Review on the Role of Planetary Factors on Habitability

Habitability Is Binary, But It Is Used by Astrobiologists to Encompass Continuous Ecological Questions

The term "habitability" is pervasive throughout the space sciences and astrobiology literature and is broadly used to describe an environment's ability to support life. Here, we argue that, while it is fundamentally a binary matter whether an organism can persist in an environment or not, these binary assessments lead to continuous ecological measurements that are often collected under the umbrella term "habitability" by astrobiologists. Although the use of habitability in this way has provided a framework for those studying the potential of environments to support life, including comparative analyses between terrestrial and extraterrestrial environments, it can also generate confusion and limit interdisciplinary understanding. Namely, differing ecological metrics used as proxies for habitability can yield differing conclusions depending upon the metrics chosen. Therefore, we suggest that in this continuous sense, the terms habitable and habitability lose meaning unless the specific scientific question and biological metric chosen to address it are defined. As a corollary, the search for universal single metrics to make habitability assessments is not to be encouraged, and as we argue, attempting to do so would oversimply analyses of the ability of environments to support life.

The Astrobiology of Alien Worlds: Known and Unknown Forms of Life

Most definitions of life assume that, at a minimum, life is a physical form of matter distinct from its environment at a lower state of entropy than its surroundings, using energy from the environment for internal maintenance and activity, and capable of autonomous reproduction. These assumptions cover all of life as we know it, though more exotic entities can be envisioned, including organic forms with novel biochemistries, dynamic inorganic matter, and self-replicating machines. The probability that any particular form of life will be found on another planetary body depends on the nature and history of that alien world. So the biospheres would likely be very different on a rocky planet with an ice-covered global ocean, a barren planet devoid of surface liquid, a frigid world with abundant liquid hydrocarbons, on a rogue planet independent of a host star, on a tidally locked planet, on super-Earths, or in long-lived clouds in dense atmospheres. While life at least in microbial form is probably pervasive if rare throughout the Universe, and technologically advanced life is likely much rarer, the chance that an alternative form of life, though not intelligent life, could exist and be detected within our Solar System is a distinct possibility.

Which Type of Planets do We Expect to Observe in the Habitable Zone?

We used a sample of super-Earth-like planets detected by the Doppler spectroscopy and transit techniques to explore the dependence of orbital parameters of the planets on the metallicity of their host stars. We confirm the previous results (although still based on small samples of planets) that super-Earths orbiting around metal-rich stars are not observed to be as distant from their host stars as we observe their metal-poor counterparts to be. The orbits of these super-Earths with metal-rich hosts usually do not reach into the Habitable Zone (HZ), keeping them very hot and inhabitable. We found that most of the known planets in the HZ are orbiting their GK-type hosts which are metal-poor. The metal-poor nature of planets in the HZ suggests a high Mg abundance relative to Si and high Si abundance relative to Fe. These results lead us to speculate that HZ planets might be more frequent in the ancient Galaxy and had compositions different from that of our Earth.