Requirements and limits for life in the context of exoplanets

Abstract

The requirements for life on Earth, its elemental composition, and its environmental limits provide a way to assess the habitability of exoplanets. Temperature is key both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems. Life can grow and reproduce at temperatures as low as −15°C, and as high as 122°C. Studies of life in extreme deserts show that on a dry world, even a small amount of rain, fog, snow, and even atmospheric humidity can be adequate for photosynthetic production producing a small but detectable microbial community. Life is able to use light at levels less than 10 −5 of the solar flux at Earth. UV or ionizing radiation can be tolerated by many microorganisms at very high levels and is unlikely to be life limiting on an exoplanet. Biologically available nitrogen may limit habitability. Levels of O 2 over a few percent on an exoplanet would be consistent with the presence of multicellular organisms and high levels of O 2 on Earth-like worlds indicate oxygenic photosynthesis. Other factors such as pH and salinity are likely to vary and not limit life over an entire planet or moon.extremophiles | Mars | astrobiology T he list of exoplanets is increasing rapidly with a diversity of masses, orbital distances, and star types. The long list motivates us to consider which of these worlds could support life and what type of life could live there. The only approach to answering these questions is based on observations of life on Earth. Compared with astronomical targets, life on Earth is easily studied and our knowledge of it is extensive--but it is not complete. The most important area in which we lack knowledge about life on Earth is its origin. We have no consensus theory for the origin of life nor do we know the timing or location (1). What we do know about life on Earth is what it is made of, and we know its ecological requirements and limits. Thus, it is not surprising that most of the discussions related to life on exoplanets focus on the requirements for life rather than its origin. In this paper we follow this same approach but later return briefly to the question of the origin of life. Limits to LifeThere are two somewhat different approaches to the question of the limits of life. The first approach is to determine the requirements for life. The second approach is to determine the extreme environments in which adapted organisms-often referred to as extremophiles-can survive. Both perspectives are relevant to the question of life on exoplanets.It is useful to categorize the requirements for life on Earth as four items: energy, carbon, liquid water, and various other elements. These are listed in Table 1 along with the occurrence of these factors in the Solar System (2). In our Solar System it is the occurrence of liquid water that appears to limit the occurrence of habitable environments and this appears to be the case for exoplanetary systems as well.From basic thermodynamic considerations it is clear that life ...

Fundamental drivers for endolithic microbial community assemblies in the hyperarid Atacama Desert

“…We also observed differential light transmission across wavelengths for all rocks, with higher transmission in the infrared, which is in agreement with red wavelengths penetrating deeper than blue wavelengths (McKay, ). Spectral tuning of photosystem II has been reported in Cyanobacteria (Croce and van Amerongen, ) and experimentally demonstrated with Synechococcus variants in microbial mats from hot springs (Becraft et al ., ), suggesting a potential adaptation to low quantum flux densities for endolithic Cyanobacteria .…”

Hyperdiverse archaea near life limits at the polyextreme geothermal Dallol area

“…Finally, we detect active silica encrustment/fossilization of cells but also abiotic biomorphs of varied chemistry in samples where there is no life. Our work has implications to circumscribe planetary habitability (11,12), raising warnings for the interpretation of morphological biosignatures on Earth and beyond.…”

Is the Traditional Pedologic Definition of Soil Meaningful in the Modern Context?

“…Our ideas concerning processes on other heavenly bodies are constantly being challenged and adjusted. Grotzinger et al (2014) recently reported on a relatively young, likely habitable environment on Mars that could have supported an ecosystem based on chemolithoautotrophy, and McKay (2014) reported that the surface of Mars has adequate carbon to support life. Levin and Straat (2014) actually contend that missions to Mars have found proof of living microorganisms on that planet.…”