In the next few years, the number of catalogued exoplanets will be counted in the thousands. This will vastly expand the number of potentially habitable worlds and lead to a systematic assessment of their astrobiological potential. Here, we suggest a two-tiered classification scheme of exoplanet habitability. The first tier consists of an Earth Similarity Index (ESI), which allows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time. The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature. For the second tier of the classification scheme we propose a Planetary Habitability Index (PHI) based on the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent. The PHI has been designed to minimize the biased search for life as we know it and to take into account life that might exist under more exotic conditions. As such, the PHI requires more detailed knowledge than is available for any exoplanet at this time. However, future missions such as the Terrestrial Planet Finder will collect this information and advance the PHI. Both indices are formulated in a way that enables their values to be updated as technology and our knowledge about habitable planets, moons, and life advances. Applying the proposed metrics to bodies within our Solar System for comparison reveals two planets in the Gliese 581 system, GJ 581 c and d, with an ESI comparable to that of Mars and a PHI between that of Europa and Enceladus.
There exists a positive correlation between orbital eccentricity and the average stellar flux that planets receive from their parent star. Often, though, it is assumed that the average equilibrium temperature would correspondingly increase with eccentricity. Here we test this assumption by calculating and comparing analytic solutions for both the spatial and temporal averages of orbital distance, stellar flux, and equilibrium temperature. Our solutions show that the average equilibrium temperature of a planet, with a constant albedo, slowly decreases with eccentricity until converging to a value 90% that of a circular orbit. This might be the case for many types of planets (e.g., hot-jupiters); however, the actual equilibrium and surface temperature of planets also depend on orbital variations of albedo and greenhouse. Our results also have implications in understanding the climate, habitability and the occurrence of potential Earth-like planets. For instance, it helps explain why the limits of the habitable zone for planets in highly elliptical orbits are wider than expected from the mean flux approximation, as shown by climate models.Subject headings: stars: planetary systems, planets and satellites: fundamental parameters, planetary habitability, equilibrium temperature, habitable zone, eccentricity.
Rational speculation about biological evolution on other worlds is one of the outstanding challenges in astrobiology. With the growing confirmation that multiplanetary systems abound in the universe, the prospect that life occurs redundantly throughout the cosmos is gaining widespread support. Given the enormous number of possible abodes for life likely to be discovered on an ongoing basis, the prospect that life could have evolved into complex, macro-organismic communities in at least some cases merits consideration. Toward that end, we here propose a Biological Complexity Index (BCI), designed to provide a quantitative estimate of the relative probability that complex, macro-organismic life forms could have emerged on other worlds. The BCI ranks planets and moons by basic, first-order characteristics detectable with available technology. By our calculation only 11 (~1.7%) of the extrasolar planets known to date have a BCI above that of Europa; OPEN ACCESSChallenges 2014, 5 160 but by extrapolation, the total of such planets could exceed 100 million in our galaxy alone. This is the first quantitative assessment of the plausibility of complex life throughout the universe based on empirical data. It supports the view that the evolution of complex life on other worlds is rare in frequency but large in absolute number.While life is known to exist with certainty only on Earth, there are compelling reasons for assuming that it could exist throughout the universe in abundance [1][2][3]. Organic molecules have been found in star forming regions [4], around protoplanetary disks [5], in meteorites [6], in comets [7], and in deep space [8]. Water is among the most common molecules in the universe, and a host of other liquids can exist at planetary temperatures [9]. Besides an abundance of light and heat in all stellar systems, many other forms of energy are locally available on probably most planetary bodies [10-14]. Thus, the prerequisites for life are commonplace throughout the cosmos. In response to the intuitive logic that life is likely to be found on other worlds like the one world where it is known to exist, the search for other worlds that could harbor life has emphasized searching for planets similar to Earth in geophysical properties and in relation to their central stars. A growing list of such planets has been confirmed [15], including Gl581c and d [16,17], GJ667Cc [18-20], Kepler-62e and f [21], HD40307g [22], and HD85512b [23]. Atmospheric modeling studies have been conducted for both , confirming the potential habitability of the former. As the number of known exoplanets has grown, the need for quantitative measures of their similarity to Earth has become apparent. As one approach to meeting this need, we have proposed an Earth Similarity Index, or ESI, which rates the similarity of extrasolar planets to Earth on the basis of mass, size, and temperature [28].There is also, however, broad if not universal agreement that life could exist in forms quite dissimilar from those on Earth [3,29,30]. Even on our plan...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.