Cartoon Alien

Extra-terrestrial Life

This essay attempts to answer the question:
"Are We Alone in the Universe?"



The answer to the above question is I don't know. This essay is not an essay of answers because nobody knows the answer. What I want to do is speculate. My speculation will be controlled and based on the knowledge we have at present. Like all science, everything I say in this essay could be proved wrong by a new theory or discovery tomorrow. However, the path that we follow will be full of interesting ideas that, I hope, will stimulate the reader.

The reason I talk about extraterrestrial life is because there is strong evidence that all life on this planet has a common origin. Chemically and genetically there are evolutionary links between all organisms that more or less reflect the more traditional attempts at linking them. This means, of course, that we have only one example of life to study. This limits the number of conclusions we can draw. The more examples of a phenomenon that you have, the more you can deduce by comparing.

I know that there are many people who believe that UFO's represent intelligent visitors from other worlds. In this essay I will assume that UFO's are not intelligent visitors from other worlds. I may be proved wrong tomorrow...

Definition of Life

The next thing to do is to define life. What is life? How can we tell a living thing from something that doesn't live. Sometimes it's easy (a tiger and a rock). Sometimes it's a little tricky. In fact, the line between living and non living is actually a very fuzzy one. Viruses are very close to that line. Away from a host cell, most viruses are crystalline solids composed of proteins. Within a cell, the material inside the virus uses the cell's chemistry to reproduce itself. So what we have to find is a working definition that will include all living things but not ones that are not living. That is actually very difficult. The one I will use for this essay is as follows.

A living organism is a system that can interact with its environment, exchanging material with the outside, and capable of reproducing itself.

Essentially, this is a chemical definition of life. Molecules pass into the organism and are built up to complex molecules distinctive of the organism by a series of chemical reactions. Waste products from this process are passed back out again. Many people believe that life involves something else apart from chemistry. This is a piece of speculation that I will ignore since I know nothing about it.

Organic Chemistry

The above chemical definition of life actually imposes severe limits on where life can be expected. Life on Earth is made up of complex molecules based on Carbon. This is no accident. Carbon is the only atom capable of forming large complex molecules. The chemistry of carbon compounds is called Organic Chemistry.

The chemical reactions of life are subtle and complex. Let us look at the conditions necessary for complex chemical reactions to occur.

In nature, matter is found in three states: gas, liquid and solid. In solids the molecules are held together in fixed positions relative to each other. In liquids, the molecules are close together but not rigidly held: they can slide over and around each other. In gases the molecules are flying around free of each other.

When solids are mixed they tend not to react together (unless it's a violent reaction like in gunpowder). Gases mix very thoroughly and any reaction goes to completion (in other words there is no subtlety when gases react). Reactions in liquids, however, can be controlled by things like concentration, acidity, and temperature. Liquids provide an excellent medium for complex and subtle chemical reactions. In fact it is difficult to see how the complex reactions of life can occur in anything other than a liquid medium. Of course, this could change with new evidence...


Unfortunately, the liquid state is the rarest in the Universe. In the depths of space, matter exists as either solid (dust, meteors, asteroids) or as gas (nebulae). Liquids are always 'leaking'. In the vacuum of space there is nothing to hold a liquid together. A drop of liquid would quickly evaporate. In the rest of the Universe, the most common unit is the star. All stars are made of gas. The temperature and pressure of even the coolest star ensures that all substances within the star are gaseous.

Liquids can only exist as the 'filling' of a sandwich. What do I mean by this? Liquids require a solid surface to hold them and a gaseous or solid topping to keep them from evaporating away. A body like the moon which has no atmosphere cannot have any liquid on its surface. Without atmospheric pressure to keep it there, a Lunar liquid would quickly (in the geological sense) leak away and be lost. The only places in the Universe where liquids can exist are solid planets with atmospheres or planets where the liquid is covered by its frozen form.

Solar System Bodies

In the Solar System there are only five places where liquids could exist.

The Sun is wholly gaseous. Jupiter, Saturn, Uranus, and Neptune are giant, mainly gaseous major planets with no solid surfaces (the Jovian Planets or Gas Giants).

The remaining major planets (Mercury, Venus, Earth, Mars - the Terrestrial Planets), their satellites and the minor planets are smaller, rockier bodies. Of these, Mercury and most of the satellites (like our Moon, Jupiter's Io and Ganymede) are small solid objects with no atmospheres. The minor planets (e.g. Ceres, Chiron, Vesta) and Kuiper Belt Objects (e.g. Pluto, Sedna) are rocky, metallic or icy bodies with no significant atmosphere.

Four of the remaining bodies in the Solar System have atmospheres. These are Venus, the Earth, Mars and Saturn's largest moon, Titan. Jupiter's Moon, Europa, is covered by ices and there is some evidence of liquid below the surface. Liquids could only exist on these five objects; this is not the same as saying liquids do exist on these objects. That depends on the chemistry, pressure and temperature.

On Venus the atmosphere is very thick, but the temperature is over 700°C. Nothing that exists in the atmosphere of Venus is liquid under the conditions found on the surface of Venus. Carbon Dioxide and Sulphuric Acid (the two constituents of the Venusian atmosphere) are both gases under those conditions. Venus is surrounded by thick clouds but infra red and radar measurements indicate a bone dry surface.

On the Earth, the atmosphere is made up of Nitrogen, Oxygen, Argon, Carbon Dioxide and Water Vapour. The average temperature is about 20°C. These conditions are right for liquid water to exist and, of course, it does.

Mars has a very thin atmosphere made up of Carbon Dioxide and Nitrogen. Neither the pressure or temperature (-50°C) on the surface is sufficient for either to exist as a liquid. No liquid has ever been observed on Mars although there is evidence that it existed in the past and may exist below the surface. Life may exist on Mars in some primitive or suspended form in areas where there is evidence of past water.

On Titan, the atmosphere is Nitrogen and Methane and the temperature is -150°C. With these conditions, it would be possible for liquid Methane to exist on the surface of Titan. The Orange atmosphere hides the surface. A recent probe discovered what look like small lakes on the surface. Saturn's Moon, Enceladus, has shown evidence of geysers of water coming from liquid water below the surface.

Jupiter's satellite, Europa has no atmosphere. It is covered by solid water ice that shows evidence of liquid welling up from below before freezing in the cold temperatures. It is likely that liquid water exists below the ice. This is another world that will be explored for life. Another of Jupiter's satellites, Ganymede, may have a similar makeup.

Neptune's satellite, Trton has geysers of liquid nitrogen which indicates the presence of that substance under the surface.

So, of all the bodies in the Solar System, one (the Earth) definitely has liquid on the surface, and five others (Titan, Enceladus, Europa, Ganymede and Triton) may have liquid on or below the surface. No wonder we haven't seen any Martians or Venusians!

Recap and Conclusion

Let's just recap on the logic of all this:

Of course, this doesn't mean that there is life on Titan, Enceladus, Ganymede, Europa or Triton.

On Titan, there may not be a large amount of liquid. Or if liquid Methane exists, this substance doesn't support life the way water does on Earth. Even if liquid Methane does support life, life may never have actually began on Titan for some reason (too cold, something else missing).

On Europa and Ganymede, liquid water may exist but there may be no energy source for life to function. Certainly, sunlight is very weak on these bodies but there may be thermal energy generated by Jupiter's intense gravitational field.

There are too many unknowns for us to say anything definite. What I can say is that I, personally, would be very surprised if life was found elsewhere in the Solar System; but if it were, I would look on Titan, Ganymede or Europa first. Evidence of past life may be found on Mars.


Before we look further, I just want to note that liquid water actually does have some very special properties (shared by liquid ammonia) that most other liquids don't have. Perhaps this factor is critical to the formation of life. I will ignore this and assume that some liquid, any liquid, is necessary. The properties of water could easily be the subject of a complete essay!

Beyond The Solar System

It is now time to look further than the Solar System for possible sites for life.

Our Sun is one star in a Galaxy of three hundred thousand million (300,000,000,000). That is 60 stars for each person on the Earth. Of course, stars are not the places to look for life. What we want are planets orbiting these stars, preferably with atmospheres that can hold liquid on their surfaces. How many are there? I don't know but I will make some 'guesstimates'.

How many stars have planets? Planets orbiting other stars (celled extra-solar planets) are not normally visible. They shine by reflected light and are close to the stars shining millions of time brighter. As of 2013, about 700 extra-solar planets have been detected around nearby stars by their gravitational effects on the parent star or by changes to the star light caused by eclipses. It appears to be a general rule that for every large object in the Universe there are dozens of smaller objects. Current technology tends to favour the discovery of large Gas Giant type planets but in recent years planets around the size of the Earth have been found.

Infra red observations have indicated accretion disks around young stars which appear to be planets in the making. Modern theories of star formation require planets to be formed as a by product. In our own Solar System, the Sun has its planets; but then the larger planets are surrounded by their moons. The balance of evidence that we have is that any object that forms in space will be surrounded by smaller objects.

We will assume for this essay that planet formation is a normal part of the formation of stars.

The Earth is now 4,700 million years old. Life apparently took about 1,000 million years to begin. Is this typical? Was life very quick to begin on this planet? Or did it begin more slowly than average? There is no way of knowing. The Earth is the only example of a living planet so I cannot make any sensible deductions about this. The easiest way to proceed is to assume that the progress of life on the Earth was average. In other words, given suitable conditions, life can be expected to require 1,000 million years before it begins. In the We Are Stardust essay, I explain how the lifetime of a star depends on its initial mass. Any star that lasts for less than 1,000 million years will probably not be around long enough for life to form on any planets around it. From a study of the stars in the Galaxy it appears that 80% of the stars (80,000 million) will last long enough for life to begin on a suitable planet around them.

Also from the We Are Stardust essay, I explain how the early stars (first generation stars) were made up of material containing only Hydrogen and Helium. Later stars (second and third generation stars), contained heavier elements. These heavy elements are the material that would give rise to solid planets.

To summarise, stars that are first generation may have planets but the chances are that the planets will not be solid. As it happens, in our Galaxy, only 20% of the stars are later generation stars. These are the ones to look at for solid planets. There are 60,000 million such stars. Recent evidence indicates that some of these stars will contain too high an abundance of heavy elements. This could cause the Gas Giants (which form at a large distance from the parent star) to migrate inwards and upset the orbits of any Terrestrial planets present. I will assume that 15,000 million stars will have the optimum abundance of heavy elements.

A solid planet in orbit around a stable long lived star requires a nearly circular orbit if there is to be a constant source of energy to drive the chemistry of life. More that 50% of the stars are binary, that means they are part of a two star system - a few stars are members of multiple systems. Close binaries (where the stars are near each other) will make the environment unsuitable for life as there may be no stable orbits for planets. If the stars are very distant from each other, each could have a stable planetary system. I will assume that the existance of binary and multiple star systems excludes a third of the stars - this still leaves 10,000 million.

Next, we require a solid planet with an atmosphere to be orbiting a stable long lived star and that planet to actually have liquid on its surface. In the Solar System there are 50 planets / moons but only two or three of them (possibly) have liquids. This is about 1 in 25. Since our Solar System is the only one we know in detail, I'll assume that there is a 1/25 chance of finding liquid on a planet. This means that there are 400 million stars with a planet containing liquid on the surface.

But will life actually form on these planets? What percentage of these suitable planets orbiting suitable stars actually evolve life? Again, I have no idea. All I can do is look at what little evidence is available. Life on Earth began when complex Carbon based molecules began to replicate themselves. There are three areas where we have some evidence on the subject.

The Abundance of Life

During the 1930s, experiments were performed where simple inorganic molecules were mixed together and sparked with electricity (to simulate lightning) or iridated with Ultraviolet light (to simulate the early Earth). After a few days an organic brew of simple organic molecules resulted. Amino Acids were formed. These are the building blocks of proteins.

Occasionally, rocks from space strike the Earth. These are called meteorites. Most are made of stone or metal (usually Iron). A few are made of carbon rich rocks. Within these, scientists have found simple organic molecules. It is possible to distinguish these organic molecules from those made by living things on the Earth. It appears that the chemistry of life occurred in space while the Sun and planets were condensing. Recent experiments suggest that comets colliding with the Earth may be able to transfer organic material.

In the space between the stars, there are the gas clouds (nebulae) from which stars form. These are mainly Hydrogen and Helium. There are however traces of other molecules. Within the last 20 years, Infra red telescopes have detected a few simple organic molecules in these clouds.

In recent years, life has been found in the bottom of the oceans living under high pressure close to super-hot water vents using sulphur as an energy source. Spores have been found deep underground in rock and in ice. Microbes have been detected in clouds. Living organisms left on the Moon for several years and later retrieved have been unharmed. Life appears to be a lot more robust than previously thought.

Number of Life Baring Planets

In all three cases above, the indication is that organic chemistry is a normal part of nature. The complex molecules found have all been based on carbon and nothing else. The indication of this is that given the right conditions, chemistry will move in the direction of complex molecules resembling those of life on this planet. This is good news for seekers of extraterrestrial life. So, how does this help us? Being conservative (with a small c), let us assume that half of the suitable planets will harbour life. This indicates that 200 million life-bearing planets may exist in the Galaxy. So where are they? Assuming that life-bearing planets are scattered at random throughout the Galaxy, the nearest one to the Earth is 1000 million million km (600 million million miles) away.

Let's put that huge figure into perspective. Light is the fastest thing possible. In one second it will travel seven times around the Earth. It takes light 1.5 seconds to travel from the Earth to the Moon. From the Sun, light takes 8.3 minutes to travel to the Earth. The Solar System (the Sun, nine planets, 50 moons, and lots of smaller bits floating around the Sun) is large enough that a beam of light takes twelve hours to travel from one side to the other. From the nearest star, light takes 4.3 years to reach us. To travel the distance of 1000 million million km to our nearest life bearing planet (if the assumptions I have made hold up) would take light over 100 years. If Einstein was correct and nothing can travel faster than light, we may never be able to travel those distances. Conversely, it is highly unlikely that another intelligent life form will ever find us.


As I said at the beginning of this essay, this is all speculation. My own personal view is that life does exist in the Universe, but the chances of us coming into contact with it are very small. I don't believe that the Earth has been visited by spaceships (either in the past or in the present). The search for extraterrestrial life is probably one of the most important quests of the century. Not finding life on the nearby planets doesn't prove anything about how common life is in the Universe. However, if we did find life (say on Mars or Titan), it would indicate that life was a very common phenomenon in the Universe.

The discovery of extraterrestrial life would be the most dramatic scientific, social and religious event in history. Many of our values would change as we realise that we are not unique. Life would then be a normal part of the evolution of the Universe rather than a special event confined to Earth. The repercussions would reverberate over all disciplines for many years.

© 1997-2013 KryssTal

This essay is dedicated to the late Carl Sagan.

Books From and

KryssTal Related Pages

An introduction to the chemistry of organic compounds.

The evolution of stars and the origin of the chemical elements.

An easy-to-understand scaling of the Universe in space. Distances in space are represented by the time light takes to travel there.

An easy-to-understand scaling of the Universe in time. The chronology of the Universe is compared to a real year.

An introduction to the theory of relativity.

Maps showing the distribution of UFO sightings around the world.

External Links

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SETI@Home Site
The Search for Extraterrestrial Intelligence (SETI). Help the search in your own home using your PC.