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

Abstract

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.

The Case (or Not) for Life in the Venusian Clouds

“…They advised that the original authors should not have evoked it, but by doing so, Greaves et al [ 2 ] caused a lot of media hype. However, as pointed out above, Venus likely had oceans on its surface in the past [ 9 ] and if life did not originate on Venus independently, it could have been transported from Earth into another habitable environment on Venus [ 13 ]. Therefore, I consider it reasonable that life existed on Venus at some point in the distant past, the more difficult hypothesis to defend is whether it could have adapted to the currently existing environmental conditions in the lower atmosphere.…”

“…Ever since Morowitz and Sagan [ 1 ] suggested that life may exist in the lower Venusian atmosphere, many authors looked into that possibility [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 ] and made incremental progress understanding the environmental conditions in the Venusian atmosphere, and the possibility of it being inhabited by microbial life. The presence of an aerial biosphere may seem strange to an observer from Earth, because on Earth, the atmosphere appears to serve only as a temporary habitat [ 12 ], as reproduction has not been demonstrated in Earth’s atmosphere [ 13 ]. This assessment is further supported by a study from Amato et al [ 14 ], which indicated the lack of evidence for bacterial cell division in the atmosphere through meta-transcriptomics analysis.…”

Evaluating the Microbial Habitability of Rogue Planets and Proposing Speculative Scenarios on How They Might Act as Vectors for Panspermia

“…Thus, nutrient cycling could occur within a rogue planet even without plate tectonics, which is the major recycling mechanism on Earth. When considering possible life on another world, including a rogue planet, we have to be open-minded for novel adaptations of how life might have dealt with the environmental challenges at hand [ 47 ], but for a Steppenwolf-type of a rogue planet, the icy moons in our solar system may be a first suitable analog model.…”

“…If so, then a putative organism could move between different layers of salinity and each way being able to harvest energy. Enough energy could be in principle gained to support an ecosystem [ 19 , 47 ].…”

Microbiota entrapped in recently-formed ice: Paradana Ice Cave, Slovenia

“…In recent years, cold habitats such as glaciers and ice sheets 2 , sea ice 3 , lake ice 4 and permafrost 5 have received most attention as regards microbiological studies, mainly because of their sensitivity to climate change. Also, there is a growing research interest related to frozen ecosystems as analogues of extraterrestrial habitats 6 . Even if the study of these habitats is supported strongly by the rapid development of new technologies (which shed more light on the gene presence, functional gene potential, gene expression, in situ identification of active microorganisms and biotechnological potential) more fundamental ecological research is needed urgently.…”