Evolution of Extraterrestrial Life

I gave a condensed version of this essay as a lecture to the Atheist Society in Melbourne, Australia on 8 May 2018.

There is a lot of speculation about whether there is life in other parts of the solar system, and in the rest of the universe. Some scientists have concluded that there must be a huge number of places throughout the universe that contain some kind of life. Other scientists have calculated that there very few places that could contain life, and there may be only one. There is also a lot of searching for evidence of such extraterrestrial life.

What are the chances of there being any extraterrestrial life, what are the chances of finding it or recognising it? What would it be like?


Answering these questions requires clarity about what a life is form and what life is.

A common biological definition is that life implies four functions:

  • metabolism – using material and energy within the body to support continued functioning;
  • reproduction – producing, from within the bodies of living parents, new separate organisms that become similar to their parents;
  • growth – increasing in size from infant to adult;
  • interaction with the environment – taking action as needed for metabolism, growth, reproduction and safety.


A generalised concept invoking the science of thermodynamics first appeared in the book What is Life written by the quantum theorist Erwin Schrödinger. He said that living beings are “the class of phenomena that are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form.” This means that life forms take in material and energy, which is used to build chemical structures, some of which become part of the body and others become like tiny charged batteries to power the body’s processes. Whatever material is left over is discarded.

An essential aspect of this definition is that taking in energy requires the availability a source of energy that is suitable to each kind of organism. Different kinds of organisms have evolved to require their particular specific kind of energy. For plants and some other organisms the energy source is sunshine or equivalent radiant energy. For many other organisms it is also the chemical energy contained in the food they eat.

This definition does not specifically address growth or reproduction. But if life is to persist, life forms must reproduce, and/or add new parts to their bodies to replace any damage, and/or be able to sustain its bodily parts against damage and shortage of food and energy.


All known forms of life contain a set of information within their structure that directs all of their actions and processes in ways that are not seen with inanimate matter – with the exception of some devices created by humans. This distinguishes life from inanimate matter. Every life form on Earth uses the same system. This system is based on two related types of molecules, DNA and RNA, which use four specific types of molecules that are regarded as “letters”.  The letters are combined into sequences of groups, all containing three pairs of letters. The groups might be thought of as words, and the sequences as sentences. The particular letters and the rules by which the system operates are not necessarily the only ones that could have been used. Scientists have already altered the genomes of some bacteria to include two more letters and make four-letter words that produce molecules that do not occur in evolved life forms. Different letters and rules, and/or different kinds of molecules to contain them, would have produced different kinds of life forms from those that now exist.


To survive, every life form, wherever it is, must have behavioural characteristics that enable it to maintain itself, including when its environment changes. It must act purposefully to select the matter and energy it needs to take in from its environment. It must be able to use the material and energy it takes in to perform its various functions. It must act purposefully to avoid harm and danger, and to reproduce.

Doing these things requires an ability to apprehend the environment in several ways. Life forms must be able to distinguish between the kind of matter and energy they should take in from their environment from what to ignore. And they must know what materials and situations are harmful and to determine how to avoid them. They must also have an urge to do these things. These behaviours and capabilities must be developed by the time its structure is functional. Each environment will produce organisms that can survive in its particular conditions.


Because the only forms of life that we know anything about are those on Earth, all of these concepts relate to life on Earth. There may well be other concepts of life, but I think it is reasonable to expect any life elsewhere in the universe to have similar kinds of needs.

As far as we know, all life forms on Earth are the products of a process of evolution from one common ancestor. They all depend on the information contained in their DNA and/or RNA to direct and control all of their functions. We would expect any life in other parts of the universe to have evolved and to be directed and controlled by some equivalent repository of information, but the chemical structures of such repositories would be dependent on the physical and chemical conditions of their environments.

There is no accepted scientific theory of how living organisms first came to exist on Earth: Biologists and other scientists have identified a lot of things that they think would need to happen for life to be able to emerge from inanimate matter. For the many necessary materials to come together and produce a living organism would have required a very unlikely series of coincidences, in which each step would have to sustain itself while the next step was being prepared.


We are fairly sure that once life began on Earth, about four billion years ago, the process of evolution of new species has always been directed by the conditions of the physical environment and the effects of competition, cooperation, hostility, and exploitation between organisms. Evolution has produced the physical bodies of all life forms, and also their behaviour.

It seems reasonable to assume that there could be similar kinds of processes elsewhere in the universe, by which life originated and evolved in conformity with the particular conditions of their locations, including conditions very different from those on Earth. This is, in fact, the implication of Schrödinger’s definition of life.

Conditions on Earth have allowed the development of organisms with complex internal processes that need to be finely coordinated. It appears that the emergence of life from inanimate matter, and also the emergence of multicellular organisms from single-cell organisms, occurred only once on Earth. This might suggest that both processes would occur very rarely, even under suitable conditions. Scientists regard the evidence of primordial life to relate to the life we know, but it is possible that life emerged more than once and only one version survived.

People tend to think of “intelligent” extraterrestrial life as being vaguely similar to Homo sapiens. But any life forms elsewhere in the universe could be enormously different from all kinds of life on Earth because the conditions almost everywhere else would be significantly different from those on Earth.


The main conditions on Earth that shaped the evolution of its life forms are:

  • The structure of the planet.
    Earth’s crust consists mainly as rock, as distinct from the “gas giants” Jupiter and Saturn, which consist mainly of hydrogen and helium, and the “ice giants” Uranus and Neptune whose solid matter is mainly water ice. Earth consists of a molten metal core, a semi-liquid mantle and a solid crust. The solid crust allows Earth to maintain very large coherent bodies of liquid water on its surface. These are the seas and the oceans, which provide a habitat for a huge variety of organisms.
  • The earth’s magnetic field
    Earth’s rotating liquid core provides the planet with a magnetic field. This has a shielding effect from the solar wind, that is, from streams of ionised particles that are continually being emitted from the sun and would be damaging to some kinds of organic tissues. The solar wind can strip the atmosphere from objects that do not have a magnetic field, such as Mars and Earth’s moon which have hardly any atmosphere
  • The Motion of the Tectonic Plates
    Earth’s crust floats on the mantle, and its components, “tectonic plates”, are continually moving slowly. The plates slowly collide and separate over periods of millions of years. This pushes up mountain ranges and causes some heavier plates to push under others. It also causes cracks in the plates, and earthquakes and volcanos. These things destroy habitats and create new ones, bring nutrients to the surface and occasionally cloud the atmosphere with volcanic dust that blocks sunlight from reaching the crust. It also allows gases to enter the ocean floor through small vents, providing food for some microorganisms, which then become the bottom level of a food chain.
  • The atmosphere
    The content and density of the atmosphere affect the amount and wavelengths of the electromagnetic radiation from the sun that reaches the surface of the earth. High up in the atmosphere there is a layer of ozone (O3) which is a form of molecular oxygen. It is the main contributor in the blocking of the ultra violet radiation. Ultra violet radiation is damaging to the tissues of most of Earth’s life forms.
    All the oxygen in the atmosphere (mainly O2) was produced by organisms, initially by bacteria from about three billion years ago, and then also by plants from less than a billion years ago.
    The content of the atmosphere also affects how much of the electromagnetic radiation emitted from the earth’s surface is redirected back to the surface. This has a significant effect on the temperature at the surface.
    The atmosphere is one of the systems that transport heat around the surface of the earth.
    The atmosphere also provides part of the habitat of many kinds of organisms and its oxygen and nitrogen that are essential for many life forms.
  • The size and mass of the planet.
    This determines the force of gravity at the surface, which is significant in determining the size and shape and functioning of the various kinds of life forms, and the density and content of its atmosphere. In conjunction with Earth’s magnetic field, the gravitational force of the mass of Earth helps to retain the atmosphere.


  • The mass, luminosity and proximity of the sun
    The particular mass of the sun, and its distance, affect its gravitational pull on the earth. This is the main determinant of the rise and falls of the tides in the oceans and the sub-surface liquid and semi-liquid rock below the crust.
    The electromagnetic radiation from the sun is the major determinant of the temperature at the earth’s surface. It also determined what kinds or eyes and other detectors of radiation evolved, and what kinds of protection from the radiation.
  • The range of temperatures at most areas on the surface of the planet
    The temperature range is dependent on Earth’s distance from the sun, and the size of the sun, and on the amount of air surrounding the planet and on the types of gases in the atmosphere. The range of temperatures on Earth is such that most of its water is liquid, but it provides for significant amounts of ice and water vapour. The distribution of temperatures around the earth is dependent mainly on currents in the oceans and on the winds, and on the Earth presenting of all of its sides equally to the sun as it rotates on its axis and revolves around the sun.


  • The tilt of the axis of rotation of earth with respect to its plane of revolution around the sun. This tilt points the planet’s north and south hemispheres differently to the sun, so that as the earth revolves around the sun there is a change of seasons, and the polar regions have alternating periods of six months of darkness and six months of continuous sunlight.
  • Earth’s short day of 24 hours gives all of the earth’s surface apart from the polar regions, about twelve hours of daylight alternating with about twelve hours of night as it rotates on its axis and revolves around the sun. This produces a comparatively small difference between daytime and night-time temperatures. In contrast, one day on Earth’s moon lasts about 28 earth days, which means about 14 Earth days of continuous sunshine and warming, and about 14 days of darkness and cooling.
  • Earth’s particular history of sequences of changes in temperature and the composition of the atmosphere, and of the tectonic movements of the surface land masses, and of the emergence of multicellular organisms and the mass extinctions, has affected the processes of adaptation, and hence the course of evolution. If the history had been different, the present range of life forms would be significantly different from what it is now.



All this amounts to a very large range of very specific conditions that determined the characteristics common to life as we know it. These conditions have led to a huge range of kinds of organisms, because the temperature, the number of locations, the kinds of terrain, and the kinds of different substances, vary greatly from place to place, and have varied from time to time.

Some of these characteristics of the earth would be similar to those in some other parts of the universe, but there will always be some significant differences in every other object in the universe. For example, the length of a day on the planet Mars is also about 24 hours, and the tilt of its axis is similar to that of Earth, but Mars has hardly any atmosphere and is further from the sun than Earth, which means it gets much colder than Earth and its range of temperatures is much greater.


Earth’s biota consists of a range of distinguishable types. But because of their diversity and their ability to mutate, and the fact that all their structures and operations depend on DNA and/or RNA which are interchangeable between all organisms, there are exceptions to all the described types. The simplest are unicellular organisms that do not have a nucleus. There are also unicellular organisms that have a more complex structure including a complex nucleus. And there are very complex organisms composed of many complex cells, almost all of which contain identical nuclei.

The complex organisms are classified into three fairly well distinguishable “kingdoms”, Animalia, Funghi and Plantae, and a diverse residue whose classifications are disputed. There is also some dispute about the three kingdoms. There is no way of predicting what kind of diversity might evolve elsewhere in the universe.


It so happened that one kind of life form on Earth, the kingdom Animalia, has a member, Homo sapiens, that has an intelligence far exceeding that of all other species, and also has a physical structure and an environment that allows it to use that intelligence in a great number of ways. Perhaps the predecessors of other kinds of animals, or even of organisms such as plants and fungi, might have had the potential to do the same under different conditions.

When most people think of “intelligent” extraterrestrial life they seem to assume that it must have characteristics similar to those of Homo sapiens. But H sapiens was produced by a combination of very many coincidences that were peculiar to planet Earth and its particular conditions, as were all our kingdoms of complex life. Any life forms on “Earth-like” exoplanets could be enormously different from all kinds of life on Earth. And life in places that were not Earth-like would be more so.


As mentioned earlier, there is no accepted scientific theory of how living organisms first came to exist on Earth. So we don’t know the sequence of events that led to the origin of life on Earth, or which of the conditions on Earth were necessary for life to emerge, or how different the conditions would have needed to be to prevent the formation of life. The oldest identifiable life forms on Earth occurred about four billion years ago, when the conditions were significantly different from what they are now, and  differently again from 100 million years ago.

For life to emerge and then evolve anywhere in the universe, its environment must allow the possibility of an unbroken series of self-sustaining steps. One requirement would be that the conditions of temperature, chemistry, radiation and physical movements would not disrupt the structural integrity of an organism during and after development, or the information that controls all its processes.


The emergence of life from inanimate material, and the sustainability of life forms, and the evolution of more complex life forms all depend on the process of synthesis, that is, combining materials to make something new. This comprises the synthesis of chemical compounds and the assembly of these compounds into structures such as cells and organisms. At every step of these processes there must be an input of some source of energy to drive the action, such as heat, electromagnetic radiation or chemical action. The types of organism that would emerge and evolve would depend on the availability and feasibility of using any sources of energy.


It has been suggested that the life on Earth originated from somewhere else. One such idea is Panspermia, according to which there are, among the gases and dust in outer space, either robust microorganisms or chemical compounds that are potential progenitors of life. If they arrive on an object that has suitable conditions they will develop into primitive life forms that will survive, reproduce and evolve. So was one “pansperm” so extremely lucky that it  survived the cosmic radiation and the electromagnetic radiation and the particles emitted from the sun, and the stronger gravitational attraction of the sun and outer planets, and to arrive on one little planet with the conditions for it to be our ancestor? But there are no reported signs of Panspermia.

There is evidence of organic chemical compounds in outer space that some scientists think would have been essential in the process by which life emerged on Earth. (In this context, the term organic does not imply that these compounds were derived from the bodies of organisms; it refers to compounds containing the element carbon, which is in every cell of every organism on Earth.) These compounds are just one small component of huge interstellar clouds of gas, plasma and dust, which are denser than most of the interstellar space, and may occupy more space than the solar system. The components of these clouds can be drawn together by gravitational attraction and eventually form star systems, like the solar system. It seems reasonable to assume these clouds could produce the ingredients for building organisms, but they would probably be too chaotic to support the necessary stages to actually produce any. But, given the available amount of time, this could not be ruled out.

There is also a suggestion that some primitive life forms from Mars might have been flung into space, landed on Earth, and then replicated and developed there. And there is a suggestion that it might have been the other way round.

Such things may be possible. Some of Earth’s bacteria are extraordinarily robust and survive temperatures well above and below what any animal can survive, and others can survive in suspended animation for thousands of years. Some life forms from other parts of the solar system and beyond could be just as hardy. But if they were to survive, and then reproduce and diversify, what they evolved into would depend on the conditions of wherever they landed. Also, throughout their time floating around in space they would be bombarded with particles and radiation from the sun, which would be likely to disrupt their inner workings. The “hardy” organisms that we know on Earth are protected from most of the radiation and particles by Earth’s magnetic field and atmosphere.



Characteristics of life on Earth and elsewhere

To consider what any life forms would be like if they were produced under the different conditions at different parts of the universe, we might start by looking at life on Earth.

All organisms on Earth are composed of cells. Cells are bags containing the basic structural and functional components of life, with appendages and openings on their surface to allow them to interact with their environment, which often includes other cells. Many organisms consist of just one cell. Complex organisms consist of integrated arrangements of cells, with almost every cell containing a copy of the version of DNA that is unique to that individual organism. Adult human bodies have more than ten trillion cells. We might assume that life elsewhere would be structured on a similar principle, but there is no reason to think that it must be.

All life forms on Earth seem to contain liquid. When considering extraterrestrial life, we usually regard the presence of large amounts of liquid water and the accessibility of carbon to be essential for the emergence of life from inert matter. Liquids seem to be ideal for many of the chemical processes that are implied by the various functions specified in the various definitions of life.

The complex organisms on Earth differ greatly in size structure and function, from plants to fungi to animals. Fungi and animals could not exist without the presence of other organisms, because they cannot rely on only inorganic materials or solar radiation for food and energy. So there is a “food chain” on Earth, with complex organisms eating other organisms. It might seem reasonable to expect this to apply elsewhere in the universe, but it might not be the only kind of arrangement. There might be places whose complex organisms all take in their requirements of energy and material directly from an inorganic environment. Some microorganisms and plants do this on Earth.

Virtually all animals and many fungi must have continuous access to molecular oxygen, i.e., in the form of O2. During the early stages of Earth’s existence there was hardly any molecular oxygen in the atmosphere or in the sea, although there was plenty of oxygen as a component of water. All the molecular oxygen on Earth has been produced by organisms, in the early stage of life by cyanobacteria, and then, after complex life forms developed, also by plants. In both cases the oxygen was the by-product of photosynthesis, using the radiant energy of the sun.

This wide range of characteristics of life on Earth suggests that any other place in the cosmos that has life on it could also have a wide range of life forms.


When scientists search for “intelligent” extraterrestrial life they are thinking of organisms that would be comparable to humans. Any such organisms might have comparable intelligence, or greater, but the conditions under which they evolved and the kinds of things that occurred in the course of their evolution would inevitably have been extremely different from those that produced Homo sapiens. So they would have very little, if any, structural similarity with us.


Evolutionary effects of different physical conditions

We know of no other place in the solar system that has conditions under which most of our kinds of life could exist without special artificial protection. The exceptions would probably be some hardy kinds of microorganisms that might be able to survive and flourish in some places on the planet Mars, or in oceans on some moons of Jupiter and Saturn. Some scientists think that life would emerge on every cosmic object that had liquid water. This may be correct, but I see no reason why it must be. Conditions that might support microorganisms would not necessarily allow such organisms to emerge or develop into complex forms.

But it should be possible for other kinds of life to emerge and evolve in some of the very different conditions in other parts of the solar system and beyond. This includes places that, from our perspective, would seem to have no chance whatever of producing and supporting life.


While liquids might be essential for the chemical processes that are required for the various definitions of life, that does not necessarily mean water. Life forms probably also need to contain solids, including flexible materials, in order to maintain the integrity of their structure.

Or perhaps strong electrical or magnetic forces might contain the liquids –  or gases or plasmas – without the need for solids, to produce forms of life that might have nothing in common with our idea of life except, perhaps, complying with Schrödinger’s description of it.

There may be other possibilities, such as organisms that contain no liquid. For example, something like highly intelligent self-regenerating robots that don’t need liquids, which we might someday invent, might evolve as organisms somewhere in the universe.

So, at least in theory, life could take a range of extremely different kinds of forms in extremely different environments. And there are enormous differences of conditions among the different kinds of cosmic objects.


The most significant physical conditions that produce specific structural characteristics as organisms emerge and evolve are temperature, gravitational forces and electromagnetic forces.



The lowest possible temperature is -273o C, absolute zero. No life on Earth, with the possible exception of a few extremophile microorganisms, could survive and reproduce outside the range of -20oC to +120oC. This includes organisms living near hydrothermal vents on the ocean floor and in hot springs. These limits are wider than the temperature range of almost all of the surface of Earth. Many other cosmic objects have surface temperatures completely outside this range. No eukaryote can go through its life cycle outside the temperature range of -2oC to +60oC. Almost all organisms require a much smaller range than this.


On Earth, water exists in the forms of ice, liquid and vapour.
At sea level, water boils at very close to 100° Celsius, but under higher pressures it will stay liquid at higher temperatures. Under lower temperatures, such as at high altitudes above the surface  of Earth it boils at lower temperatures. It also evaporates into the air at temperatures below its boiling point. Under the very low atmospheric pressures on places such as on Mars and the moon water evaporates much more readily.

At the very low temperatures such as at the outer parts of the solar system, water can exist only as ice and is physically like a hard kind of rock.

One of the liquids that is present in several places in the colder areas of the solar system is ammonia, which becomes liquid at about -80° C under Earth’s atmospheric pressure, and under lower pressure at lower temperatures. The planet Neptune has a lake containing a mixture of water and ammonia whose mass is greater than ten times the mass of Earth. The chemistry of ammonia has some similarities and some differences  with that of water. It reacts with water to form the alkaline liquid ammonium hydroxide.

Another common chemical is methane, which on Earth is a gas but becomes liquid at temperatures below -161° C. It is mainly found as a gas on the gas giants and as a solid on Pluto. Liquid methane is very different from water, for example is a non-polar substance, that is, it does not split into ions when an attempt is made to pass an electric current through it.

So there can be a mixture of solids, liquids and gases on planets and moons at these low temperatures, and the same could apply at temperatures significantly higher than those on Earth, but the way that these substances interact would be very different from their interactions on Earth.


The planet Venus, which is next to Earth and closer to the sun, has an average surface temperature of about 460° C. That is hot enough to melt lead. Its atmosphere is mainly carbon dioxide and the high temperature is the result of the greenhouse effect. None of Earth’s life could survive at this temperature or even 300° C cooler. (Some bacteria on Earth thrive in environments containing sulphuric acid, and might survive in the cooler outer atmosphere of Venus, which contains sulphuric acid.) Many rocky planets elsewhere in our galaxy are much hotter than Earth because they are close to their star or their star is hotter than our sun.

It might be hard to accept the possibility of life existing under such hot or cold conditions. No organisms suited to conditions on Earth could survive them. At the very low temperatures the chemical reactions could be a lot slower than on Earth. But this need not prevent the development of processes similar to those that must have led to the emergence of life on Earth.


Planets that have low temperatures are either further from their star than Earth is from the sun, or their star is a lot cooler or smaller than our sun. In either case, there would be a lower concentration of incoming energy for organisms to use. Some organisms on Earth, including most animals, get their energy from chemical reactions of various complex materials that they take in. But these complex materials have been built up through a food chain dependent on energy from the sun. With a low input of such external energy, all organisms would have to rely directly or indirectly on other forms of energy, which would probably be chemical from an inorganic source. Energy radiated from a star keeps on coming, while chemical energy might get used up. So a low external supply of energy might make it difficult for very complex organisms to evolve or be sustained for a long enough time to produce a sophisticated society.


At higher temperatures, chemical processes are faster because the atoms and molecules vibrate more vigorously. In fact, the heat energy of all materials consists in the energy of the  vibrations of the particles they are composed of. So at very hot locations the chemical reactions and/or the thermal vibrations might be too violent for there to be enough stability for life to emerge or to develop. High temperatures can cause the break-up of chemical compounds by weakening or breaking their chemical bonds. But any organisms that might live under these conditions (and had sufficient intelligence and knowledge of the cosmos) would probably think that the conditions on Earth would be so sluggish as to inhibit life emerging or developing very far, just as we might think that of very cold locations.



While temperature has a very strong effect on the characteristics of life forms, gravitational and other forces are also important. The gravitational force at the surface of any object depends on its mass, its size, and its shape. Some smaller cosmic objects such as comets and asteroids are irregular in shape so the force of gravity varies significantly over their surfaces. Objects the size of planets and the larger moons are roughly spherical because the force of gravity has drawn them in before they had fully consolidated. So the gravitational force is about the same everywhere around their surface.

The force of gravity at the surface of a fairly spherical object is proportional to its mass divided by the square of its radius. For example, the mass of a planet with the same density and structure as Earth but twice the volume would be twice that of the mass of Earth. The force of gravity on its surface would be about 1.3 times than that on Earth. (It is not twice that of the earth because its radius would be about 1.3 times that of the earth and the force of gravity at the surface is inversely proportional to the square of the radius.) The force of gravity on the surface a similar planet twice the diameter of Earth, which would have eight times Earth’s mass, would be twice Earth’s surface gravity.

The size of the force of gravity on a planet or other object influences the sizes and shapes of the organisms that might evolve on it. Very small organisms can be safely structured with more slender body parts than larger organisms. This is because the mass, and the weight, of an organism is roughly proportional to the cube of its height or its length. Larger organisms with larger masses are therefore less nimble.


An illustration of the effects of gravity and size is the jumping abilities of four kinds of mammals on Earth. The height that mice can jump is many times their body length. The world record for a human high jump is 2.45 metres, and most humans are unable to jump as high as their body height. The world record for a horse is about 2.5 metres, and most horses are unable to jump much higher than one metre. An elephant is too heavy to jump. The same principle applies to other kinds of animals, such as lizards and insects. It applied to the dinosaurs. (Differences in the structure and processes of different kinds of animals produce different limits.)

Elephants on a planet twice the mass of Earth, with twice the gravitational force, would find it harder to move. Their legs would have to be more powerful and thicker to support their greater weight, and this would further increase their weight. Any life forms there would evolve with their size, shape and materials adapted to the greater gravitational force and other relevant aspects of their environments.

In contrast, we have seen videos of astronauts on the moon, bouncing very high as they walked because of the moon’s smaller size and gravity. On the moon they can jump higher but they fall back more slowly than on Earth. And on the moon, elephants (wearing suitable spacesuits) would be able to jump.

Organisms that have an ocean for a habitat are much less affected by the force of gravity of their planet because they are supported by the water or other liquid. The degree to which they are affected depends on their average density compared with that of the liquid of their ocean. On Earth, whales can grow bigger than elephants. They, and other large active sea organisms can move quickly and easily through and also up and down in the ocean, because their weight is supported by the water. They are in real trouble when beached and lying on the sand, even though they may breathe air.


As mentioned earlier, having a strong gravitational force helps a planet or a moon to retain its atmosphere, which can shield the planet from ultra violet and other radiation from its star, which may also have a greenhouse effect.




Some planets and moons in the solar system have atmospheres and oceans. If there is a substantial amount of atmosphere it exerts a pressure at the solid or liquid surface of the planet or moon. The amount of pressure depends on the gravitational force at that point and on the amount of atmosphere and its density. At higher altitudes above the surface the pressure is lower because the gravitational force is lower, and there is a lesser amount of atmosphere above. Also, as an atmosphere is mostly gas, and gases are compressible, at higher altitudes the atmosphere becomes increasingly less dense. Organisms that fly get better buoyancy in the higher pressure atmospheric levels closer to the surface, because gas is very compressible, and therefore denser under higher pressure. So on Earth, insects can fly higher than birds. Microorganisms don’t need to fly: they get carried up by the wind.

The atmospheric pressure on Venus and on the four giant planets is greater than on Earth.

The conditions of oceans are different. Liquids are much denser than gases and they are much less compressible. At any point in an ocean the pressure is proportional to the depth. On Earth, the pressure at the average ocean depth, a bit less than 4km, is about 350 times the atmospheric pressure at sea level.

On Earth, all land animals can tolerate atmospheric pressures somewhat higher than that at sea level, and at lower pressures it is the corresponding lower quantity of oxygen that will cause great discomfort or death. At pressures greater than about five times that of sea-level atmospheres, it is again not the pressure itself that is a problem but the pressurised absorption of atmospheric gases into the blood stream. This causes great discomfort or death to human divers unless they slowly decrease their depth on returning to the surface.

Animals that evolved to live at great depths of the ocean have difficulty and often die when brought to the surface, because some of the substances it their bodies are solids under the high pressure at their habitat but are liquid at lower pressures. Also, their muscles are adapted to the support of the buoyancy. That is why whales that are beached are in real trouble. Most animals that evolved on land and in shallow depths of water would be crushed by the high pressure at greater depths.

High pressure makes some gases become liquid. On Earth, carbon dioxide is usually a gas at pressures below about five times ground level atmospheric pressure. Below this pressure it can be a gas at temperatures down to about minus 70o C, and a solid, commonly known as “dry ice”, below minus 70o C. At pressures higher than five times atmospheric, it becomes liquid at temperatures above minus 70o C. Then, under increasingly high pressures it starts to become solid, depending on the temperature. Carbon dioxide is common to most of the planets and moons in the solar system.


Other conditions

There is evidence that some moons of Jupiter and Saturn have oceans of liquid water under ice that might be kilometres thick. Scientists conjecture that there might be life in these oceans, because their temperatures would be similar to that of some of the oceans of Earth.

The surface of these moons is cold enough for ice to form and stay frozen. In some of them the water below the ice is kept liquid because the moons are pulled by the very strong gravitational force of their planets. This force varies as the moons move around their elliptical orbits, which causes tides, including in the rocky solid parts of the moons. The friction of the tides causes heat, which can keep some of the water liquid.


In some of the lakes the surface ice may be so thick that hardly any sunlight gets through. On Enceladus, one of Saturn’s moons, there appears to be liquid water under a layer of ice whose thickness is about 30km to 40km thick. In the water below this ice most of the radiation would be infra red, emitted by the heat of the water and the adjoining solid parts of the moon. Any organisms evolving in such lakes may have a minimal sense of vision, or none. It might rely on any infra red light, or use other methods of detecting the features of their environment, as do organisms on the deeper floors of Earth’s oceans. But they would have no apparent way of knowing what was beyond their ocean. Even if they were to develop a high level of intelligence, they would have no way of investigating, or even knowing about, some of the phenomena that we are familiar with, such as many aspects of the physics of electricity or cosmology, or gravitation.

Similarly, there may well be phenomena that we are intrinsically unable to know about because the conditions of our part of the universe prevent us from being able to evolve a way to detect them.

And there may be parts of the universe where organisms can evolve to know about such phenomena and be able to do things that we think are physically impossible and to solve our scientific mysteries.

So, might we ever meet these or other aliens – in either their homeland or ours?



Extraterrestrial Visits and Visitors

Before space probes discovered the conditions on other planets and their moons, people used to talk about Martians and Venusians, wondering which would visit or invade Earth. But women are not from Venus, and men are not from Mars.

We now speculate whether there ever was, or still is, life on Mars, but so far only inconclusive signs of possible previous organic activity have been detected. But that is evidence of primitive life as we know it, and conditions on Mars are very different from those on Earth. We have no data on life on Venus.

For decades we have searched the cosmos for evidence of signals, in the form of electromagnetic radiation that might have been emitted by extraterrestrial civilisations. We have identified exoplanets that might support life, and looked for signs of substances that we think would not be there in the absence of life. We have sent unmanned space vehicles to or around all the known planets and some of their moons. We have not yet encountered any extraterrestrial organisms or plausible evidence of their existence.


A few people have travelled from Earth to the moon and returned after very short stays. Several people have stayed for months on the International Space Station, which is about 400km away, orbiting the earth.

Some people are very confident that human beings will soon visit and colonise other parts of the solar system, and eventually beyond it. Some people are very certain that they have already met extraterrestrial visitors.

How feasible are these ideas? The issues, for both us and the aliens, are survival on arrival, how long it would be possible to stay, how long the trip would take, and how to get back home.

Humans taking short visits in deep sea craft and space stations are confined in protective enclosures. Colonising such places would be very different. More distant places would be even more difficult.


Some countries have plans to set up research stations on Earth’s moon. Its atmosphere is extremely thin. A lunar day lasts about 29.5 Earth days and the temperature ranges from above 120oC during the daytime and about minus 150oC during the night. So the NASA moon landings had to be timed to arrive at a part of the day that was sunlit but neither too hot nor too cold, and the astronauts’ time on the moon was always short. A colony would need to have a means of living with this temperature range, or keep moving around the circumference at about 40km per day to stay in a comfortable location.

Much of the moon’s surface is covered with a fine dust. The dust rises easily because of the low gravitational force, and it will lodge in any accessible crevice. These conditions would severely restrict the everyday things any colonising humans would be able to do.

It is thought that there is accessible water but not much of other materials that are essential for maintaining the life of humans. The water is thought to be useful as a source of energy.

Trips the moon, which is about 385,000kilometres away, and to the space station, are direct flights. Trips beyond these are more complicated.


The planet Mercury is the closest to the sun. It has no atmosphere. One solar day on Mercury lasts as long as 176 Earth days, so there are about 88 continuous (Earth) days of sunlight and 88 continuous days of night. The maximum daytime temperature reaches 427 o C and the night-time minimum gets down to minus 180 o C. (Mercury’s sidereal day, which is the time it takes to make one rotation on its axis, is about 59 Earth days.)

Mercury is continually bombarded by particles and strong radiation, including ultra violet from the sun. It has water ice at the bottom of some of its deep craters. Some kind of life might exist in the vicinity of this ice, but it would be unable to survive far from its crater.

Mercury’s elliptic orbit takes is as close as 47 million kilometres to the sun and as far as 70 million. This means that, near Mercury, the gravitational pull of the sun ranges from about four and a half to ten times that near Earth, making it fairly difficult for space vehicles to visit or leave the planet without being pulled into the sun or into permanent orbit around it.

Any organisms that evolved in the absence of molecular oxygen, such as on Mercury, would find our atmosphere toxic and corrosive.


Venus, which is the second planet out from the sun, is the closest planet to Earth. At about 450 degrees hotter than Earth, it could not be considered as a place for any earthly organism to visit. Lead would be a liquid, similar to (the chemical element) mercury on Earth, and mercury would be a gas on Venus. Atmospheric pressure at the surface of Venus is 92 times that on Earth. It would be highly unlikely that any Venusian organism could exist on Earth, or anywhere else, without being kept at temperatures comparable with those on Venus.


With an orbit about 79 million kilometres away from Earth’s orbit, Mars is the next closest. However, this distance is not necessarily the length of the trip. All the planets are moving around their orbits, so any two of them will spend a large amount of time on opposite sides of the sun. So you may need to wait some months for the best starting date, and then also for the trip home. If we were to visit Mars we would need to take enough air, food and water for the entire trip. Mars is more than 200 000 times as far as the International Space Station, which gets regular supplies from Earth, and more than 200 times as far as the moon.

So the cost of getting people to Mars, along with all the supplies of food, equipment and shelter they would need, would be enormous. Irrespective of cost, the technology for providing enough rocket power does not yet exist. The load would be many times that of putting an un-manned rover on Mars, and with present technology the point would be reached where the weight of any extra fuel would require more energy than it produced.

But more efficient propulsion technologies are expected to be developed, architectural drawings of proposed buildings for a colony on Mars have been produced, and suggestions have been made that bacteria be sent soon to Mars to prepare the soil for agriculture for when a colony is established. One proposed likely date for the first colonisation is about 2030, but many people think that is very optimistic.

So far, there have been no sightings of life on Mars. The atmosphere of Mars is 95 percent carbon dioxide and about 100 times thinner than Earth’s. It has enough atmosphere to have dust storms that can blanket the entire planet and last for months, but not enough to block ultra violet rays from the sun.

It has ice cliffs on the surface, ice caps at the poles, and underground ice. It also has a large muddy salty liquid lake of water more than one kilometre below one of the polar ice caps. There is speculation that the lake might harbour microorganisms. The surface temperature of Mars ranges from minus 125o C to +20 o C with an average of about minus 60 o C, which means that  our kinds of plants would not grow in outdoor martian conditions. Colonisation would need a lot of infrastructure and the development of edible species that could thrive under martian conditions. If we were to visit Mars we would need to take enough air, food and water for the entire trip. Because of the low atmospheric pressure the water on Mars evaporates very readily during the warmer season. This would include water in the soil of any gardens that were not inside pressurised buildings.

Splitting water or other material containing oxygen into its components to produce molecular oxygen would be possible, but it would not be feasible to create a planetary atmosphere of oxygen sufficient to support human life. If it were practicable to produce enough oxygen, there would be no possibility of retaining it in an outdoor atmosphere because Mars has no magnetic field to protect it from the solar wind, which deprived it of most of its atmosphere long ago. Any oxygen produced would need to be contained within sealed structures. This would apply also on Earth’s moon.

Colonising Mars (and most other locations) would mean living in airtight enclosures with air pressure similar to Earth’s atmospheric pressure. Going outside on foot would be nothing like as carefree as on Earth. It would always entail wearing a spacesuit or being in a sealed vehicle that was thermally insulated and regulated, and contained oxygen (which might be diluted with another gas, but not carbon dioxide).

Spacesuits must protect against high and low external temperatures, ultraviolet radiation and very fast-moving particles emitted from the sun. They must compress the body to provide the equivalent of Earth’s atmospheric pressure. They must provide a sufficient supply of oxygen for the period to be spent outdoors, and dispose of exhaled carbon dioxide and water vapour, and all the potentially smelly substances exuded from the skin. They must also prevent the build-up of heat generated by the body from raising the temperature inside the spacesuit to unsafe or uncomfortable levels.

The spacesuits would have gloves, which would make any fine manipulation of anything fairly difficult. They would need to be flexible at all of the places where the human body is flexible.

The effectiveness of all of these provisions would determine how long the person could remain outdoors. Colonists and visitors would probably go outside mostly in vehicles that provided all the necessary protections, and unless they were wearing a spacesuit they would have to stay inside the vehicle.


The habitats of most extraterrestrial species would be so different from anywhere on Earth that we would be unable to colonise them. And we should think twice about colonising any extraterrestrial site that does not presently support life. We would not have enough time to evolve to suit its conditions. And Earth would need to be in an extremely dire condition for it to be less habitable for us than elsewhere in the solar system.


Since it seems that any extraterrestrial life in the solar system would be very different from life on Earth, and unlikely to develop into a very complex form, and since any that might become highly intelligent would be unable to survive on Earth, I don’t think we can expect any visits from them.

Any life forms beyond the solar system would need to be more intelligent than us and have vastly better technology to reach us and to colonise us. The closest ones would have to travel a distance of more than four light years (or 40 trillion km as the crow flies) to get here from the planets of our nearest star, Alpha Centauri, and their spaceships would be travelling at a fraction of the speed of light. Since Earth is such an insignificant speck in the solar system, let alone the cosmos, it seems very unlikely that Earth would be their favoured destination.

But despite Earth’s small size, some of its features may be discerned by some far distant civilisation, and arouse its interest in us. We continually emit radio signals carrying video, audio and data that are not confined to the surface of the planet. We have also sent information about Earth and how to find it, and instructions for reading the information, on four space vehicles. Pioneer 10 was launched in 1972, Pioneer 11 in 1973, and Voyager 1 and Voyager 2 in 1947. This information, encoded onto playable discs, was a very minor part of the purposes of all missions, but it was hoped (but not very confidently) to be the beginning of dialogue with alien civilisations. But many more decades will elapse before these vehicles could get near the regions of our nearest star.

Another deliberate kind of contact was launched in November 2017, this time using a specially coded radio signal directed to an exoplanet in the star system Alpha Centauri, about 12 light years away. The signal contains mathematical and scientific statements and other information. The same signal was sent on three consecutive days. If the signal is received, quickly decoded and quickly replied to, all of which might not be very probable, we could receive the reply in about 25 year’s time. We might, of course completely miss the reply.

Some people might think that this attempt to alert a specific civilisation to our existence is more dangerous than the undirected messages carried by the Pioneer spaceships. It is hard to envisage a trip of 12 light years, more than a hundred trillion kilometres or more than two hundred million times the distance from Earth to the moon. And it might not be a direct flight: it would have to negotiate the gravity of the outer planets of the solar system and other possible planets of its home star. Perhaps the crew would go into a decades-long hibernation to minimise the amount of sustenance they would need to carry. Or they may have a very slow metabolism and a lifespan of a few hundred Earth years.


It is not only distance that separates us from other possible civilisations. Time may also be a factor. The universe is about 14 billion years old. Some star systems that might hold life forms could vary in age by billions of years from our solar system.

Most objects in the universe undergo progressive changes and disintegrate or are consumed by larger objects. There would be periods when a particular object contained life and periods when it did not.

On Earth, microorganisms comprise a significant proportion of the life forms. For more than a billion years, microorganisms were its only form of life. So there might be only microorganisms in places that seem suitable for life. Some microorganisms could possibly get swept up into space and drift around, but it is hard to envisage them developing long-distance spaceships.

The proportion of the time of a cosmic object’s existence during which it contained life forms that were capable of travelling to a nearby planet, and particularly beyond the system of its star, would probably be small. Some people think that humanity will cause its own extinction because of injudicious use of its increasingly powerful technology, and that alien civilisations would do so also. While these are just assumptions, particularly concerning what extraterrestrial evolution might produce, they should be being part of assessment of the likelihood of extraterrestrial meetings. Furthermore, Earth has suffered natural mass extinctions, and life elsewhere might also be subject to mass extinctions.


We could have had a visit a billion or so years ago – but no evidence of any visit seems to have survived. Or we may receive a visit in a billion or so year’s time.

The best we might hope for would be that an un-crewed alien space probe goes astray and enters our region in our time. But don’t hold your breath.




The orbit of the International Space Station ranges from 330km to 435km, being re-boosted using its engines or by visiting spacecraft. It completes 15.54 orbits per day.

The geosynchronous orbit for satellites around Earth is 35,786 km above the Earth’s equator, following the direction of the Earth’s rotation.

Earth’s moon is about 385,000km away.