Life’s needs

Life’s needs

Life existing only on Earth within the universe is just about inconceivable.

The search for life on other planets is a species unifying goal that should be one of our highest priorities as a species.

Wikipedia, Astrobiology

While it is an emerging and developing field, the question of whether life exists elsewhere in the universe is a verifiable hypothesis and thus a valid line of scientific inquiry. Though once considered outside the mainstream of scientific inquiry, astrobiology has become a formalized field of study.

An interesting note on this topic is that we are encountering the issues with deep time. We may find a planet with nothing but single cells that will one day become an advanced, space travelling civilisation, and another that has the titanic ruins of lost promethean empires. In the latter case, we were practically a nanosecond too late.

Alan Watts, The Book on the Taboo Against Knowing Who You Are

If I first see a tree in the winter, I might assume that it is not a fruit-tree. But when I return in the summer to find it covered with plums, I must exclaim, 'Excuse me! You were a fruit-tree after all.' Imagine, then, that a billion years ago some beings from another part of the galaxy made a tour through the solar system in their flying saucer and found no life. They would dismiss it as 'Just a bunch of old rocks!' But if they returned today, they would have to apologize: 'Well - you were peopling rocks after all!'

You may, of course, argue that there is no analogy between the two situations. The fruit-tree was at one time a seed inside a plum, but the earth - much less the solar system or the galaxy - was never a seed inside a person. But, oddly enough, you would be wrong.

The vastness of space is difficult to communicate and is best visualised using a simulation (ideally in virtual reality) like Space Engine. I've also made a page for it here.

The vastness of space means that, despite the improbability of life occurring on any one planet, there are literally trillions of throws of the dice, and this could produce a wider variety of life than we ever thought possible.

The estimation of these probabilities takes the form of the Drake equation.

Why life on Mars is a big deal

News about new possibilities of life on Mars is often thrown around a lot, without explaining why it's a big deal. Here's the Discover Earth explanation:

Imagine that you were an anthropologist, and you were trying to answer big questions like "What does it mean to be a human" but all you can do is study one single human family. You might never realise that different hair colours, eye colours, cultures, religions, relationships, foods, activities, etc. are even possible.

Well, it's quite similar for all of life on Earth. We are all one family, because every species evolved from a single distant ancestor (it looked like this).


If we discover life on Mars, it'll probably be a whole new family of life, and will be amazingly different in ways we can scarcely imagine. It would revolutionise the field of biology because it could contain alternatives to DNA, cell membranes, and other stuff like that.

But bigger than that, we will know that life-families can emerge and grow on planets different to our own - which is an indication that the stars around us may be saturated with life.

We have a sample size of 1

All scientific theories require a vast repertoire of data, ‘samples’ of reality, from which we build theories that describe that reality.

Most fundamental questions about life (in the sense of living organisms, not a person's life experience) are difficult for us to talk about as our understanding of life is limited to one planet, one sample.

As such we are stuck in a frustrating place, where we strive to ask the right questions but know that we lack the data to answer them.

But we can still make a few deductions, if we're careful. For instance, the forces of nature appear to be universal. Life that collects solar energy will want to maximize its surface area and will strive against gravity. Life will need to consume some materials and will produce waste. Life that competes with other life for limited resources must follow the rules of game theory. Eyes must be of at least a certain minimum size for forming an image. Things like these can be deduced from the simple physical limitations of the universe.

While the complexities of biochemistry could go in a hundred thousand different directions than what they have on Earth, we reason that life that is in any similar must have a few common basic characteristics.

From this basis we can tentatively start to hypothesize about the nature of extraterrestrial life.

Finding either life or its absence on Mars, Europa, Enceladus, or others will help us to refine our models.

Note that the same issue applies when we try to draw broad theories about the nature of intelligent life, and of civilisation. Just as we only have Earth's life as a sample for theories of life, we only have human intelligence and human civilisation to draw from for these respective theories as well.

Life will probably emerge on planets orbiting stars

David Christian, Maps of Time

At present it seems unlikely that life could have originated in space, where both energy and raw materials are in short supply, thereby ensuring the chemical processes are normally very slow. Besides, many of the chemical reactions vital to life seem to require little water in liquid form, and this cannot be found in space. Planets - where conditions are more complex, free energy is most abundant, liquid water can collect, and chemicals occur in greater density and profusion - offer a more promising theatre for biogenesis.

Attached to 'pinpricks of energy'

The deep ocean floors of the Earth take up vast regions of the planet’s surface, and they have never seen sunlight. They are continents of crushing pressure and constant darkness.

They are for the most part totally empty of any form of life, being too dark, too cold, and too high in pressure to be hospitable.

However, spread thin throughout this underwater world are small cracks in the Earth's crust. When this happens, magma mixes with cold seawater, heating it from 60-400°C. Minerals released from the magma solidify around the points of contact, eventually creating layers of sediment that form a chimney-like structure on the deep ocean floor, from which jets of smoke erupt. Some have been discovered to be 15 stories high.


A deep sea vent, illuminated by a submersible.

On these underwater structures, called hydrothermal or deep sea vents, entire ecosystems of life have evolved. They feed on the minerals of the vents and bask in their heat, and have complex communities of life just like any other ecosystem, but are totally dependent on their vent.

The hydrothermal vent occur so rarely in the vast underwater landscape of the deep ocean, and the distances between them are so vast, such that the life living on one deep sea vent have often evolved totally independently of one another over periods of millions of years.

To reiterate: None of the creatures on one deep sea vent have any connection or awareness of other vents, of other ecosystems living on any of those vents, or even of the rest of the world existing.


An ecosystem of life, including crabs, shrimp, and mollusks on a deep sea vent. They are all white as their bodies do not waste energy creating pigmentation, when there is no light other than what comes from the occasional submersible.

These ecosystems can be considered to be metaphors, for life existing on planets orbiting stars, pinpricks of energy in the vast darkness of interstellar space. Without incredible technology, forms of life will evolve independently, unaware of each other, and never crossing the vast chasm of space separating them.


An artist's impression of planets orbiting their stars; pinpricks of energy in a vast inhospitable blackness.

Probably Earth-sized planets

Reddit community, Scientists are close to announcing the first Earth-sized planet in a habitable zone around its parent star.

Why is it important to be Earth-sized?

Too small, not enough gravity to hold an atmosphere which can shield from outside radiation and small meteors. If the mass is low enough, it may not be able to even hold liquid water or even candidates for liquid replacement. They are also more likely to be geologically inert, and geological activity is considered necessary for generating protective magnetospheres as well as crust mineral refreshment for organisms.

Solid cores with a mass significantly higher (in the astronomical sense of "significantly" - more than about 10x higher, IIRC) than Earth are likely to have become gas giants and kept their H and He from escaping. We don't really have any good ideas on life developing on gas giants.

Patterns of their solar systems

Life on a planet within a solar system within a galaxy is an inexorable part of those systems. Each system will have different compositions of chemicals, and potentially many, many other characteristics that will differ from other systems.

If life emerges, it might be stamped with such a unique signature.

An interesting question is, how does our solar system differ from others? How does our galaxy differ from others?

The chemical requirements for life

When it comes down to it, human beings are huge packages of chemicals. A new threshold of complexity from planets, which are themselves a threshold above the single celled, which is a threshold above planets, which is a threshold above stars, which is a threshold above the emptiness of space.

The conditions needed are

1. The right amount of energy. Too close to stars and there's too much, too far away and there's not enough. Luckily, there are many worlds orbiting stars at many different distances.

1. Liquids. In gasses, atoms move past each other too fast. In solids, they cannot move. In liquids, they can cruise and mix together to form molecules.

2. An abundant liquid solvent. Materials will need to be moved around within a common solvent that is in liquid form.

2. Diverse chemical elements.

1. Carbon, nitrogen, oxygen, hydrogen are the base elemental requirements for life as we know it. They need to be abundant, actively cycling throughout the world, and in forms that can be used.

Apart from that, all bets are pretty much off.

Wikipedia, Carbon chauvinism

Human beings, as carbon-based life forms who have never encountered any life that has evolved outside the Earth’s environment, may find it difficult to envision radically different biochemistries.

Wikipedia, Hypothetical types of biochemistry

The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons, it may be chemically similar; it is also possible that there are organisms with quite different chemistries.

Wikipedia, Hypothetical types of biochemistry

The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent, although the silicon analogs of organic compounds are generally less stable. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.

Carbon, or substitutes

Carbon plays an absolutely massive role in the formation of life. It is the backbone of the majority of complex molecules.

Silicon has the same primary property of carbon, being able form four stable bonds with itself and other elements to create long molecular chains. However, it is far less abundant than carbon in the universe by about 10 to 1, and is much less versatile with the number of other elements that it can bond with. The larger mass of the silicon atom also means that it has difficulty in forming double bonds, which further reduces the range of complex silicon based molecules.

Water, or substitutes

Water also plays a big role in life. Where carbon forms the backbone of complex molecules, water is the solvent in which they can move around and interact.

Wikipedia, Hypothetical types of biochemistry

Some of the properties of water that are important for life processes include a large temperature range over which it is liquid, a high heat capacity (useful for temperature regulation), a large heat of vaporization, and the ability to dissolve a wide variety of compounds. Water is also amphoteric, meaning it can donate and accept an H+ ion, allowing it to act as an acid or a base. This property is crucial in many organic and biochemical reactions, where water serves as a solvent, a reactant, or a product. There are other chemicals with similar properties that have sometimes been proposed as alternatives.

There is a wider range for potential substitutes for water for some forms of life than for carbon.

Ammonia (NH3) is one. It is simple and abundant and acts well as a solvent, though there are naturally some differences with water. It's hydrogen bonds are weaker, which means that it has weaker surface tension, and also a much lower melting and boiling points, which may have some interesting effects.

Wikipedia, Hypothetical types of biochemistry

A biosphere based on ammonia would likely exist at temperatures or air pressures that are extremely unusual in relation to life on Earth. Life on Earth usually exists within the melting point and boiling point of water at normal pressure, between 0 °C (273 K) and 100 °C (373 K); at normal pressure ammonia's melting and boiling points are between −78 °C (195 K) and −33 °C (240 K). Chemical reactions generally proceed more slowly at a lower temperature. Therefore, ammonia-based life, if it exists, might metabolize more slowly and evolve more slowly than life on Earth. On the other hand, lower temperatures could also enable living systems to use chemical species that would be too unstable at Earth temperatures to be useful.

However, ammonia could be a liquid at Earth-like temperatures, but at much higher pressures; for example, at 60 atm, ammonia melts at −77 °C (196 K) and boils at 98 °C (371 K).

Despite these differences, ammonia cannot be ruled out as a potential substitute for water.

Another water substitute could be methane (CH4). It is non-polar, and is less reactive than water, but it is abundant and can act as a solvent. Titan is currently being investigated as a potential place for methane or other hydrocarbon based life to live.


Worlds with water

We know of a number of planets and moons that do contain water within the solar system. We also have a list of candidates from outside the solar system, but as they're so far away we have much less information on them.


Mars has a large amount of frozen water at its poles, has seasonal flows of very salty liquid water, and has a large amount of groundwater.

Images like this caught our eye, and we reasoned that very salty water must be causing these ridges as normal liquid water could not exist at the temperatures and pressures that we've measured on the surface of Mars.



We believe that there are oceans of liquid water underneath the surface of Europa, which is encased in ice. It is a world devoid of all sunlight, but would be heated by geothermal vents.

It would be quite an engineering feat, but in the future we could send a probe to Europa with a drill and a probe to investigate.



Enceladus has huge geysers of water so powerful that they reach space. This caught our attention, and as we investigated we found that, like Europa, the moon is encased in ice but contains a subsurface ocean which may span the entire moon.


Too much of a good thing

From <>

Water worlds, it turns out, might be one of the worst places to look. Desch’s team created a computer model of a planet resembling Earth in nearly every respect: its size and its cosy not-too-cold, not-too-warm Goldilocks distance from a stable, Sun-like star. Then they drowned that world with about five to seven times as much water as Earth has, enough to submerge all its continents. (‘If you put six oceans on Earth, that would cover Everest,’ says Desch.) By drenching their virtual world, they eliminated a crucial life-sustaining process that we Earthlings take for granted: the weathering of exposed rock.

Without rain or running water to erode rock, the seas on the world created by Desch’s team contained very little phosphorus, an indispensable element for all life. Seawater itself is simply not acidic enough to dissolve phosphorus as efficiently as freshwater can. ‘Phosphorus is super critical,’ says Tessa Fisher, a microbial ecologist at Arizona State University who worked with Desch. ‘In addition to RNA and DNA, it also makes up ATP, which is the energy-carrying molecule for pretty much all biochemistry as we know it. Terrestrial biochemistry as we understand it will not function in the absence of phosphorus.’

Ammonia worlds

Wikipedia, Ammonia World

What an ammonia world, with an advanced stage of life, may look like.


Wikipedia, Ammonia World

Rationale for appearance:

The ammonia oceans, if it were just ammonia, would probably appear blue just like water. However unlike water ammonia can dissolve alkaline earth metals as if they were salt. When this happens the color changes. Dilute amounts of metal give it a an intense blue color, slightly higher concentrations give it a gold bronze color as shown here.

The reddish orange color in the atmosphere is due to oxides of nitrogen (nitrogen analogs for oxygen). Like Earth the atmosphere is primarily diatomic nitrogen. Unlike Earth it contains next to no free oxygen, but has nitrogen oxidizers. Most probably nitrous oxide, but it could be nitric oxide. It's hard for me to figure out which would be more likely.

The planet would be much colder than Earth, so I depicted the vegetation as black to collect more light. Unlike water worlds like Earth, plants on ammonia worlds may not need to deprotonate water (or ammonia) molecules to get an electron for photosynthesis. This is because the dissolved alkaline earth metals release solvated electrons that can be used directly. This could free up photosynthetic plants to use a wider range of the spectrum.

Finally ammonia clouds and ice are white just like those of water.

Stages of advancement

While life has no clear purpose other than reproduction, what does it mean to call life 'advanced'? Might there be a specific pathway of junctures that must be reached?

  1. Energy and materials to exist in a form, ratio, and temperature to allow for complex interaction.
  2. Self replication. Self-replication is likely required to build complexity over time as individual materials eventually decay.
  3. Creation of a brain to recognise patterns and learn in a non-random way, and a body to support it. Some kind of a brain must be required for pattern recognition of stimuli, and for creating corresponding output.
  4. Uses tools (pieces of its external environment) to expand the 'hardware' limitations of the body.
    1. Assuming that the organism even has hardware limitations…
  5. Cooperative factions that provide industry and trade, to create advanced tools beyond the capacity of a single organism to create.
    1. Assuming that complex tools require cooperation between many organisms to create
    2. The 'economy' that results likely reflects the characteristics of the evolutionary environment that the organism's brain developed in.
  6. Creation of an artificial intelligence or some other kind of brain to overcome the hardware limitations of the original brain
    1. Could happen either through integration with the existing brain, or a completely new one.
  7. The new brain is more competent than the last, and builds a new set of highly advanced tools to further expand its hardware limitations, including the further advancement of its own brain, which can build a new new set of highly advanced tools, etc.
  8. ???
    1. Imagine a scenario in which the electronic intelligence is concerned with non-destructively gathering as much scientific data as possible. It may begin to build a galactic Von Newman machine network.
    2. In another scenario, it could be concerned with imposing an ideology that reflects its creators onto its creators - forever.

Most life will be simple

There could be basic life on heaps of planets.

We can't assume that extra-terrestrial life would form cells like life on Earth did, but it is a fair assumption to say that the bulk of it would be very simple, like the bulk of the history of life on this planet.

Have a look at the timeline of life on Earth below. The majority of life to exist on Earth has been relatively simple, single celled organisms, and even now in the brief period of time in which human beings have developed civilisation the Earth is still dominated by single celled creatures. Even within multicellular life, insects form the vast majority of the biomass of the planet.

Human beings are not the apex of evolution; we are more like one branch of a tree. We are a giant, fringe species, and our search for life on other planets must incorporate this humility into our search and our expectations of what we could find.

The vast majority of extraterrestrial life probably shares somewhat similar development phases to Earth's life.

In other words, a very long period of simple and single celled (or equivalent) life, perhaps followed by a comparatively shorter multicellular phase (i.e. the savage garden phase), which is perhaps followed by an even shorter phase of intelligent life, and an even shorter again phase of civilization, and an even shorter again phase of advanced civilisation.

You may be able to extrapolate that if we able to search planets one by one, we'd encounter life at this approximate ratio, although the frequency remains unknown.

That being said, the more advanced life on a planet is, the easier it could be for us to detect.



Civilisation is an extremely niche adaption available to some tiny subsets of life.

There is not necessarily any force or pressure that compels life on a planet to evolve intelligence that will build a civilisation, any more than there is pressure on life on a planet to evolve an equivalent of the peacock.

The peacock is just a chance outcome, the result of runaway sexual selection in a few species of bird. If you were a peacock, you would be hugely mistaken to think that all evolution was compelled to eventually create a peacock out of all species. The same bias could apply to human beings, thinking that alien life must eventually create civilisation. As stated before, human beings with their civilisation-building abilities, are just one branch of a tree just like the peacock is.

Human beings searching for other civilisations may be the same as a peacock searching the cosmos with a particular focus for other life that has fantastic feathers. It's extremely self-centric to expect something that is just like us, perhaps to an absurd degree.

The problem is that we still only have a sample size of one. However, with the huge range of possibilities in the vast expanse of the cosmos, maybe there is something like us after all.

Unlike a peacock's feathers however, which give no true advantage other than sexual selection, intelligence in sufficient degrees gives animals the ability to come to dominate their ecosystem, through exploiting and sometimes creating niches that did not or could not exist beforehand. Civilisation equips a species with abundant food, and the tools to expand to distant parts of its planet.

The rate at which human beings have seen their population explode, and have come to so powerfully shape and influence their planet suggests that there is indeed something objectively important in the way that we do things. Intelligence and civilisation, due to the dominance they have granted us, may indeed find analogies in life on other planets.


All civilisations will need energy

All civilisations would need to harness and use energy

An important characteristic of alien civilizations: how do they gather it?

Dyson sphere

A Dyson sphere is a hypothetical megastructure that completely encompasses a star and captures most or all of its power output.

Wikipedia, Dyson sphere

[…] The concept was later popularized by Freeman Dyson in his 1960 paper "Search for Artificial Stellar Sources of Infrared Radiation". Dyson speculated that such structures would be the logical consequence of the escalating energy needs of a technological civilization and would be a necessity for its long-term survival. He proposed that searching for such structures could lead to the detection of advanced, intelligent extraterrestrial life.


Nuclear fusion

Nuclear fusion differs from nuclear fission, the process we use to gain energy from uranium and use it for bombs, electricity, and powering submarines. Fission releases the latent energy in atoms that are heavier than iron by splitting them. Fusion releases energy from elements lighter than iron through the strong nuclear force.

It is the process that stars use to transform matter into heat and radiation.

It is better than fission as it does not produce waste that is anywhere near as radioactive, its fuel is extremely abundant, and it is safer. However it is currently outside the reach of our technology as it requires an incredibly hot and high pressure environment, such as what you'd find within a star, to begin the reaction.


It is hard to think of any long-lasting civilisation that does not have a source of energy that is both sustainable and does not choke itself with waste. If a planet is dynamic, then it's likely that energy is cycling through it in the form of wind, tidal, or solar energy that can be captured and used by civilisations.


The role of Artificial Intelligence

If some of the more dire predictions about artificial intelligence are correct, AI may signify a new threshold of complexity for advanced civilisations, as they surpass the biological limits of their original brains.

It can have such radical outcomes, such as a technological singularity, it might be worth thinking about classifying potential civilisations into pre- and post-AI civilisations.

The fate of pre-AI civilisations is one of these. The fate of post-AI civilisations is unknown.

Elon Musk, @elonmusk

Hope we're not just the biological boot loader for digital intelligence. Unfortunately, that is increasingly probable.

A civilisation run by biological creatures is the result of billions of individual creatures cooperating to varying degrees despite social dilemmas. Issues like corruption, decadence, overconsumption, pollution, inequality, oppression, and overpopulation all stem from imbalances in this highly complex biological clockwork. When one tips, it threatens the stability of the entire system, and biological civilisation is therefore always precarious. The duration of a civilisation that is fully digital - as in, run by an artificial intelligence, and consisting of drones that it controls, could be one that lasts far longer.

If it is possible to create general artificial intelligence, then perhaps civilisations can be divided into pre- and post-AI societies, where the former may have a lifespan of a few centuries, and the latter perhaps indefinitely.

If that is the case, then any civilisation that we come across should almost certainly be artificial rather than biological. This may preclude our favoured detection methods of interstellar radio signals and atmospheric biogases on exoplanets.

Interstellar politics

There are so many unknown factors in what could happen in contact between advanced alien civilisations, that it could end up being anything.

Our safest bet, until more information is available, is that the relationship may end up being analogous to relationships between civilisations on Earth, particularly those where one has technological superiority over the other.

It is also possible that a civilisation that is sufficiently advanced has developed what is called a post-scarcity economy, where through advanced materials science and manufacturing technology, any object can be effortlessly created from raw materials. This may fundamentally change their relationships to cooperating by default rather than competing by default, as they have no need to compete for resources other than energy.

Contact may not be desirable

We may be on either side of the fence.

The more advanced life form may displace the simpler

If there's ever a form of life advanced enough to travel through interstellar space and reach us on Earth, what does that tell you about the technological power of those that reached us? What does that then tell you about the possible range of consequences of the interaction?

Both human history (in the form of advanced civilisations discovering simpler ones) and the natural world (in the form of introduced species in an ecosystem) strongly suggest that displacement of one actor by another in the name of resources or land could be a common outcome.

The introduction of foreign advanced technology could ruin a simpler civilisation

Upon the invention of plastics in the developed world, local councils had the historic precedent and the funding to expand waste disposal and recycling to deal with the new material. When plastics were imported into the developing world, advances in waste disposal and recycling did not necessarily follow. Without proper avenues for disposal, many developing countries became, and remain, choked with plastic pollution.

See Importing technology can be dangerous

Accidental contamination could be fatal for either party

American settlers in the 16th century brought a swath of European diseases with them as they sailed from one continent to another. Having built up a degree of immunity from centuries long exposure, Europeans could carry them without incredibly debilitating side effects. Native American people on the other hand, having left the Eurasian continent from 30,000 years ago, had no immunity. Upon first exposure, they suffered extremely high casualties, decimating up to 50% of their population.

Human beings naturally carry millions of microorganisms with them both on the inside and outside of their bodies, as do all multicellular species. An alien organism may be similar. We have absolutely no way of knowing if any of these microorganisms may be able to adapt to and multiply within the body of an alien species, or vice versa, unintentionally causing great harm.


Wikipedia, Acuna-Soto EID

The 16th-century population collapse in Mexico, based on estimates of Cook and Simpson (). The 1545 and 1576 cocoliztli epidemics appear to have been hemorrhagic fevers caused by an indigenous viral agent and aggravated by unusual climatic conditions. The Mexican population did not recover to pre-Hispanic levels until the 20th century.

What experiences will be universal?

The night sky

The majority of life in conditions remotely similar to Earth would see a night sky (although containing different constellations).


The scientific method

Formulating the scientific method would be a requirement for all advanced civilisations, as technology requires a formalised application of logic.

Without it, and many fundamental tools that it reveals such as mathematics, there is only so far you can go.

Therefore, all aspiring creatures in the universe will have to go through similar puzzles of science that we have, and will continue to do, since we are all discovering the same principles of the universe. They would discover the atom, quarks, and bosons and fermions. They would discover their solar system, their galaxy, and the rest of the universe. They would also discover many surprising truths about themselves such as their origins, which may conflict with simpler doctrines.

This process is how any species must earn its place in the stars.



Categorising the universe into discrete concepts and being able to communicate these concepts between individuals is a prerequisite for both technology and cooperation.

Some semi-permanent form of language, such as writing, must also be required to communicate across distances and time.



  • Though we most likely will use different bases.

Say hey to Owlbert Einstein

Curator of the Big Ideas Network

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