Keywords

Introduction

Environmental adaptation is a hallmark of life as we know it on Earth, and the nature of life is such that its evolution anywhere in the universe must occur in a way that generates forms of life adapted to their particular environments. In speculating on the nature of alien life, therefore, we need to consider the environments from which that life has emerged (Schulze-Makuch et al. 2013; Irwin and Schulze-Makuch 2011).

Any definition of life is fraught with philosophical difficulties and subjective judgments, so any essay on the existence or distribution of life will inevitably reflect the definition of life that is assumed. With that in mind, this chapter accepts the broad consensus that at least three conditions must be met for any entity to be regarded as a living organism: It must be (1) a bounded system in thermodynamic disequilibrium with its environment that (2) consumes energy to maintain its low entropic state and perform work and (3) can reproduce itself autonomously (Schulze-Makuch and Irwin 2008).

Our solar system consists of a wide range of planetary habitats – most of which are mirrored by the range of exoplanets discovered to date. While the distribution of planetary types is certainly skewed by the sampling bias responsible for the greater number of discoveries of large planets orbiting near their central stars, still quite a variety of sizes and inferred temperatures have been revealed within the inventory of exoplanets now tabulated (Mendez 2017). These include (with solar system examples in parentheses) cold rocky planetary bodies (Mercury, Mars, Io), warm to hot rocky planets (Venus, Earth), and cold gas giants (Jupiter, Saturn, Neptune). Not yet confirmed but surely existing in other planetary systems are small, icy planets, dwarf planets, and moons (Pluto, Triton, Titan, Enceladus, Ganymede, and many others). The one category of exoplanets not present in our solar system is warm to hot gas giants, orbiting close to their central stars.

Where water exists in liquid form, either on or beneath the surface, life composed of sequestered carbon-based biochemical systems in an aqueous medium has distinct advantages over other forms of life (Schulze-Makuch and Irwin 2004, 2018). The full range of organisms known on Earth – a planet close enough to make effective use of sunlight and chemically rich enough to provide oxidation-reduction cycling as energy sources – constitutes life as we know it. For the remaining habitats, which account for the majority of exoplanets known to date, exotic forms of life unfamiliar to us must be envisaged.

Life on Cold Rocky Bodies

Planets such as Mercury and moons like Io which lack a significant atmosphere are said to be cold because the space above their substrate is always frigid, as is the substrate itself when it faces away from or is distant from the sun. Planets like Mars that are large enough to hold a tenuous atmosphere can be warmer when near enough and facing their central stars. Whether any life can exist on bodies such as these may depend on whether they are able to retain any liquid.

Mars represents the most promising case of the three examples above. Strong evidence suggests that Mars was warmer and wetter at earlier stages of its planetary history (Carr 1996; Kargel 2004); and substantial water is known to lie beneath its surface today (Schulze-Makuch et al. 2005; Titus et al. 2003). Lithotrophic microbes in the substrate that are able to hygroscopically hold on to water may lie dormant in such an environment most of the time, but when their central star warms the surface enough for metabolism to be activated, they could come to life as long as the temperature allows it (Irwin and Schulze-Makuch 2011). Assuming ample time for macroscopic life to emerge early in the history of Mars, descendent forms could still persist in sequestered environments like the lava caves known to be abundant on Mars (Irwin and Schulze-Makuch 2011).

If a planet is as barren and susceptible to extreme temperature as Mercury seems to be, the chances of any form of life would appear to be meager. But a tiny amount of frozen water is known to exist even on Mercury (Slade 1992), so hygroscopic lithotrophs brought out of their dormant state briefly as the terminator (boundary between darkness and daylight) passes through their location long enough to bring the surface temperature into the metabolizable range are a theoretical possibility.

On a volcanically active body such as Jupiter’s moon Io, the periodic sheets of lava that pour across the surface, temporarily heating it enough to support metabolic activity of organisms embedded in the substrate, could serve a similar role to that of the terminator on Mercury, in activating otherwise dormant lithotrophs (Irwin and Schulze-Makuch 2011). On a sulfur-rich world like Io, life based on polymeric chemistry using sulfur-based rather than carbon-based building blocks is conceivable. Silicones, consisting of (R2SiO)n repeating units, are stable at much higher temperatures and could serve as the backbone for biopolymers on which the biochemistry of such lithotrophs could be based (Schulze-Makuch and Irwin 2008). Even more exotically, Feinberg and Shapiro (1980) suggested the possibility of “lavobes,” existing in lave flows, and “magmobes” that could harvest energy from thermal gradients or chemical energy within molten rock. At temperatures cooler than molten lava but well above the tolerable range for exclusively carbon-based polymers, silicate-based zeolites could form semipermeable membranes with selective filtering characteristics comparable to the phospholipid-protein membranes familiar to us (Bowen et al. 2004).

Life on Icy Planetary Bodies

Our solar system includes some worlds warm enough for salty water or water/ammonia mixtures to persist beneath frozen shells and others so cold that hydrocarbons in liquid form are found on their surface. These two conditions pose different challenges for the existence of living organisms, so each will be considered in turn.

Life in Subsurface Aquatic Habitats

This includes planets too cold for water to exist in liquid form on the surface, though it may be liquid beneath an ice shell. Jupiter’s moon Europa is the iconic example of this type of habitat, though recent evidence supports similar habitats on Enceladus, Ganymede, and Ceres (Phillips 2014; Prettyman et al. 2017). With sunlight unavailable beneath the thick ice crust, energy is presumed to be derived from chemical redox reactions. Microorganisms that rely on this form of energy on Earth are well characterized and likely would be recognizable to us. The overall ecosystem, however, would probably be much more static and plantlike than marine life on Earth.

Irwin and Schulze-Makuch (2011) have envisioned several forms of life that could exist beneath the icy crust of Europa that would have few if any analogs on Earth. Producers serving as food at the base of the ecosystem could include lithotrophic (rock-dwelling) microorganisms deriving energy from the oxidation of hydrogen or sulfur, algae coating the undersurface of the ice shell supported by oxygen from water dissociated by radiation at the surface, ciliates on the bottom of the ocean or the underside of the ice shell energized by the kinetic motion of currents, and organisms using alternating cycles of inward and outward ionic flux as they migrate from hyperosmotic benthic regions to the hypoosmotic environment at the top of the ocean. A variety of larger secondary and tertiary consumers could then be envisioned to feed on the producers, and grazing detritivores (scavengers and decomposers) could subsist on dead organisms as they fall to the ocean floor. At the consumer level, life might be vaguely familiar to us, though it would probably be smaller, slower, and more fungal- and plantlike than what we see in our own oceans, due to the persistently lower temperatures of such aquatic environments.

Life in Nonaquatic Habitats

Some planetary bodies too cold for water to exist in liquid form on their surfaces may harbor habitats with pools of hydrocarbons with freezing points much lower than water. The best known case in our solar system is Saturn’s moon, Titan, which has documented lakes and seas of liquid ethane and methane on the surface (Moskowitz 2014; Raulin 2008), beneath a nitrogen-rich atmosphere 1.5 times denser than on Earth (Coustenis and Lorenz 1999).

A dynamic, energy-processing metabolic system based in a solvent other than water is totally alien to the forms of life with which we are familiar. In principal, however, some combination of organic compounds that are liquid at extremely low temperatures could have solvent properties capable of supporting biomolecules and biochemical interactions capable of carrying out the dynamic functions of living organisms. Methanogens possibly analogous to oil-dwelling methanogens found on Earth might exist in the petroleum lakes of Titan (McKay and Smith 2005). Schulze-Makuch and Grinspoon (2005) suggested that metabolic reactions might include the catalytic hydrogenation of photochemically produced acetylene or involve the recombination of radicals created in the atmosphere by ultraviolet radiation. In a similar vein, McKay (2016) proposed that photochemically produced organics, particularly acetylene, could be a source of biological energy by reduction with atmospheric hydrogen. He further suggested that life on Titan could make use of trace metals and other inorganic elements from meteorites and that hydrogen bonding with H2O molecules could serve in a way that metals are used by enzymes on Earth.

Because a semipermeable boundary between the metabolic processes of an organism’s interior and the surrounding environment is considered a fundamental part of the definition of a living organism (Schulze-Makuch and Irwin 2008), consideration of life in a nonaqueous medium often starts with the question of what a stable membrane in a nonpolar solvent would look like. The phospholipid core of biomembranes on Earth would dissolve in the hydrocarbon habitats on Titan, so an alternative chemical structure would be expected for cellular boundaries in hydrocarbon solvents. Nitriles may provide such a structure. Studies of the low-temperature chemistry of compounds like acetonitrile (CH3CN), acrylonitrile (CH2CHCN), cyanoacetylene (HCCCN), and cyanogen (NCCN) suggest that they could function as membrane components in the frigid petrochemical environments on Titan (Hudson and Moore 2004). Molecular simulations by Stevenson et al. (2015) have demonstrated that membranes made with these small compounds have an elasticity in cryogenic solvents equal to that of lipid bilayers in water. As a proof of concept, these workers demonstrated that azotosomes stable at cryogenic temperatures could be made of such membranes.

We have no evidence that such structures exist on Titan or if cellular life making use of them could be assembled into organisms above the microbial level, but their theoretical plausibility enlarges our view of the possibilities for exotic forms of life under conditions alien to life on Earth.

Life in the Atmosphere

Viable representatives of all the major microbial taxa are found in the upper atmosphere of the Earth (Smith 2013). The concept of permanently airborne life on other worlds, therefore, is not implausible. Whether any forms of life larger than unicellular microbes can permanently thrive and reproduce in the atmosphere is an open question. The variety of exoplanets known to date provide both habitats familiar in our solar system, as seen in the case of Venus and the gas giants, and those for which no analog is present in the solar system, such as gas giants orbiting very close to their central stars.

Life in the Clouds of Rocky Planets

Atmospheres beyond trace densities are found on three rocky planets in our solar system: Venus, Earth, and Mars. Venus is enshrouded in a very dense atmosphere of CO2 to an altitude of about 38 km, with a layer of sulfuric acid haze on top of that up to 48 km, and a thick cloud deck of CO2/H2SO4 extending from the top of the haze to about 95 km above the surface (Hunten 1999). The barometric pressure of Earth’s predominantly N2/O2 atmosphere is 1/90th of that on Venus. Mars has a very thin atmosphere of CO2, less than 1% of that on Earth (Barlow 1997).

No forms of life have been detected in the atmospheres of either Venus or Mars, but serious attempts to do so have not been made. Informed speculations about the plausibility of microbial life in the clouds of Venus have been offered for decades (Grinspoon 2003; Irwin and Schulze-Makuch 2001, 2011; Morowitz and Sagan 1967; Sagan 1961; Schulze-Makuch and Irwin 2004). Those clouds arguably provide a more likely habitat for microbial life than Earth’s relatively thin atmosphere, in which microorganisms are known to be abundant. The advantages of the Venusian atmosphere include its density, long-term stability, benign temperatures and pressures at certain altitudes, abundant energy from the sun, reasonably dense water vapor, and oxygenic species like SO2 and O2 in thermodynamic disequilibrium with reducing species such as H2S and H2 (Schulze-Makuch and Irwin 2018). These are circumstances that reasonably could exist on many exoplanets.

Schulze-Makuch and coworkers (2004) proposed a sulfur-based survival strategy for airborne microbes on Venus, in which sulfur allotropes, particularly S8, could serve as a UV sunscreen and an energy-converting pigment or as a means for converting UV light to lower frequencies that can be used for photosynthesis. The extreme acidity of the sulfuric acid/water droplets in the clouds admittedly presents harsh conditions, but extreme acidophiles are known on Earth. Exoplanets with dense atmospheres consisting of less sulfur could be even more amenable to airborne life. An ecosystem of various autotrophic and heterotrophic microbes has been envisioned in the clouds of Venus (Irwin and Schulze-Makuch 2011), providing a possible template for life in dense atmospheres anywhere.

Life in the Clouds of Gas Giants

Unlike rocky planets, which have a hard boundary between their atmospheres and substrates, the gas giants consist of nothing but atmospheric gasses that grade continuously into regions of higher temperature and pressure with increasing depth. Eventually, liquid phases are reached, with a rocky core at the center, but both of which exist at temperatures and pressures incompatible with chemical-based living systems. As on Venus, however, there may be strata at which conditions for living organisms are tolerable.

Sagan and Salpeter (1976) proposed that photoautotrophs metabolizing CH4 could account for the spectral absorption properties in the Jovian atmosphere. They envisioned hypothetical macroorganismic consumers in the form of “thin gas-filled balloons.” They suggested that small powered organisms (“hunters”) could seek one another out and coalesce into “sinkers” and “floaters,” the latter being 1 m to 1 km in diameter. Such an ecosystem would likely depend on the availability of producers deriving energy from photosynthesis or some other energy source, such as the radiation generated by the planets themselves, or some exotic oxidation-reduction cycling dependent on the particular features of the atmospheric chemistry on the planetary body. Another possible source of energy would be the temperature differential between upper and lower strata of the atmosphere. A long, thin, cigar-shaped organism weighted at the bottom could float through the atmosphere in a vertical orientation, tapping the warmth of gasses at the lower level for energy that is harvested as it circulates upward toward the cooler top of the organism.

Challenges for Life in the Atmosphere

In principle, life in a dispersed medium such as an atmosphere should be just as feasible as life in a liquid, like the fresh waters and oceans of Earth. But gas has properties that present greater challenges and fewer advantages than liquids. In atmospheres, the range of temperatures is greater, radiation exposure is more intense, winds – especially on gas giants – are generally more forceful than currents in water, and the lack of buoyancy of living organisms in the lower densities of atmospheres where temperatures are tolerable for biomolecules are all challenges for the persistence of organisms in the atmosphere (Schulze-Makuch and Irwin 2018). While buoyancy of organisms in air could possibly be dealt with through secretion of H2 or another light gas into vacuoles that decrease overall organismic density, as air bladders do in fish on Earth, the other challenges remain.

The greatest difficulty in accepting the plausibility of a robust ecosystem in any planet’s atmosphere is envisioning the evolutionary trajectory that could have led to populating that habitat. On rocky planets, organisms in the atmosphere could be descendants of substrate-dwelling ancestors (Schulze-Makuch and Irwin 2006). On gas giants, however, which had no solid substrate to support the origin of life by any mechanism recognized today as likely, the only plausible way that life could be seeded in the atmosphere would be transport from another point of origin and that life most likely would have evolved in a very different habitat and therefore be poorly adapted for survival in a suddenly very different environment.

Of the two cases discussed here, evolution of life in the atmosphere of a rocky planet seems more probable than life in the clouds of gas giants. Indeed, the proximity of Venus and the possibility of life in its atmosphere are receiving too little attention, in the view of some astrobiologists (Hand 2011). To correct this oversight, specific strategies for a sample collection mission to Venus have been proposed (Schulze-Makuch and Irwin 2002; Schulze-Makuch et al. 2002b).

Non-encapsulated Entities with Lifelike Properties

Growth of inorganic structures in appropriate solvents in the presence of the required chemical building blocks is a lifelike property that long has been suggested as an analog process to prebiotic replication. Inorganic templates which could induce formation of organic polymers in a defined configuration by complimentary binding of organic building blocks to the template are a common feature of speculations about the origin of life (Cairns-Smith 1982; Davis and McKay 1996; Miyakawa et al. 2006).

If a structural feature existed in nature that maintained a consistent, finite shape, was more complex than its surrounding environment and consumed energy to maintain its lower entropic state, and could generate near-exact copies of itself, it would fulfill the minimal requirements for being “alive” (Schulze-Makuch and Irwin 2008). No such structure has been found on Earth – perhaps because organic systems are so much more efficient at degrading energy than less complex mineral-based systems (Beck and Irwin 2016). However, advances in materials science may be shedding light on the possibility that such entities could exist under different circumstances on other worlds.

Complex plasmas may naturally self-organize themselves, based on nontrivial physical mechanisms of plasma interactions, into stable interacting helical structures that exhibit features normally attributed to organic living matter (Tsytovich et al. 2007). Amphiphilic compounds prepared by coupling tailored hydrophilic and hydrophobic branched segments, when injected into water containing appropriate building blocks, can generate a rich repertoire of shapes capable of self-assembly (Percec et al. 2010). Mutually attractive nanoparticles that are deformable owing to flexible surface groups have been shown to spontaneously order themselves into strings, sheets, and large vesicles; and anisotropic colloids with attractive patches can self-assemble into open lattices and the colloidal equivalents of molecules and micelles (Evers et al. 2016). These authors further showed that a modest change in the building blocks can result in much greater complexity of the self-assembled structures. Ben-Zion and colleagues (2017) programmed the self-assembly of micron-sized colloidal clusters with structural information from DNA origami in conjunction with the structural rigidity of colloidal particles, to achieve the parallel self-assembly of three-dimensional microconstructs with highly specific geometries. Wong and co-workers (2016) engineered a unique Janus (two-faced) bilayer architecture of soft materials that generated an efficient transformation of surface energy into directional kinetic and elastic energies. The resulting structure was able to respond to pinpoint water stimuli by a rapid, self-assembly several centimeters in length, reminiscent of the leaflet folding in response to touch by Mimosa pudica. This entity was thus able not only to replicate its structure but respond to an environmental stimulus.

The laboratory successes outlined above suggest that structural entities of appropriate composition under specific environmental conditions can self-assemble and respond to stimuli. While they are not bounded structures in the conventional mode of living cells, they have a finite shape at a lower entropy state than their environment. An interesting feature of these creations is that they appear not to be confined to sizes of a single magnitude; so forms ranging from microscopic to macroscopic could exist. Were they able to self-replicate in the near-exact dimensions of their antecedent structures (embodying the design information that would carry forward into their replicated forms), there would be no obvious basis for distinguishing them from a living organism. Structurally and metabolically – if indeed they can be said to have a metabolism – would be totally alien to living forms on Earth; but they would fulfill the technical definitions of a living organism and could be a form of life on other worlds with very different habitats from our own.

Exotic Forms of Matter with Supraorganismic Lifelike Properties

Brief mention should be made of ideas put forth by serious scientists who have suggested that matter on a planetary scale or beyond can display the characteristics displayed by individual organisms. In order of descending plausibility, they include (1) life as a global entity, (2) life as a formless cloud in space, and (3) intelligent life in binary stars.

The idea that the Earth is a living organism as a whole traces from speculations by Bruno (who also conceived of multiple solar systems and exoplanets) in the sixteenth century. The modern form of this notion was revived by Vernadsky (1997), who argued that the biosphere in its geophysical and biophysical entirety is a living system for absorbing and transforming free energy from the sun. The best known and most elaborated version of this view is the Gaia Hypothesis (Lovelock 1979, 1995), which argues that the Earth in its entirety operates as a living, self-regulating, homeostatic system with properties deriving from and defining the nature of life itself.

First introduced in his 1959 novel, The Black Cloud, Fred Hoyle envisioned a formless mass drifting through space, absorbing energy from starlight, and communicating within itself and with other similar forms through radio waves (Hoyle 1983). He speculated that in such clouds, by analogy with complex animals on Earth, gas would serve as “blood,” pumped throughout the system by an electromagnetic “heart,” with radio waves constituting a neurological system. A~related idea was proposed by Tamulis and associates (2003) in the form of a molecular quantum computing cloud that could absorb electromagnetic energy from planets and stars, compute information, and be moved through space by pressure from light.

Vidal combined thermodynamics and systems theory with extrapolations from the development of civilizations in search of an alternative to organic life as a source of extraterrestrial intelligence. This led him to conclude that some binary stars might actually constitute forms of intelligent life. Since these hypothetical entities would feed on other stars, he named them “stellivores” and presented a thermodynamic argument for their existence, with a metabolic interpretation of their binary interactions.

Fitting all three of these concepts into a conventional definition of a living organism is a challenge. The Gaia Hypothesis has intuitive appeal at a functional level, but the boundaries of the supposed living system are imprecise, and it fails the requirement that a living entity be able to generate a near-exact, independent copy of itself. The formless cloud envisioned by Hoyle as being alive cannot be construed as a living organism precisely because it is formless (unbounded) and lacks a reasonable explanation of how it could arise. In an admission that is understated at best, Vidal (2016) grants that “the jury is still out” on his concept of intelligent life in the form of binary stars but claims that the hypothesis is empirically testable with existing astrophysical data. The scientific community is awaiting such a test.

The thrust of this article is that chemical and even physical systems unknown to us could constitute living organisms on other worlds. But for the concept of “life” to have scientific utility, it must be defined as precisely as possible, and any chemical or physical system hypothesized to be alive must fit within the limits of that definition. While the novel ideas cited in this section represent imaginative examples of dynamic megastructural entities and exotic physics, their failure to fit within the constraints of a bounded system capable of self-replication renders them unrecognizable as a form of life in any meaningful sense (Schulze-Makuch and Irwin 2008).

Conclusions

Exoplanets are now known to occur throughout our galaxy, and presumably the entire universe, in forms both familiar and unfamiliar to us. Within our own solar system, every other planetary body and moon differs, substantially to drastically, from conditions on Earth. The life that almost surely exists on other worlds, therefore, may exist in forms unknown on our planet. A formal definition of life leaves room for solvents, molecular structures, chemical interactions, and morphologies totally alien to our experience. Planets with dense atmospheres could harbor organisms perpetually floating in their clouds. Frigid worlds with subsurface oceans could be home to entire biospheres dependent on energy sources other than light. And worlds so cold that water could never exist in liquid form may contain cellular life encased in membranes of exotic composition, thriving in petrochemical lakes and seas. Even structural entities lacking anything like the internal organization of living cells on Earth could be regarded as novel but nonetheless living creatures by virtue of their structural distinctness from their surroundings and their ability to self-assemble descendants in their own likeness and absorb energy for maintaining their structural complexity and replicating themselves. Any suggestion that the habitable zone around a star is restricted to regions where water can exist as a liquid is therefore incompatible with the full range of possibilities for life in the universe.

Cross-References