energy balance that balances the net incoming solar radiation ( F s ) and the net outgoing radiation ( F IR ), both of which are calculated at the top of the atmosphere (TOA). This leads to the common definition [ 1 ] (Equation (1)): S EFF = F IR F s = S S o (1) Here, S is the flux received by the planet, whereas S o
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