## The Rare Earth hypothesis

Ever since the pioneering work of Frank Drake (1930-) in 1960, astronomers have been looking for radio signals from extraterrestrial civilisations and have failed to find anything. This could be because Earth-like planets containing complex life forms (such as ourselves) are rare in the Universe and only a series of highly improbable events led to the evolution of intelligent life on Earth. In 2000 geologist and palaeontologist Peter Ward and astronomer Donald Brownlee published a book in which they explained the term ‘Rare Earth Hypothesis’ which they had coined to describe this viewpoint.

However, they were not the first people to arrive at this conclusion. These ideas had been circulating for decades before the publication of their book. For example, the astronomers John Barrow and Frank Tipler discussed them in detail in their 1987 book ‘The Anthropic Cosmological Principle’.

A little background – the Drake Equation

Back in 1961, Frank Drake invented an equation to estimate the number of intelligent civilizations within our galaxy with whom we could potentially communicate, to which he gave the symbol N.  His equation, known as the Drake equation, consists of seven numbers multiplied together:

N=  R* x FP x NE x FL x FI x FC x L

in which

• R* is the average number of stars formed per year in our galaxy. Current estimates are that this has a value of around 10.
• FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1. A value of 1 means that all stars have planets. 0 would mean that no stars have planets. Current estimates are that FP is very close to 1.
• NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life. For this to happen liquid water must exist somewhere on the planet.  A reasonable value which many astronomers would agree with is 0.4, meaning that out of every 10 stars which have planets, 4 have bodies which could support life.
• FL is the fraction of bodies with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1.  A value of 1 means that on all  planets with the right conditions life will evolve. There is no consensus among astronomers about the value of FL.
• FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, again expressed on a scale of 0 to 1. Again, there is no consensus among astronomers about what this value is.
• FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space. For example, on Earth TV and radio signals escape into space and could be picked up by a nearby alien intelligence with a sensitive enough receiver tuned to the right frequency. No one knows what the value of FC is.
• L is the average lifetime of a civilisation in years.  Again, there is no consensus on this point.

As the values of many of the terms in the Drake Equation are not known to any degree of accuracy, it cannot be used to provide a meaningful estimate of N.  However, it is still very useful to illustrate the factors involved

Could we be alone? Could FI could be very very low?

Perhaps the term with the biggest degree of uncertainty in the Drake equation is FI, the fraction of bodies having life on which life has evolved into intelligent civilisations. I will outline the arguments for believing that this number is very low indeed.

The Earth is roughly 4.6 billion years old. Simple single-celled lifeforms emerged 300 million years later, a relatively short time after the Earth had cooled enough for liquid water to exist. These simple cells, called prokaryotes, cannot form complex organisms where different types of cells perform different functions. However, individual prokaryotes  can group together in colonies, forming a kind of slime.

Colonies of prokaryotes

All complex life on Earth is based upon cells called eukaryotes. These cells have a nucleus (containing the genetic material of the cell), structures called mitochondria, which regulate the cell’s energy and other specialised units known as organelles.

The first eukaryotes didn’t emerge until 2 billion years after prokaryotes, indicating that it was a much bigger step in evolution than the emergence of the first simple cells. It could well be the case that the large number of sub-steps needed in the evolution from prokaryotes to eukaryotes means that even where simple lifeforms have emerged,  in the vast majority of cases there has been no further evolution beyond this stage.

Even after the emergence of these complex eukaryotes, it would take over one billion years before multi-cellular life forms such as the first plants and animals appeared. In these organisms cells are specialised, so different types of cells perform different functions within the organism. Given the large amount of time taken to move from eukaryotes to complex organisms, it might well be the case that even if something akin to eukaryotes emerge on a planet, which have the potential to eventually evolve into multi-celled organisms, evolution proceeds no further. The average time taken to evolve from complex cells to multi-celled organisms given favourable conditions might be 2, 5 or even 10 billions years.

The role of mass extinction events

It has taken 600 million years from the appearance of the first animals to the emergence of Homo sapiens in Africa, around 200,000 years ago. Over this vast amount of time the vast majority of species have disappeared and have been replaced by other species which are better suited to the changing environment. However, the disappearance of species and emergence of new ones doesn’t occur at an even rate. Every 50-100 million years there have been catastrophic events which have caused periodic mass extinctions when a large number of species failed to survive. The most dramatic of these was at the end of the Permian period, around 230 million years ago, when 95% of land and 70% of sea species became extinct.

Fossil of a Trilobite – one of many species to disappear in the mass extinction at the end of the Permian Period

Perhaps the best known mass extinction occurred 65 million years ago when a massive comet or asteroid 10 km in diameter hit the Earth at a speed of up 50,000 km per hour. Its high speed coupled with its huge mass meant that it smashed into the Earth in the area now known as the Yucatan peninsula with an energy 6 billion times greater than the atomic bomb dropped on Hiroshima at the end of the Second World War.

The impact melted much of the local crust and blasted molten material outwards. Any object near to the impact site would have been instantly vapourised. Such was the energy of the impact that some of the Earth’s crust was thrown upwards with so much velocity that it went out into space. Over the next few hours molten rock, dust and ash rained down on an area millions of square kilometres in area. This hot material would have ignited fires, destroying plant and animal life within a large area.

The impact 65 million years ago occurred in what is now the Yucatan peninsula in Mexico.  The outer ring shows the area which become covered in debris.

A fine cloud of dust from the impact circled the entire Earth. This blocked out sunlight, causing the Earth’s temperature to fall by about 15 degrees Celsius. The sudden drop in temperatures and lack of sunlight getting through meant that plant growth stopped for several months.  As a result, many species of herbivorous animals which fed on those plants became extinct and the carnivores which fed on the herbivores became extinct too. Those species which did survive had their numbers drastically reduced.  Nearly 75% per cent of the species of animals alive before the impact became extinct, including every species of dinosaur. After the disappearance of the large reptiles, mammals became the dominant land animals.

Although we as a species are now probably sufficiently advanced as to be able to survive a mass extinction event, even if only in small numbers, this would not have been the case if one had occurred earlier in our development. If one had happened 150,000 years ago, when Homo sapiens were few in number and less equipped to withstand a major famine – those hunter gatherers would have had nothing to hunt and nothing to gather for many months – we would have become extinct long before we could develop civilisation.

Indeed,  if mass extinctions were to occur every million years, rather than every 50 million years, it is difficult to see how any intelligent species could ever evolve. It would be wiped out before it became fully established. The reason why mass extinctions occur infrequently is due to the unusual layout of the Solar System, and I’ll talk about this next.

Our special Solar System

The Sun is a single star in an uncrowded region of space, in the outer regions of the Milky Way galaxy. The nearest star to the Sun is over 4 light years (40 trillion km) away, more than ten thousand times further than the outermost planet Neptune. If the Sun were in a more crowded region of the galaxy, a passing star’s gravity could easily disrupt the Earth into a different orbit, which might be closer to Sun, making it too hot to support life, or further away, making it too cold to support life.

The giant planet Jupiter is more than 300 times the mass of the Earth. As comets enter the inner Solar System from its outer reaches, Jupiter’s gravity slings most of these fast-moving ice balls out of the Solar System before they can get close to Earth.  Without Jupiter, comets like the one which hit the Earth 65 million years ago would collide with our planet much more frequently.

Image from NASA

Observations of planets detected around other stars have shown that arrangements of planets similar to our Solar System, with small inner rocky planets surrounded by massive giants in the outer reaches, are relatively rare.

Another factor which may be essential to the emergence of complex lifeforms is the presence of a large moon close to the planet. The Moon is 25% of the Earth’s diameter and is only 400 000 km away, a very short distance in astronomical terms.

Relative sizes of the Earth and the Moon

The Moon  is much larger in comparison to its parent planet than any other moon in the Solar System. This means that the Moon’s gravity stabilises the Earth’s tilt so that it doesn’t vary too much from its current value. Without the Moon there would be massive swings in the tilt between 0 degrees, when there would be no seasons, and 50 degrees, where there would be extreme seasons where much of the planet would be in darkness or full daylight for months at a time.

The Earth’s magnetic field

The Earth’s magnetic field is generated by convection currents stirred up by rotation in its liquid outer core, which is made out of iron. In fact for any planet to have a strong magnetic field, part of its interior must consist of a liquid which conducts electricity and it must be rotating rapidly enough to generate convection currents.

Without a magnetic field the planet would not have a protective ozone layer shielding its surface from deadly ultraviolet radiation. It would be very difficult for advanced lifeforms to exist on the surface.  It is not known how many planets around other stars satisfy the criteria to have a magnetic field. However, the Earth is the only one of the inner planets in our Solar System (Mercury, Venus, Earth and Mars) to have a strong magnetic field.

Planetary size

In order for life to evolve, the planet must be large enough to retain a significant  atmosphere, without which liquid water cannot exist. Mars is roughly one tenth the mass of the Earth and its gravity is so low that its atmospheric pressure is only 0.6% of that of Earth – too low for liquid water to exist.

Man has been lucky to survive

In addition to periodic mass extinctions which wipe out most species, all populations come under pressure due to sudden changes in climate, such as ice ages, major volcanic eruptions, loss of habitat, disease and competition from other species for food and shelter. The first Homo sapiens were confined to a single region of Africa and were few in number, and thus vulnerable to becoming extinct. 70,000 years ago a super-volcano erupted in Indonesia causing the Sun to be partially blocked out and resulting in a fall in global temperatures. Only an estimated 15,000 humans are thought to have survived (Edwards 2010).

Summary

If we take all the factors I’ve talked about into account, for single-cell organisms to evolve into intelligent lifeforms capable of building civilisations, all of the following must happen.

• Single cell creatures must have evolved in into complex cells, something akin to eukaryotes.
• To reduce the frequency of mass extinctions, the planetary system must have a large giant planet in its outer reaches.
• To ensure the planet remains at a habitable temperature, the parent star must be a single star in a less crowded region of the galaxy.
• To ensure the planet remains at a habitable temperature over a long period of time, the parent star’s energy output must not vary too much.  This constraint rules out advanced life around the most common type of star in the galaxy, red dwarfs .
• To reduce the frequency of dramatic changes in climate leading to mass extinctions, the planet must have a large satellite.
• The planet must rotate rapidly enough to generate a magnetic field, as without this complex life would not be possible on its surface.
• The planet must be large enough to hold an atmosphere.
• Multi cellular organisms must have evolved.
• The planet must have a significant fraction of its surface as dry land. The assumption is that although aquatic creature such as dolphins are undoubtedly intelligent, only land based creatures could build civilisations.
• A land-based intelligent species has emerged during a survival of the fittest.
• The intelligent species has managed to survive major environmental changes such as ice ages and has built a civilisation.

Putting all these together, followers of the Rare Earth hypothesis believe that the probability of all the above happening on a planet on which simple life has evolved could be as low as a billion to one.

If it were this low, then the expected number of intelligent communicating civilisations in a galaxy such as ours would be very low.  If the disputed figures within the Drake equation were guessed to be on the low side, N could be as low as 0.000016.  This would mean that not only are we the only intelligent communicating civilisation in our own Milky Way galaxy, but also that there are none in any galaxies within tens of millions of light years of us.

If this is the case then the Earth would not be just be an ordinary planet orbiting an ordinary star in an ordinary galaxy.  It would be a very special place indeed!

Reference

Edwards, L. (2010) Human were once an endangered species, Available at: https://phys.org/news/2010-01-humans-endangered-species.html#jCp (Accessed: 7 April 2018).

## The zoo hypothesis

This post is about the zoo hypothesis, a term coined in 1973 by the astronomer John Ball, but the idea had been in existence for decades before then.  The zoo hypothesis states that there are many advanced and intelligent alien civilisations out there, but they hide their existence from us so that they they do not interfere with our development. Human beings are effectively in a cosmic zoo being observed by more advanced aliens.

As I’ll talk about later, the zoo hypothesis has formed the basis for many science fiction stories.

Origins of the zoo hypothesis

As discussed in a previous post, the Sun is one of 400 billion stars in our Milky Way galaxy and there are hundreds of billions of galaxies in the observable Universe. In 1961 Frank Drake invented an equation to estimate the number of intelligent civilisations within the Milky Way with whom we could potentially communicate, to which he gave the symbol N.  Drakes’s equation consists of seven numbers multiplied together:

N=  R*  x  FP  x  NE x FL x FI x FC x L

where

• R* is the average number of stars formed per year in our galaxy
• FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1
• NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life
• FL is the fraction of bodies with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1
• FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, expressed on a scale of 0 to 1
• FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space
• L is the average lifetime of a civilisation in years.

However, because there is no consensus among astronomers what these value are, the Drake equation cannot tell us with any degree of certainty how many alien civilisations there are. If the values of one or more of these numbers are very low, then the value of N will be very low and we could be the only intelligent life form in the galaxy.

What our Milky Way galaxy would look like from outside – Image from NASA

On the other hand, if most of the values in the Drake equation are very high, intelligent life would be very common and there could be as many as 80 million intelligent communicating civilisations in the galaxy. If this is the case, many of these civilisations will have been around for a long time.

On Earth civilisation is only a few thousands of years old and in the last 100 years,which is an incredibly short time compared to the lifetime of the galaxy, we have discovered nuclear energy, made huge advances in microelectronic and computing, sent space probes to explore the entire solar system and sent astronauts to the Moon. Within the next 30 years humans may walk on the surface on Mars. If a civilisation were millions of years old it is likely that it would have colonised the neighbourhood where it first emerged and would have spread throughout the galaxy, perhaps in cooperation with other advanced civilisations.

It might even be the case that in certain civilisations intelligent machines have become so advanced that they have taken over from the creatures who originally built those machines. It would be easier for aliens to disperse themselves outside their own solar system if they had evolved into machines which are more robust and longer lasting than carbon-based lifeforms.

Assuming such civilisations exist and are widespread throughout the galaxy, they would almost certainly have visited the Earth or at the very least be watching us and monitoring our development. For nearly sixty years astronomers such as Frank Drake have been searching for radio signals with sensitive radio telescopes.  However, no transmission has ever been detected from any alien intelligence and there is no scientifically verified evidence that aliens have ever visited Earth (see notes).

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Frank Drake – image from Wikimedia Common

If aliens were to contact us it would be the greatest event in human history. If they chose to do so, aliens could give us the huge advances in technology which we had not yet discovered for ourselves. This would interfere with our development, which would not then follow its natural course. So, in the zoo hypothesis, aliens choose to remain hidden and place the Earth ‘out of bounds’.

In some versions of the zoo hypothesis the aliens watching us will reveal themselves when civilisation on the Earth has advanced to a certain level, for example when humanity has been able to achieve interstellar travel.

The zoo hypothesis in science fiction

Unsurprisingly, the zoo hypothesis has been a popular topic for many science fiction writers.

In one of my favourite Arthur C Clarke novels, Childhood’s End (published in 1953), an alien civilisation known as the Overlords have been observing the Earth’s evolution and human history for thousands of years.

At the beginning of the book, when mankind is about to achieve spaceflight, the Overlords reveal themselves. They then supervise humanity’s development to ensure that we do make any mistakes such as destroying ourselves in a nuclear war.

Those of you who follow Star Trek may recall a rule of the Federation called the ‘Prime Directive’. This prohibits Starfleet personnel from interfering with the internal development of alien civilisations which are below a certain threshold of technological, scientific and cultural development.

In ‘The State of the Art’ a novel by the Scottish science fiction writer Iain M Banks (1954-2013), an advanced interstellar civilisation called ‘The Culture’ secretly visit the Earth. They decide to leave the Earth without revealing themselves to its inhabitants so that they can watch its development as if it were a control group in an experiment.

In ‘Cancelled’, an episode of the adult cartoon series South Park, the whole of Earth is the subject of a reality show watched by an advanced alien intelligence. When the aliens realise that humans have discovered that they are participants in a show, the aliens consider whether or not to cancel the show. This is because, now that humanity realises what’s happening, the show’s quality will be reduced. Unfortunately, cancelling the show means destroying the Earth. In the end a deal is done whereby the show is allowed to continue but everyone on Earth has their memory wiped of the knowledge that they are participants.

These are just four examples, but there are many other works of fiction which are based around the zoo hypothesis.  Feel free to mention your favourites in the comments!

Problems with the zoo hypothesis

Although it has provided an interesting basis for many science fiction writers, only a very small minority of astronomers believe in the zoo hypothesis. One problem with it is that if intelligent alien life is commonplace within our galaxy, then it will have arisen independently in many places. Each of these civilisations is likely to have very different rules and values. It is difficult to see how all of them would adhere to something akin to the prime directive. It would only take one of these alien civilisations to try to contact us for the zoo hypothesis to break down.

I am convinced by the arguments made by scientists (such as the British theoretical physicist and popular science writer John Barrow) who believe that some of the values in the Drake equation should be set very low. There are a large number of steps which occurred between the emergence of the first primitive single-celled life forms and the evolution of man. Each of the individual steps may have a very low probability. So FI – the probability of life evolving into intelligent civilisations – would be extremely small and mankind is probably the only intelligent civilisation not only in our galaxy but in the thousands of neighbouring galaxies as well.  This is sometimes called the rare Earth hypothesis and is such an interesting topic that I’ll talk about it in a future post.

Notes

This is sometimes called the Fermi Paradox,  named after the Italian-American physicist Enrico Fermi (1901-1954) who first made this point in 1950.

Image from Wikimedia Commons

Fermi’s argument is as follows:

• There are billions of stars in the galaxy that are similar to the Sun, and many of these stars are billions of years older than Earth.
• It is highly probably that some of these stars will have Earth-like planets, and some might develop intelligent life.
• Some of these civilisations might develop interstellar travel.
• When a civilisation develops interstellar travel, it may visit vast numbers of place in the Milky Way within a few million years.
• Therefore, the Earth should have already been visited by aliens.
• So where is everybody?

## Life in our galaxy?

With the recent discovery of three planets orbiting the red dwarf star Trappist-1 which have a similar size, mass and average surface temperature as the Earth, there has been considerable speculation as to whether one or more of these planets supports life.

What the surface of Trappist 1f, one of the planets orbiting Trappist 1, might look like – Image from NASA

Although there are challenges to complex lifetime forms  evolving on a planet around a red dwarf – which I discussed in a previous post – red dwarfs are the most common type of star in our galaxy. In this post I’ll discuss the likelihood that life has evolved in other places within our galaxy, including on planets around red dwarfs.

The Drake Equation

Frank Drake (1930-) is an American astronomer who is known as the ‘father of SETI’ – the Search for Extra Terrestrial Intelligence. Beginning in 1960, he was the first person to search for radio signals from aliens.

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Frank Drake- Image from Wikimedia Common

In 1961 he invented an equation to estimate the number of intelligent civilizations within our galaxy with whom we could potentially communicate, to which he gave the symbol N.  This equation, which is known as the Drake equation, consists of seven numbers multiplied together:

N=  R*  x  FP  x  NE x FL x FI x FC x L

As I’ll explain below, some of these numbers are known to a reasonable accuracy, whereas others are not well known and astronomers differ widely their views of what the values should be.

• R* is the average number of stars formed per year in our galaxy.  Current estimates are that this has a value of around 10.
• FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1. A value of 1 means that all stars have planets. 0 would mean that no stars have planets. Planets are difficult to detect around other stars, because they are far too faint to be seen directly and have to be detected by other techniques. In 1961 Drake estimated that FP lay in the region of 0.2 to 0.5 (i.e between 20% and 50% of stars had planets). Current estimates are somewhat higher and that FP is very close to 1.
• NE is for the average number of bodies, either planets or moons of planets, with the right conditions to support life. Current estimates for this vary considerably.  If most stars were like Trappist-1 then this value would be as high as 3. A reasonable value, which many astronomers would agree with, is 0.4, meaning that out of every 10 stars which have planets, 4 have bodies which could support life.

The Trappist-1 system – image from NASA

• FL is the fraction of bodies, with the right conditions to support life, on which life actually evolves, expressed on a scale of 0 to 1.  A value of 1 means that on all  planets with the right conditions life will evolve. There is no consensus among astronomers about the value of FL. If, in the future, life is found in many other places in our solar system which have the right conditions  e.g. Mars, or in the warm underground oceans of Saturn’s moon Enceladus (see here for more information) then it would be reasonable to assume that, given the right conditions, in general life will evolve and FL is nearly 1 (see note 1).

A geyser of warm water erupting from an underground ocean on Enceladus. Image from NASA

• FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, expressed on a scale of 0 to 1. Again, there is no consensus among astronomers about what this value should be. Enthusiasts for extra terrestrial intelligence such as Drake believe that the value is close to 1, meaning that intelligent life will always evolve. Others, who believe that it was a highly improbable chain of events which led to the eventual evolution of man from single celled creatures, believe the value is very low.
• FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space. For example, on Earth TV and radio signals escape into space and could be picked up by a nearby alien intelligence with a sensitive enough receiver tuned to the right frequency. No one knows what the value of FC is, but current estimates are around 0.2.
• L is the average lifetime of a civilisation in years. This could be very short if civilisations end up destroying themselves once they have discovered nuclear weapons – or it could be hundreds of millions of years.

The Optimists’ View.

As said previously, no one really knows what the values of most of the terms in the Drake equation are. If we go for values at the high end (FP= 1, NE=0.4, FL=1, FI=1, FC=0.2, L= 100 million) then we get the following:

N= 10 x 1 x 0.4 x 1 x 1 x 0.2 x 100,000,000

which works out as 80 million intelligent communicating civilisations in our galaxy!

One of the problems with such a large number is that we would expect a significant fraction of civilisations to be more advanced than us. Humans have only been civilised for a few thousand years and have already travelled into space.  If a civilisation had been around for more than 1 million years, for example, it is likely that they would have developed the ability to travel the vast distances to other planetary systems and would have already attempted to make contact with us. The fact that they haven’t may mean that civilisations much more advanced than us are rare.

It is also possible (although in my opinion extremely unlikely) that intelligent civilisations do exist and have been observing our planet for a long period of time. They deliberately do not contact us to avoid interfering with our development although they may decide to reveal themselves to us when we reach a certain level of development. This is known as the zoo hypothesis and has appeared in many science fiction stories.

What is clear is that for nearly 60 years, since the pioneering work of Drake in 1960, astronomers have been looking for radio signals from nearby civilisations over a wide range of radio frequencies and have failed to find anything.

Could we be alone ?

Other astronomers believe that some of the values in the Drake equation are very low. There are a large number of steps which occurred between the emergence of the first primitive single-celled life forms and the evolution of man. Each of the individual steps may have a very low probability. So FI the probability of life evolving into intelligent civilisations would be extremely small. For most of the Earth’s lifetime there were only single-celled organisms and, perhaps on most places where life emerges, it never gets beyond this point.

Another point is that mammals only become became the dominant life form after the extinction of the dinosaurs 65 millions years ago. Before that large small-brained reptiles were the dominant life form. Having greater intelligence does not always give an advantage over other traits such as size, speed and physical strength in the survival of the fittest.  There is therefore no guarantee that evolution will result in life forms with the intelligence necessary to develop civilisations.

In addition, dramatic events such as sudden changes in climate can cause any species to become extinct. Roughly 70,000 years ago, an enormous eruption occurred in what is now Sumatra, leaving behind Lake Toba. This triggered a major environmental change which caused the near extinction of the human race.  Humanity could have easily disappeared at this point. Although this has been recently disputed (BBC 2010).

Lake Toba, site of a supervolcano eruption 70,000 years ago – Image from Wikimedia Commons

For these reasons some scientists, such as the British theoretical physicist and popular science writer John Barrow, believe that FI could be around 0.000000001 or even lower. If it were this low, and we take the low end values for for the other parameters, then the expected number of intelligent communicating civilizations in the galaxy would be 0.000016. What this means that if we took 60,000 galaxies similar to our own Milky Way we would on average expect to find only one communicating civilisation. Ourselves!

If this is the case then the Earth would not be just be an ordinary planet orbiting an ordinary star in an ordinary galaxy.  It would be a very special place indeed, for it would be the only place for tens of millions of light years where intelligent life exists.

Notes

1 The Earth was formed about 4.6 billion year ago, and at first its temperature was thousands of degrees – far too hot for life to exist.  The first life forms appeared relatively early in the Earth’s history, when it was less than 1 billion years old when conditions became cool enough for life to exist.  This might seem to indicate that, if conditions are right, then life will evolve relatively quickly. Indicating that, perhaps, FL is close to 1.

Reference

BBC (2010) Toba super-volcano catastrophe idea ‘dismissed’, Available at:http://www.bbc.co.uk/news/science-environment-22355515 (Accessed: 15 Apr 2015).

## Breaking news!

News is just reaching us of the most amazing astronomical discovery which I am ever likely to experience in my lifetime, one which I never thought I would see.  As regular followers of this blog will know, the SETI (Search for Extraterrestrial Intelligence) Institute has been searching for evidence of life in the universe by looking for some signature of its technology. Using the most  powerful astronomical equipment humanity has managed to invent, Frank Drake and his associates have been hunting the universe for alien life forms for well over fifty years. Having narrowed down the search using the mathematical formula known as the Drake equation, astronomers are now thrilled to announce the presence of not only life, but intelligent life, on a hollow planet far, far away.  Pictures just arriving back at Earth from Voyager 2 have stunned scientists all over the world.  The slightly blurred video footage shows a race of small pink knitted creatures living in peace and harmony, thriving on a diet of green soup and blue string pudding.  They use a primitive but decipherable language to communicate with each other and have clearly developed a wide range of problem-solving strategies.  It was only a matter of time before they developed long-range space flight.  Negotiations are at this moment taking place at the highest level between world leaders and a representative from the planet, the Soup Dragon, to plan a trip to Earth as soon as possible.  For exciting film footage of the new discoveries, please click here!

## This is Earth Calling

Sending Messages to be picked up by alien civilisations is called METI (Messaging to Extraterrestial Intelligence) and is an activity which has taken place with increasing frequency over the past few decades. Advocates of METI argue that in addition to searching for extraterrestrial signals we can increase our chances of making contact with aliens by sending messages to stars which have planets on which intelligent life may have evolved.  The alien life forms, if they receive our signal from Earth, should recognise it as a message from another civilisation and, in theory at least, be able to extract the information contained in the message.

One of the challenges of such a message is writing it in such a way that it could be understood by an alien civilisation receiving it. The recipients would have no knowledge of Earth or of any of its culture and would in it all likelihood look and act very different from human beings.

The difficulties faced when trying to decipher a message in an unknown language are illustrated by the hundreds of stone tablets on the Greek island of Crete written in a language which archaeologists call Linear A. All these tablets were produced around 3,500 to 4,500 years ago (roughly the time of ancient Egyptian civilisation) but nobody has managed to decipher them. Most of the letters in the Linear A alphabet appear in no other known language. Huge efforts have been made over the years to decipher the Linear A, but to no avail.  What is needed is a clue like the Rosetta Stone, which contains the same text in three different languages and its discovery provided the key to finally understanding Egyptian hieroglyphics.

A Stone tablet with an inscription written in Linear A -Image from Wikimedia Commons (Ursus)

Frank Drake and The Arecibo Message

The first and the most famous message sent from Earth intended for alien civilisations was the Arecibo Message. This message was constructed by Frank Drake(1930-) who, as mentioned in my previous posts, was one of the pioneers of the search for extraterrestrial intelligence.

Frank Drake author of the Arecibo Message – Image From Wikimedia Commons (Raphael Perrino)

Drake decided to convey basic information about humanity by sending a simple picture which someone receiving the message could easily construct. The message was transmitted from the Arecibo telescope at a wavelength of 12.6 cm and consisted of a sequence of 1,679 zeros and ones (see the notes at the end of this post). The number 1679 was chosen because its only factors (numbers which divide into it without giving a remainder) are 23 and 73. Drake felt that when the message was interpreted by an alien civilisation they would naturally write out the message in a 23 by 73 or a 73 by 23 grid.

Understanding the message

The message consists of seven separate components. This is shown more clearly if we colour code the different parts of the message as shown below.

The Arecibo Message colour-coded -Image from Wikimedia Commons. The message is written out in a grid of 73 rows and 23 columns, which is the way it was intended to be read.

All zeros in the message are coded black. The ones in the message are colour coded to show the component they belong to.  A civilisation receiving this message would have to work out the components themselves.

1. The white blocks at the top show the numbers from one  to ten. To understand how to read these numbers from the white blocks see the notes at the bottom of the post.
2. The purple blocks gives the numbers 1, 6, 7, 8 and 15. These are the atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus, which make up DNA. DNA  holds the genetic information of all living organisms found on Earth except viruses. If the civilisation reading this message had their genetic information held in a similar way then this part of the message should make sense to them.
3. The green blocks give the chemical formulae, i.e. the number of atoms of hydrogen, carbon, nitrogen, oxygen and phosphorus of each of the basic building blocks of DNA.
4. The blue blocks in the middle show the double helix which is the basic structure of DNA. Superimposed upon this are the white blocks which show a massive number approximately 4.3 billion which is the number of basic building blocks in DNA.
5. The red figure shows a graphic of a human.  To its left, in white,  is a number which gives the height of an average human. Clearly this could not be expressed in any Earth-based units such as metres or feet and inches, because these would not make sense to an alien civilisation. Drake thought the only natural unit to use was multiples of the wavelength the Arecibo message was transmitted on (12.6 cm).  So the value given is 14 because 12.6 cm multiplied by 14 gives 176.4 cm the aforementioned average height of a human. In white, to the right of the human, is a very large number, just over 4 billion, which was the human population of Earth when the message was transmitted.
6. The yellow blocks near the bottom show a plan of the Sun and the nine planets – the Solar System. The third block is raised to indicate which of the planets the message is coming from. Interestingly, this is now rather out of date as there are now considered to be only eight planets. Since 2006 Pluto has no longer been classified as a planet because of its very small size.
7. The bottom of the message shows a picture  of the Arecibo telescope and a number giving its diameter in multiples of 12.6 cm.

Transmission

The message was transmitted on 16 November 1974 and  was aimed a globular cluster, a dense collection of stars, about 25,000 light years away. It was sent a total of three times. It will take about 25,000 years to reach the position where its target was in 1974, by which time the original target will have moved, although, in theory, it could be intercepted by any alien civilisation along the way.

The Arecibo messages was re-sent to 8 nearby stars, which were known to have planets, in 1999 and 2003.

Subsequent messages to alien civilisations

In addition to the Arecibo message there have been a number of other attempts to send messages to extraterrestrial civilisations. Perhaps the most famous was the “A Message from Earth” transmission. This was sent in October 2008 towards Gliese 581c, an  Earth-like planet which orbits the star Gliese 581, and is  about 20 light years from Earth. The message is a digital time capsule containing 501 different messages that were selected through a competition on the social networking site Bebo. It contained a variety of information, such as details about the contributors’ own lives and ambitions and images of famous landmarks and famous people – for more information click here.

The large radiotelescope used to transmit “A Message from Earth” – Image from Wikimedia Commons

I think that if an alien civilisation on Gliese 581c were to receive this message they would find it impossible to decipher.  For one thing, most of the text messages are in English!

However Seth Shostak, an astronomer who has long been involved in SETI, believes that whether or not aliens who might receive the messages would understand them is beside the point. They would at least recognise that there was some sort of message coming from Earth. In 2008 he told the BBC:

“The point might simply be: well, here we are; we’re clever enough to build a radio transmitter”.

Summary

As yet although numerous message have been sent from Earth we have not received a reply. This might be expected given the vast distances involved. For example, if  “A Message from Earth” reaches its target, and the alien civilisation decides to reply by sending a message back to Earth we will not hear anything before the year 2049.

Another very interesting point to make is that it may not be a good idea to draw the attention of a very advanced civilisation to ourselves. Indeed in a 2010 documentary the British theoretical physicist Stephen Hawking said:

“We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet,” …. “I imagine they might exist in massive ships … having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach.”

He went on to say that a visit by extraterrestrials to Earth might well be like Christopher Columbus arriving in the Americas, “which didn’t turn out very well for the Native Americans.”

Stephen Hawking -Image from NASA

Opinion vary widely about how widespread intelligent life is. My view, as outlined in my previous posts, is that we won’t ever hear any replies from any of these messages. I am convinced  by the argument that although life may be common in our galaxy, intelligent life leading to advanced civilisations are very rare and we are very likely to be the only such civilisation in our galaxy.

Next post

I hope you have enjoyed reading this. My next post which will be the final one on communicating with extraterrestials will be about the messages sent on five spacecraft which are currently in space and are due to escape from the Solar System into interstellar space over the next few decades.

Notes

1) Content of the Arecibo Message in zeros and ones

The number 1,679 is equal to 23 multiplied by 73 and if we arrange the 1679 zeros and ones into a grid of 73 rows by 23 columns we get the following:

```00000010101010000000000
00101000001010000000100
10001000100010010110010
10101010101010100100100
00000000000000000000000
00000000000011000000000
00000000001101000000000
00000000001101000000000
00000000010101000000000
00000000011111000000000
00000000000000000000000
11000011100011000011000
10000000000000110010000
11010001100011000011010
11111011111011111011111
00000000000000000000000
00010000000000000000010
00000000000000000000000
00001000000000000000001
11111000000000000011111
00000000000000000000000
11000011000011100011000
10000000100000000010000
11010000110001110011010
11111011111011111011111
00000000000000000000000
00010000001100000000010
00000000001100000000000
00001000001100000000001
11111000001100000011111
00000000001100000000000
00100000000100000000100
00010000001100000001000
00001100001100000010000
00000011000100001100000
00000000001100110000000
00000011000100001100000
00001100001100000010000
00010000001000000001000
00100000001100000000100
01000000001100000000100
01000000000100000001000
00100000001000000010000
00010000000000001100000
00001100000000110000000
00100011101011000000000
00100000001000000000000
00100000111110000000000
00100001011101001011011
00000010011100100111111
10111000011100000110111
00000000010100000111011
00100000010100000111111
00100000010100000110000
00100000110110000000000
00000000000000000000000
00111000001000000000000
00111010100010101010101
00111000000000101010100
00000000000000101000000
00000000111110000000000
00000011111111100000000
00001110000000111000000
00011000000000001100000
00110100000000010110000
01100110000000110011000
01000101000001010001000
01000100100010010001000
00000100010100010000000
00000100001000010000000
00000100000000010000000
00000001001010000000000
01111001111101001111000
```

2) How to read the numbers in the message

The first four rows show the numbers from 1 to 10  in binary format. The fourth row marks the beginning of each number and is to be ignored.  The first, second and third rows give the value of the number as shown in the table below.

## SETI

As I discussed in my last post, the search for signs of extraterrestrial life has caught the interest of many people for a very long time, and the specific search for signals from other life forms has been a particular source of fascination over the last 50 years.

In the 1997 movie Contact Jodie Foster’s character detects an alien broadcast from a nearby star while listening to radio signals.  It was a huge box office hit.

Poster for the 1997 Warner Bros. film contact

In the poster above you can see that Jodie Foster is wearing a pair of headphones. However, I’m sorry to disappoint you, but astronomers do not actually sit around looking cute and wearing big headphones. All analysis is done by computers which monitor millions of channels simultaneously, but this wouldn’t make such a good movie!

What types of signals are we looking for ?

The search for extraterrestrial intelligence (SETI) is almost entirely carried out using the large radio telescopes which are used to identify and examine astronomical objects like galaxies, gas clouds or stars.  These astronomical entities transmit radio signals over a wide range of frequencies at the same time, and this can be clearly picked up and identified by the powerful telescopes.  Astronomers have, however, reached a consensus that an alien civilisation trying to make contact with other life forms would be likely to transmit radio signals at one particular frequency, rather like a radio station.  The other characteristic of a deliberate attempt at communication would be that the signals contained some identifiable patterns.  Signals from something like a gas cloud would not contain any such patterns or information.

Radio waves from a natural source such as a gas cloud (A) are spread over a much larger range of frequencies than an artificial broadcast such as a radio message (B).

Essentially there are two types of signal we could in theory receive from an extraterrestrial intelligence.

1. A signal which has been sent out by an alien civilisation not intended to received by us. Radio and TV signals generated by humans on Earth escape into space and have been slowly spreading out in all directions into a larger and larger volume of space since the first radio transmissions over 100 years ago. If another civilisation elsewhere had developed similar technologies, we could in theory pick up these signals here on earth.  However, with our current technology these signals would just too weak to pick up on Earth even from the nearest stars. So sadly we cannot yet tune into alien TV broadcasts 😉 – but maybe we will be able to do so in the next ten years or so – if there is anyone out there sending them!
2. A signal sent by an alien civilisation using a powerful transmitter, deliberately beamed at the Earth, with the intention of letting us know that they are there and probably revealing information about itself.  (Although we cannot possibly know for certain what another civilisation would choose to disclose, we could perhaps speculate that it would contain similar information to the message that we ourselves broadcast into space from the telescope in Arecibo in 1973, in the hope that extraterrestrial beings might hear and understand it.  I will talk about this at a later date.)   This second type of signal would be many millions of times stronger than the first type and could be picked up from stars up to one thousand light years away with our existing technology.  (See notes at the end for what is meant by a light year.) Deciphering such a message constructed by an alien intelligence might prove to be incredibly difficult, but there would be no shortage of people who would be happy to devote their entire life to carrying out this amazing task.

The second type of signal (B) is many millions of times stronger than the first type (A) because all the energy is concentrated in a  narrow beam.

Drake’s Experiment

As mentioned in my previous post he first search for radio signals from alien civilisations was performed by a team led by one of the pioneers of SETI, Frank Drake (1930-), in 1960.

The Father of SETI- Frank Drake -Image from Wikimedia Commons (Flanker)

Drake’s team used a radio telescope with a diameter of 85 feet (26 metres) to examine two nearby Sun-like stars that he thought could have planets with intelligent life. He decided to search for signals at a wavelength of 21 cm, which is the wavelength of the radio waves emitted by hydrogen gas. Because hydrogen is the most common element in the universe and the 21 cm radio waves easily pass through the atmosphere of the Earth and any other Earth-like planets, Drake thought that this would be the wavelength that aliens would naturally use to transmit messages to us.

In total he scanned 4,000 narrow radio channel over a four month period. When the data was analysed no detectable alien signals were found.

The telescope in Greenbank West Virginia used by Drake in 1960 to look for extraterrestrial signals. – Image credit NRAO

Later Work

Since Drake’s project there have been a large number of searches for signals from extraterrestrial civilisation by astronomers from many countries in the world. Over the years the sensitivity of the searches has improved by using larger telescopes and cooling the receivers to very low temperatures, which cuts out some of the background noise generated by the telescope itself. This has meant that, when using the largest telescope such as the 305 metre dish in Arecibo, Puerto Rico, we can now look for signals out to the aforementioned distance of about 1,000 light years. Improvements to electronics have allowed more channels to be scanned at the same time.

The giant radio telescope in Arecibo, Puerto Rico- Image from US Government (The Science Geek took some of the observations for his PhD here in the 1980’s 🙂 )

For example, from 1995 to March 2004, a study known as Project Phoenix conducted observations in New South Wales in Australia, Green Bank in West Virginia, and Arecibo. The project observed around 800 sun-like stars over wavelengths between 10cm and 30cm.  Around 2 billion narrow channels were analysed for signals. When the project concluded in 2004, no extraterrestrial signals had been detected, leaving the project leader to conclude that the Earth must be in a quiet corner of the universe.

Funding

Funding for SETI has always been limited. Compared to activities such the space programme, the amount of public money made available has been very small. Until the Allen telescope (see below) was completed there was no radio telescope allocated for SETI. SETI researchers had to compete for time on telescopes used for conventional radio astronomy. In general conventional radio astronomy has always been given priority.

In 1994 Congress cancelled all public funding for SETI, believing it to be a waste of money. For this reason Project Phoenix, and many subsequent studies have been privately funded by the non-profit making SETI institute (www.seti.org).

The Allen Telescope

SETI finally got its own telescope when the Allen telescope (www.seti.org/ata) was built in California. It is named after Paul Allen, the co-founder of Microsoft, who has contributed more than \$30 million to the project  It consists of 42 separate dishes, each 6.1 metres in diameter. The signals from the individual dishes are combined to give the same sensitivity as a large single dish telescope at a fraction of the cost. The Allen telescope, which started operation in 2009, will be used for both conventional radio astronomy and for SETI.  Unlike other telescopes, rather than having to look at an object multiple times to cover different wavelengths it allows a large number of channels to be searched at the same time over the 3-cm to 30-cm wavelength range.  This greatly speeds up the search. It will survey 1,000,000 stars for non-natural extraterrestrial signals with enough sensitivity to detect a strong signal out to 1000 light-years.

Some of the 42 individual telescopes which make up the Allen telescope- Image from SRI International

Will we find anything?

Despite over fifty years of searching, no extraterrestrial signal has ever been detected. It is becoming clear that the locality of our own galaxy is not full of advanced civilisations communicating with each other! My own feeling is that the large searches performed by the Allen telescope won’t find anything and that we will come to the realisation that intelligent life is rare.

It may well be the case that  the Earth is not just an ordinary planet orbiting an ordinary star in an ordinary galaxy, but rather a very special place indeed, for it is the only place for tens of millions of light years where intelligent life exists.

Next Post

I hope you have enjoyed reading this (and there is much more chance of this now that Mrs Geek has corrected my terrible English), and for my next post I’ll been talking about that Arecibo message from Earth to extraterrestrial civilizations.

Notes

When measuring the vast distances to stars, astronomers sometimes use a unit called a light year (ly). One light year is the distance light travels in a year and is equal to 9.46 trillion (9,460,000,000,000) km. To give some idea of scale:

• The distance from the Earth to the Sun is 0.000016 light years.
• The nearest star other than the Sun is 4.2 light years away.
• The centre of our galaxy is 26,000 light years away.
• The gap between my English and Mrs Geek’s is 4.2 billion light years (she wrote this bit).

## Is There Anyone Out There ?

As a child I was fascinated by the idea of people from Earth encountering alien lifeforms. I was an avid watcher of Star Trek and the British science fiction TV shows Dr Who and Blake’s 7.  I am not alone in my fascination – the idea of humans making contact with intelligent aliens from other planets has intrigued people for hundred of years and has been taken up by huge numbers of writers and film-makers. Films such as the Star Trek series, Star Wars, Close Encounters of the Third Kind, Alien and E.T. have been great successes at the box office.

An alien from the 1970s BBC TV science fiction series Blake’s 7

The Earth is one of eight planets which orbit the Sun. The Sun is an ordinary star among the 100 billion or so stars in our Milky Way galaxy.  The Milky Way itself is an average-size galaxy. To me one of the most fascinating questions is this: how likely is it that there are other intelligent civilizations within our galaxy ?

The Milky Way- Image from ESO

Our galaxy the Milky Way. The picture above shows what our galaxy would look like if we were to look at it face on from  a distance of hundreds of thousands of light years away. See notes at the end of the post for what is meant by a light year.

Drake’s Equation and the seven numbers

Frank Drake (1930-) is an American astronomer who is known as the ‘father of SETI’ – the Search for Extra Terrestrial Intelligence. In 1960, he was the first person to search for radio signal from aliens.

.

Frank Drake- Image from Wikimedia Common (Raphael Perrino)

In 1961 he invented an equation to estimate the number of intelligent civilizations within our galaxy that we could communicate with, which he gave the symbol N.  To arrive at N,  Drake multiplied together seven other numbers.

N=  R*  x  FP  x  NE x FL x FI x FC x L

Drake’s seven numbers are follows:

• R* is the number of average number of stars formed per year in our galaxy. This has a value of about 10.
• FP is the fraction of the stars within our galaxy which have a planetary system with one or more planets, expressed on a scale of 0 to 1. A value of 1 means that all stars have planets. 0 means that no stars have planets. Current estimates are around 0.2 to 0.5.
• NE is the average number of bodies, either planets or moons of planets, with the right conditions to support life. Current estimates for this vary widely, but it is sometimes considered to have a value of 0.4, meaning that out of every 10 stars which have planets, 4 have bodies which could support life.
• FL is the fraction of bodies with the right conditions to support life on which life actually evolves, expressed on a scale of 0 to 1.  A value of 1 means that on all  planets with the right conditions life will evolve.There is no consensus among astronomers about the value of FL. If, in the future, life is found in other places in our solar system which have the right conditions  e.g Mars, or in the warm underground oceans of Saturn’s moon Enceladus (see here for more information) then it would be reasonable to assume that, given the right conditions, in general life will evolve and FL is nearly 1.

A geyser of warm water erupting from an underground ocean on Enceladus. Image from NASA

• FI is the fraction of bodies having life, on which life has evolved into intelligent civilisations, expressed on a scale of 0 to 1. Again, there is no consensus among astronomers about what this value should be. Enthusiasts for extra terrestrial intelligence such as Drake believe that the value is close to 1, meaning that intelligent life will always evolve. Others who believe that it was a highly improbable chain of events which led to the eventual evolution of man from single celled creatures believe the value is very low.
• FC is the fraction of bodies with intelligent life which develop a technology that releases signs of their existence into space. For example, on Earth TV and radio signals escape into space and could be picked up by a nearby alien intelligence with a sensitive enough receiver tuned to the right frequency. No one knows what the value of FC is, but current estimates are around 0.2.
• L is the average lifetime of a civilisation in years. This could be very short if civilisations end up destroying themselves once they have discovered nuclear weapons – or it could be hundreds of millions of years.

The Optimists’ View.

As said previously, no one really knows what the values of most of the terms in the Drake equation are. If we go for values at the high end (FP= 0.5, NE=0.4, FL=1, FI=1, FC=0.2, L= 100 million) then we get the following:

N= 10 x 0.5 x 0.4 x 1 x 1 x 0.2 x 100,000,000

which works out as 40 million intelligent communicating civilisations in our galaxy!

One of the problems with such a large number is that we would expect a significant fraction of civilisations to be more advanced than us. Humans have only been civilised for a few thousand years and have already travelled into space.  If a civilisation had been around for more than 1 million years, for example, it is likely that they would have developed the ability to travel the vast distances to other planetary systems and would have already attempted to make contact with us. The fact that they haven’t may mean that civilisations much more advanced than us are rare.

Indeed for over fifty years, since the pioneering work of Drake in 1960, astronomers have been looking for radio signals from nearby civilisations over a wide range of radio frequencies and have failed to find anything.

Could we be alone ?

Other astronomers believe that some of the values in the Drake equation are very low. There are a large number of steps which occurred between the emergence of the first primitive single-celled life forms and the evolution of man. Each of the individual steps may have a very low probability. So FI the probability of life evolving into intelligent civilisations would be extremely small. For most of the Earth’s lifetime there were only single-celled organisms and perhaps on most planets where there is life, it never gets beyond this point.

Another point is that mammals only become became the dominant lifeform after the extinction of the dinosaurs 65 millions years ago. Before that large small-brained reptiles were the dominant life form. Having greater intelligence does not always give an advantage over other traits such as size, speed and physical strength in the survival of the fittest.  There is therefore no guarantee that evolution will result in life forms with the intelligence necessary to develop civilisations.

In addition, dramatic events such as sudden changes in climate can cause any species to become extinct. Roughly 70,000 years ago, an enormous eruption occurred in what is now Sumatra, leaving behind Lake Toba. This triggered a major environmental change which caused the near extinction of the human race. At one stage there were only 2000 individual humans alive on the planet.

Lake Toba, site of a supervolcano eruption 70,000 years ago – Image from Wikimedia Commons

For these reasons, some scientists, such as the British theoretical physicist and popular science writer John Barrow, believe that FI could be around 0.000000001 or even lower. If it were this low, and we take the low end values for for the other parameters, then the expected number of intelligent communicating civilizations in the galaxy would be 0.000016. What this means that if we took 60,000 galaxies similar to our own Milky Way we would on average expect to find only one communicating civilisation. Ourselves!

If this is the case then the Earth would not be just be an ordinary planet orbiting an ordinary star in an ordinary galaxy, it would be a very special place indeed, for it would be the only place for tens of millions of light years where intelligent life exists.

Next Post

I hope you have enjoyed reading this.  In my next post, I’ll be talking about the work which has been done so far to search for extraterrestrial intelligence.

Notes

When measuring the vast distances to stars astronomers sometimes use a unit called a light year (ly). One light year is the distance light travels in a year and is equal to 9.46 trillion (9,460,000,000,000) km. The distance from the Earth to the Sun is 0.000016 light years and the nearest star other than the Sun is 4.2 light years away. The centre of our galaxy is 26,000 light years away and the galaxy itself is 100,000 light years in diameter.