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Unlike the other forms of speculative biology, exobiology offers a complete freedom in design, being uncostrained by the history of evolution on Earth. However, while the organisms featured in exobiological projects can show a great variety in form and function, they rarely are fundamentally different from those we see on Earth, which is not surprising, Earth life being the only one that we know of. Yet it's possible to conceive life of very different kinds, with a different chemical composition or even different physics, and able to live in environments where no terrestrial organism could ever survive.

Definition of life

The condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, functional activity, a form of genetic material, and continual change preceding death.

Limits of familiar life

Main article: Extreme environments

While life as we know it has proven itself extremely versatile, there are physical limits for the survival of carbon-water organisms. The most obvious one is temperature: if it gets too high, thermal energy can dissolve the bonds between atoms, breaking molecules apart; for example, mitochondria stop working at 45°C. Some bacteria and archaea survive using only proteins that withstand higher temperatures, and water can even be kept liquid above 100°C if it's under enough pressure. Even the most adapted carbon-based systems, very different from those used by life on Earth, would probably fall apart before reaching 500°C.

No such limit exists for low temperatures, though any life that uses a liquid solvent (see here for life that doesn't) would necessarily be impossible when it freezes solid. Many solvents, including water, can be kept liquid at a lower temperature than usual by mixing it with ions or other chemicals; for example, a liquid water/hydrogen peroxide mixture is speculated to exist under the surface of Mars.

Here's a brief summary of the most extreme conditions in which known life forms can survive:

Extremes of survival [ 1 ] [2, page 31-32]
Condition Limit Organism
Heat 121°C Methanopyrus (Archaea)
Cold -15°C various bacteria
Acidity pH 0 Picrophilus (Archaea)
Alkalinity pH 13 Plectonema (Eubacteria)
Pressure 1000 atm Halomonas (Eubacteria)
Acceleration 403 627 g Escherichia (Eubacteria)
Salinity 35% salt Halobacterium (Archaea)
Radiation 11 000 Gy Deinococcus (Eubacteria)
Toxic metals 2.5 mM (Ni), 5 mM (Cd),
12 mM (Zn), 20 mM (Co)
Ralstonia (Eubacteria)

Exotic biochemistry

Main article: Alternative biochemistry

All life on Earth is surprisingly homogeneous from a biochemical point of view. Every organism known to us is primarily built with carbon-based molecules that also contain hydrogen and oxygen, mainly proteins, fats and carbohydrates; short-term energy storage occurs in the phosphate-phosphate bond of ATP, and long-term storage in the carbon-oxygen double bonds (C=O) of complex carbohydrates; information is stored in the sequences of nitrogenous bases of DNA and RNA; chemical exchanges occur through liquid water; of the 92 natural elements, only few others play a role in known biochemistry (sulfur in certain aminoacids, calcium in shells and bones, iron in blood, magnesium in chlorophyll, sodium in nerve cells, etc).

However, it's likely that very different arrangements of atoms could sustain life elsewhere in the Universe. Just working on the planets and moons of the Solar Systems, exobiologists have conceived of microscopic acidophiles living in droplets of sulfuric acid in the clouds of Venus, algae surviving in a mixture of water and hydrogen peroxide on the surface of Mars, abyssal thermotrophs in the dark ocean of Europa, hydrogen-breathing methanogens in the hydrocarbon lakes of Titan, silicon-based life in pools of liquid nitrogen on Triton, and many others. Some have replaced carbon with silicon or with boron, water with ammonia or hydrogen fluoride or liquid methane (see the article linked above for more details).

The argument given above for extreme temperatures can be expanded to different biochemistries: Xenology argues with Carl Sagan that life based on certain chemical bonds can exist up to a temperature where the random thermal motion of atoms becomes enough to break over the 0.0001% of these bonds. The table on the linked page contains the energy value for many different chemical bonds measured in eV; the breaking temperature in K can be approximated as 1000*E, where E is the energy measured in eV.

Chemistry based on the very weak hydrogen bonds could still be stable up to 400 K (130°C), and the even weaker van der Waals interactions could survive up to 40 K (-230°C), though this doesn't necessarily mean that such chemistry would be enough to sustain life. Conversely, many important chemical bonds survive up to thousands of degrees, but again this doesn't mean they could form a complex biochemistry.

Different states of matter

Despite the solid appearance of most Earth organisms, most of their volume is actually liquid water. Biochemistry requires a liquid solvent - be it water, ammonia or sulfuric acid - for chemical reactions to occur in, as it would permit the motion of particles while at the same time keeping them relatively concentrated. However, if we hold a clear separation between the space inside and outside an organism as a necessary feature of life, then some sort of solid boundary seems inevitable.

Solid-state life

See here, page 77.

While the diffusion of chemicals through liquids is definitely faster and easier, it's still possible for it to occur through a solid matrix. Of course, any life that relied on pure solid-state chemistry would have extremely slow rhythms (metabolism, perception, reproduction etc.) by terran standards, but this isn't necessarily a concern over the lifespan of a planet. Clathrates, substances composed by a solid lattice that contains isolated molecules in its gaps, would be a useful building block for solid-state life.

One can conceive of living crystals that absorb new atoms from the ground and energy from sunlight or other sources, reproducing by gemmation. Crystals of potassium hydrogen phthalate are known to transmit information through generations, with "daughter crystals" inheriting the cuts inflicted on "mother crystals": impurities such as motes of dust could thus produce inheritable variations that would allow evolution, perhaps adapting so that the molecular structure is less likely to be modified by impurities or more likely to benefit from it. A slow locomotion might be accomplished by dismantling old parts of the crystal and rebuilding it elsewhere. Given the high resistence of crystals based on covalent bonds, life forms such as these might be active even at a temperature of thousands of degrees.

Liquid-state life

In a porous medium (mostly solid, but rich in very small pores and channels), such as pumice stone, living globs of liquid could move by capillarity, perhaps controlling the direction of movement by altering their local density and contact angle.

Even in a liquid medium, life composed entirely by liquid matter could still retain its shape and size by exploiting surface tension, the contractive force occurring on the surface of liquid bodies. Two immiscible liquids can coexist, with the least abundant one being dispersed as microscopic bubbles within the other (an emulsion, or something like micelles): this could be accomplished with a polar solvent (e.g. water, ammonia, sulfuric acid) in a non-polar thalassogen (e.g. methane, chloroform, nitrogen) or with a non-polar solvent in a polar thalassogen. At the origin of life on Earth, it's likely that bubbles of lipids in water were early precursors of cells.

Another possibility, which still require a solvent immiscible with the medium, is floating on the top of it; if the liquid organism is not large enough to cover all the medium, it would form a film only a single molecule thick. Such an organism could move forward by secreting surfactants that decrease surface tension behind itself.

Gas-state life

Also see here, page 77.

The main handicap of hypothetical gaseous life is the high dispersion of molecules, which makes harder for them to interact and to stay close without dissipating in the environment; the presence of solid grains could help keeping some molecules together and act as a catalyst for reactions. The Black Cloud, a 1957 novel by the astrophysicist Fred Hoyle, describes a huge molecular nebula acting as a living organism in space, where it performs biological functions through the manipulation of magnetic fields and electrically charged dust part. Unencumbered by the gravity of a planet, the cloud can grow to such a size that it's held together by its own gravity, thus avoiding dissipation, while still having a very low density; since the fragmentation makes its surface area so much bigger than its volume, it can sustain itself by absorbing light from the stars[1].

Different fundamental forces

Also see here.

All the life forms described above, based on carbon or silicon, in water or in ammonia, crystalline or gaseous, base their physiology on electromagnetic interactions: cells communicate through the flow of ions (electrically charged particles); they are composed by atoms, which are bound to each other by electrical attraction; energy is stored, in chemical form, in these bonds.

Electromagnetism is one of the four fundamental forces or interactions, together with gravity, the strong nuclear force and the weak nuclear force. If we define life in the broadest sense - an entity that processes energy to mantain its structure and copy itself - one can conceive very different sorts of life based on different physical interactions.

Electromagnetic life

All chemistry can be described as a system of electromagnetic interactions, but properly electrical and magnetic phenomena are not unknown in familiar life: for example, electroception and electrical synapses. Hoyle's "black cloud" described above lacks true chemistry, but it performs its biological functions by manipulating charged particles with magnetic fields.

Nuclear life

Strong nuclear life

The strong nuclear force, which acts between the particles in the nucleus of an atom, is by far the strongest of all the fundamental forces, and it acts on extremely small and fast scale. It's similar to electromagnetism, but rather than two charges (positive and negative) it's based on three "colors" (red, blue and green) with the respective anticolors, carried by quarks; it takes all three colors or all three anticolors to make a neutral particle. Different types of quarks are combined into larger particles called hadrons, which are also called mesons when they're made up by a quark and an anti-quark, and baryons when they're made up by three quarks. Protons and neutrons, which form atomic nuclei, are two kinds of baryons. Other particles, gluons, act as carrier of the force, in the same way that electrons and photons are carriers of the electromagnetic force.

There are several kinds of hadrons, and they could take the role of atoms and molecules on a scale millions of times smaller, while gluons would bind them together or move from one to the other, thus carrying energy. On such a scale, masses are so small that they can be significantly altered by the transfer of energy: hypothetical nuclear organisms would store energy converting it directly into mass, creating quarks, which they'd then destroy in order to release it again. Also, the strength of the strong nuclear force doesn't vary at all within a range of 10-15 m, 25 000 times smaller than a hydrogen atom, but over that distance it stops working altogether: thus, any nuclear "biochemistry" would have to occur within that scale. Hadrons are not as diverse as atoms, but given that most Earth biochemistry uses only a few elements this isn't necessarily a problem.

The best known nuclear organisms are the cheela from Robert Forward's Dragon's Egg (1980): they live on the surface of a neutron star, 20 km wide, composed by compressed iron nuclei and free neutrons, with mountains a few cm tall. The cheela are 5 mm long and only 0.5 mm high; their body, composed entirely by atomic nuclei, is so flattened because of the star's strong gravity. Since nuclear reactions are so much faster than chemical reactions, their metabolism and perception is also much faster: the lifespan of a cheela is about 40 minutes. They appear just 5000 years after the origin of life on the neutron star, and their entire civilization develops in a few days.

Weak nuclear life

The weak nuclear force is, obviously, weaker than the strong one, and it works only in a range a hundred times smaller. It's also far less likely than it, and than electromagnetism, to produce biological systems, as it's the only fundamental force that can't hold matter together. It works through the exchange of W and Z bosons between fermions, a class of particles that includes quarks, electrons, and neutrinos; it's involved in phenomena such as radioactive decay and nuclear fusion.

Gravitational life

Gravity is by far the weakest of all the fundamental forces, and it's not much more promising than weak nuclear force, as it's based only on mass, which can never be negative, and therefore it can only attract and not repel. Hoyle's "black cloud" is held together by gravity because of its sheer size, but all its physiology is of an electromagnetic nature. Hypothetical gravitational life would probably be extremely large, with very slow processes, and it would absorb energy by the gravity field of stars and planets, benefiting from the abundance and high efficiency of this form of energy conversion.

Life in other Universes

Life with different physics

In a 2010 issue of Scientific American, the physicists Alejandro Jenkins and Gilad Perez have outlined possibilities for the development of life in hypothetical Universes where the laws of physics are different from those we know.

In the first case, the weak nuclear force doesn't exist. If the absence of the weak force were the only difference from our Universe, regular nucleosynthesis would be impossible; however, if the ratio of matter to antimatter right after the Big Bang had been different, some form of nucleosynthesis at the beginning would still occur. It would produce deuterium (an isotope of hydrogen the nucleus of which contains a proton and a neutron) rather than regular hydrogen (which lacks neutrons); stars could fuse deuterium and free protons to form helium-3.

These stars would be smaller and cooler than the Sun, and they're expected to last about 7 billion years, but they could still produce carbon, oxygen and nitrogen. Supernovae would also be impossible, but the elements could still be dispersed in space by thermonuclear explosions. Elements heavier than iron would be extremely rare. Planets would have to be closer to their stars, and since their core wouldn't contain radioactive elements such as uranium, which release heat with their decay, they would have to be much bigger in order to have tectonic activity. None of these differences precludes life.

In other Universes, they change the mass of quarks. There are six kinds of quarks, called "flavors": two of them are light and stable (up and down, they make up baryons such as protons and neutrons), and the other four are heavier and tend to decay into the lighter flavors.

  • In our Universe, the down quark is twice as heavy than the up, and neutrons are thus slightly heavier than protons; if the masses were inverted, making protons heavier, the lightest stable element would be again deuterium. In this case, our carbon-12 would be replaced by carbon-14.
  • If the strange quark, one of the unstable flavors, were about as heavy as the up, and thus stable, protons wouldn't exist: they'd be replaced by oher baryons, called Σ- ("sigma minus"). They would still form stable elements such as "sigma hydrogen" and "sigma carbon", and even in this case chemistry wouldn't be much different.
  • If the up were the only light quark, a baryon called "delta" would be the only possible, and the only element, having a delta for a nucleus, would be "delta helium". No chemistry would be possible.
  • If up, down and strange were all stable and of similar mass, there would be eight possible baryons. Elements such as regular hydrogen and sigma hydrogen would exist, but there wouldn't be any stable analogue of carbon or oxygen. This diversity might be promising for nuclear life, though.

In The Gods Themselves (1972), Isaac Asimov imagines life in a Universe where the strong nuclear force is a hundred times weaker than it is in our Universe. There, nuclear fusion is much faster, and therefore stars are smaller, colder and shorter-lived (they last only a few million years). Some of its organisms are composed by matter where atoms are farther apart, allowing them to change shape and density and to overlap with each other's bodies.

n-dimensional life

An even more unusual proposal for exotic organisms is life in a different number of dimensions other than the three of our Universe. Life in a 1-dimensional (linear) Universe, or any kind of chemistry, is probably impossible, as each particle could only interact with its immediate neighbours, much less in a 0-dimensional Universe, a point where only one particle could exist at all. However, a 2-dimensional Universe should allow for interactions complex enough for life. We can also try to imagine some features of life in a 4-dimensional Universe, but any number of dimensions greater than that is probably too alien to be imagined in significant detail.

2-dimensional life

Physics in a 2-dimensional Universe would obviously be radically different from the one we're accustomed to in many ways. For example, in a 3-d space the force of gravity decreases proportionally to the square of the distance: this leads satellites to move in an elliptical orbit, as ellipses are described by a quadratic equation (one with a squared variable). Yet, stable orbits are still possible in a 2-d plane: they would trace a vaguely star-shaped hypotrochoid (see here, figure 2), of which the ellipse is a special case.

Light and other electromagnetic radiations would also be very different, as in our Universe they have two components oscillating in planes perpendicular to each other; they might be replaced by something similar to particle beams. Also, fluids, if they exist at all, would probably appear denser and more viscous, as they have less directions in which to be displaced by objects, and thus would exert more resistence.

2-dimensional fish

A 2-dimensional fish-like creature.

There are yet more direct obstacles to life itself: for example, a tube or channel going through a body from end to end would cut it in half. However, the transportation of matter through the body could be accomplished by a flow of vesicles, which could carry food inside and waste outside, and even merge together to form organs. In this thread on the forum, the user Kain proposes organisms composed by two jagged ribbons that fit into each other like a zipper, pushing "water" backwards. Also, intelligence could be severely limited by the fact that 2-d neurons would have less neighbours to connect with, but in his The Planiverse (1984), A. K. Dewdney proposes crossovers that allow signal-carring wires to cross each other, thereby increasing the possible connections.

The square-cube law would presumably still apply, even though it would become a "line-square law": for example, the body mass would be determined by its area, and the strength of bones and muscles by their width. 2-dimensional "animals" would probably have an external skeleton, and an open circulatory system with no blood vessels, but only heart(s) that keep fluids in movement: this is because internal bones and closed tubes couldn't pass around each other.

4-dimensional life

Also see: Four-dimensional space

Space in four dimensions, or even simple physics - let alone life - is not easy to think about. Yet, there are some features of 4-dimensional life that we can imagine by extrapolating what we know about the relation between the 3-d space and the 2-d plane. For example, gravity would be inversally proportional to the 3rd power of distance, while fluids would probably be much thinner and less viscous, as particles would have more space to be displaced into.

Computed tomography of human brain

CAT scan of a human head. This is what it would look like to a 2-d observer.

Consider what a 3-dimensional organism, such a us, would look like to a 2-d observer if we were passing through its plane. It could see only a thin slice of our body, something like a CAT scan; but since it couldn't look at it top-down, it would only see it covered in skin in all the directions. As we move through space, our 2-d section would appear to change size and shape, and even to merge or to split in the points where limbs meet the trunk.

In the same way, if a 4-dimensional organism were to pass through 3-d space, we'd most probably see an amorphous blob growing, shrinking, changing size, merging and splitting in apparently unconnected appendages - but in reality, what we'd see is only a series of sections of its body. Furthermore, it would appear entirely covered in skin, or whatever integument is found on it, while its organs, if we were to cut it open, would appear completely closed and without openings. Just like we could see every internal detail of a 2-dimensional creature, like the 2-d fish above, the 4-d organism could see inside our bodies, and inside every closed container.

Examples

Notes and references

  1. The black cloud is more precisely composed of plasma, a state of matter similar to gas in which particles have a non-neutral electric charge.

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