A Complex Life Habitable Zone Based On Lipid Solubility Theory

Rebuilding the Habitable Zone from the Bottom up with Computational Zones

Computation, if treated as a set of physical processes that act on information represented by states of matter, encompasses biological systems, digital systems, and other constructs and may be a fundamental measure of living systems. The opportunity for biological computation, represented in the propagation and selection-driven evolution of information-carrying organic molecular structures, has been partially characterized in terms of planetary habitable zones (HZs) based on primary conditions such as temperature and the presence of liquid water. A generalization of this concept to computational zones (CZs) is proposed, with constraints set by three principal characteristics: capacity (including computation rates), energy, and instantiation (or substrate, including spatial extent). CZs naturally combine traditional habitability factors, including those associated with biological function that incorporate the chemical milieu, constraints on nutrients and free energy, as well as element availability. Two example applications are presented by examining the fundamental thermodynamic work efficiency and Landauer limit of photon-driven biological computation on planetary surfaces and of generalized computation in stellar energy capture structures (a.k.a. Dyson structures). It is suggested that CZs that involve nested structures or substellar objects could manifest unique observational signatures as cool far-infrared emitters. While these latter scenarios are entirely hypothetical, they offer a useful, complementary introduction to the potential universality of CZs.

High pCO2 Reduces Sensitivity to CO2 Perturbations on Temperate, Earth-like Planets Throughout Most of Habitable Zone

The nearly logarithmic radiative impact of CO₂ means that planets near the outer edge of the liquid water habitable zone (HZ) require ∼10⁶ × more CO₂ to maintain temperatures that are conducive to standing liquid water on the planetary surface than their counterparts near the inner edge. This logarithmic radiative response also means that atmospheric CO₂ changes of a given mass will have smaller temperature effects on higher pCO₂ planets. Ocean pH is linked to atmospheric pCO₂ through seawater carbonate speciation and calcium carbonate dissolution/precipitation, and the response of pH to changes in pCO₂ also decreases at higher initial pCO₂. Here, we use idealized climate and ocean chemistry models to demonstrate that CO₂ perturbations large enough to cause catastrophic changes to surface temperature and ocean pH on temperate, low-pCO₂ planets in the innermost region of the HZ are likely to have much smaller effects on planets with higher pCO₂, as may be the case for terrestrial planets with active carbonate-silicate cycles receiving less instellation than the Earth. Major bouts of extraterrestrial fossil fuel combustion or volcanic CO₂ outgassing on high-pCO₂ planets in the mid-to-outer HZ should have mild or negligible impacts on surface temperature and ocean pH. Owing to low pCO₂, Phanerozoic Earth's surface environment may be unusually volatile compared with similar planets receiving lower instellation.

Habitability Models for Astrobiology

Habitability has been generally defined as the capability of an environment to support life. Ecologists have been using Habitat Suitability Models (HSMs) for more than four decades to study the habitability of Earth from local to global scales. Astrobiologists have been proposing different habitability models for some time, with little integration and consistency among them, being different in function to those used by ecologists. Habitability models are not only used to determine whether environments are habitable, but they also are used to characterize what key factors are responsible for the gradual transition from low to high habitability states. Here we review and compare some of the different models used by ecologists and astrobiologists and suggest how they could be integrated into new habitability standards. Such standards will help improve the comparison and characterization of potentially habitable environments, prioritize target selections, and study correlations between habitability and biosignatures. Habitability models are the foundation of planetary habitability science, and the synergy between ecologists and astrobiologists is necessary to expand our understanding of the habitability of Earth, the Solar System, and extrasolar planets.