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The Rare Earth hypothesis

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Ever because the pioneering work of Frank Drake (1930-) in 1960, astronomers have been on the lookout for radio alerts from extraterrestrial civilisations and have failed to seek out something. This might be as a result of Earth-like planets containing complicated life kinds (comparable to ourselves) are uncommon within the Universe and solely a sequence of extremely unbelievable occasions led to the evolution of clever life on Earth. In 2000 geologist and palaeontologist Peter Ward and astronomer Donald Brownlee revealed a e-book by which they defined the time period ‘Uncommon Earth Speculation’ which they’d coined to explain this viewpoint.

Nonetheless, they weren’t the primary folks to reach at this conclusion. These concepts had been circulating for many years earlier than the publication of their e-book. For instance, the astronomers John Barrow and Frank Tipler mentioned them intimately of their 1987 e-book ‘The Anthropic Cosmological Precept’.

Slightly background – the Drake Equation

Again in 1961, Frank Drake invented an equation to estimate the variety of clever civilizations inside our galaxy with whom we might probably talk, to which he gave the image N.  His equation, generally known as the Drake equation, consists of seven numbers multiplied collectively:

N=  R* x FP x NE x FL x FI x FC x L

by which

  • R* is the common variety of stars shaped per yr in our galaxy. Present estimates are that this has a price of round 10.
  • FP is the fraction of the celebrities inside our galaxy which have a planetary system with a number of planets, expressed on a scale of zero to 1. A worth of 1 implies that all stars have planets. zero would imply that no stars have planets. Present estimates are that FP may be very near 1.
  • NE is for the common variety of our bodies, both planets or moons of planets, with the fitting situations to assist life. For this to occur liquid water should exist someplace on the planet.  A affordable worth which many astronomers would agree with is zero.four, that means that out of each 10 stars which have planets, four have our bodies which might assist life.
  • FL is the fraction of our bodies with the fitting situations to assist life, on which life truly evolves, expressed on a scale of zero to 1.  A worth of 1 implies that on all  planets with the fitting situations life will evolve. There isn’t a consensus amongst astronomers concerning the worth of FL.
  • FI is the fraction of our bodies having life, on which life has developed into clever civilisations, once more expressed on a scale of zero to 1. Once more, there is no such thing as a consensus amongst astronomers about what this worth is.
  • FC is the fraction of our bodies with clever life which develop a expertise that releases indicators of their existence into area. For instance, on Earth TV and radio alerts escape into area and might be picked up by a close-by alien intelligence with a delicate sufficient receiver tuned to the fitting frequency. Nobody is aware of what the worth of FC is.
  • L is the common lifetime of a civilisation in years.  Once more, there is no such thing as a consensus on this level.

Because the values of lots of the phrases within the Drake Equation usually are not recognized to any diploma of accuracy, it can’t be used to offer a significant estimate of N.  Nonetheless, it’s nonetheless very helpful for example the components concerned

May we be alone? May FI might be very very low?

Maybe the time period with the most important diploma of uncertainty within the Drake equation is FI, the fraction of our bodies having life on which life has developed into clever civilisations. I’ll define the arguments for believing that this quantity may be very low certainly.

The Earth is roughly four.6 billion years outdated. Easy single-celled lifeforms emerged 300 million years later, a comparatively brief time after the Earth had cooled sufficient for liquid water to exist. These easy cells, referred to as prokaryotes, can not kind complicated organisms the place various kinds of cells carry out totally different capabilities. Nonetheless, particular person prokaryotes  can group collectively in colonies, forming a sort of slime.

Colonies of prokaryotes

All complicated life on Earth is predicated upon cells referred to as eukaryotes. These cells have a nucleus (containing the genetic materials of the cell), buildings referred to as mitochondria, which regulate the cell’s power and different specialised items generally known as organelles.

The primary eukaryotes didn’t emerge till 2 billion years after prokaryotes, indicating that it was a a lot larger step in evolution than the emergence of the primary easy cells. It might properly be the case that the massive variety of sub-steps wanted within the evolution from prokaryotes to eukaryotes implies that even the place easy lifeforms have emerged,  within the overwhelming majority of instances there was no additional evolution past this stage.

Even after the emergence of those complicated eukaryotes, it will take over one billion years earlier than multi-cellular life kinds comparable to the primary crops and animals appeared. In these organisms cells are specialised, so various kinds of cells carry out totally different capabilities inside the organism. Given the massive period of time taken to maneuver from eukaryotes to complicated organisms, it would properly be the case that even when one thing akin to eukaryotes emerge on a planet, which have the potential to finally evolve into multi-celled organisms, evolution proceeds no additional. The typical time taken to evolve from complicated cells to multi-celled organisms given beneficial situations could be 2, 5 and even 10 billions years.

The position of mass extinction occasions

It has taken 600 million years from the looks of the primary animals to the emergence of Homo sapiens in Africa, round 200,000 years in the past. Over this huge period of time the overwhelming majority of species have disappeared and have been changed by different species that are higher suited to the altering surroundings. Nonetheless, the disappearance of species and emergence of recent ones doesn’t happen at a good charge. Each 50-100 million years there have been catastrophic occasions which have triggered periodic mass extinctions when numerous species did not survive. Essentially the most dramatic of those was on the finish of the Permian interval, round 230 million years in the past, when 95% of land and 70% of sea species turned extinct.

Fossil of a Trilobite – one in every of many species to vanish within the mass extinction on the finish of the Permian Interval

Maybe the perfect recognized mass extinction occurred 65 million years in the past when an enormous comet or asteroid 10 km in diameter hit the Earth at a pace of up 50,000 km per hour. Its excessive pace coupled with its enormous mass meant that it smashed into the Earth within the space now generally known as the Yucatan peninsula with an power 6 billion occasions better than the atomic bomb dropped on Hiroshima on the finish of the Second World Battle.

KT extinction asteroid

The affect melted a lot of the native crust and blasted molten materials outwards. Any object close to to the affect website would have been immediately vapourised. Such was the power of the affect that a number of the Earth’s crust was thrown upwards with a lot velocity that it went out into area. Over the subsequent few hours molten rock, mud and ash rained down on an space tens of millions of sq. kilometres in space. This sizzling materials would have ignited fires, destroying plant and animal life inside a big space.

Kt ejecta map

The affect 65 million years in the past occurred in what’s now the Yucatan peninsula in Mexico.  The outer ring reveals the realm which change into coated in particles.

A wonderful cloud of mud from the affect circled the whole Earth. This blocked out daylight, inflicting the Earth’s temperature to fall by about 15 levels Celsius. The sudden drop in temperatures and lack of daylight getting via meant that plant development stopped for a number of months.  In consequence, many species of herbivorous animals which consumed these crops turned extinct and the carnivores which consumed the herbivores turned extinct too. These species which did survive had their numbers drastically diminished.  Almost 75% per cent of the species of animals alive earlier than the affect turned extinct, together with each species of dinosaur. After the disappearance of the massive reptiles, mammals turned the dominant land animals.

Though we as a species are actually in all probability sufficiently superior as to have the ability to survive a mass extinction occasion, even when solely in small numbers, this may not have been the case if one had occurred earlier in our improvement. If one had occurred 150,000 years in the past, when Homo sapiens have been few in quantity and fewer geared up to face up to a significant famine – these hunter gatherers would have had nothing to hunt and nothing to collect for a lot of months – we’d have change into extinct lengthy earlier than we might develop civilisation.

Certainly,  if mass extinctions have been to happen each million years, reasonably than each 50 million years, it’s troublesome to see how any clever species might ever evolve. It could be worn out earlier than it turned absolutely established. The rationale why mass extinctions happen sometimes is as a result of uncommon format of the Photo voltaic System, and I’ll speak about this subsequent.

Our particular Photo voltaic System

The Solar is a single star in an uncrowded area of area, within the outer areas of the Milky Approach galaxy. The closest star to the Solar is over four mild years (40 trillion km) away, greater than ten thousand occasions additional than the outermost planet Neptune. If the Solar have been in a extra crowded area of the galaxy, a passing star’s gravity might simply disrupt the Earth into a distinct orbit, which could be nearer to Solar, making it too sizzling to assist life, or additional away, making it too chilly to assist life.

The enormous planet Jupiter is greater than 300 occasions the mass of the Earth. As comets enter the inside Photo voltaic System from its outer reaches, Jupiter’s gravity slings most of those fast-moving ice balls out of the Photo voltaic System earlier than they will get near Earth.  With out Jupiter, comets just like the one which hit the Earth 65 million years in the past would collide with our planet far more continuously.

Picture from NASA

Observations of planets detected round different stars have proven that preparations of planets just like our Photo voltaic System, with small inside rocky planets surrounded by huge giants within the outer reaches, are comparatively uncommon.

One other issue which can be important to the emergence of complicated lifeforms is the presence of a giant moon near the planet. The Moon is 25% of the Earth’s diameter and is barely 400 000 km away, a really brief distance in astronomical phrases.

Relative sizes of the Earth and the Moon

The Moon  is way bigger compared to its mum or dad planet than some other moon within the Photo voltaic System. Which means the Moon’s gravity stabilises the Earth’s tilt in order that it doesn’t differ an excessive amount of from its present worth. With out the Moon there could be huge swings within the tilt between zero levels, when there could be no seasons, and 50 levels, the place there could be excessive seasons the place a lot of the planet could be in darkness or full daylight for months at a time.

The Earth’s magnetic subject

The Earth’s magnetic subject is generated by convection currents stirred up by rotation in its liquid outer core, which is made out of iron. Actually for any planet to have a powerful magnetic subject, a part of its inside should encompass a liquid which conducts electrical energy and it should be rotating quickly sufficient to generate convection currents.

Earth Interior

With out a magnetic subject the planet wouldn’t have a protecting ozone layer shielding its floor from lethal ultraviolet radiation. It could be very troublesome for superior lifeforms to exist on the floor.  It isn’t recognized what number of planets round different stars fulfill the standards to have a magnetic subject. Nonetheless, the Earth is the one one of many inside planets in our Photo voltaic System (Mercury, Venus, Earth and Mars) to have a powerful magnetic subject.

Planetary dimension

To ensure that life to evolve, the planet should be massive sufficient to retain a big  environment, with out which liquid water can not exist. Mars is roughly one tenth the mass of the Earth and its gravity is so low that its atmospheric strain is barely zero.6% of that of Earth – too low for liquid water to exist.

Man has been fortunate to outlive

Along with periodic mass extinctions which wipe out most species, all populations come beneath strain as a result of sudden modifications in local weather, comparable to ice ages, main volcanic eruptions, lack of habitat, illness and competitors from different species for meals and shelter. The primary Homo sapiens have been confined to a single area of Africa and have been few in quantity, and thus weak to changing into extinct. 70,000 years in the past a super-volcano erupted in Indonesia inflicting the Solar to be partially blocked out and leading to a fall in world temperatures. Solely an estimated 15,000 people are thought to have survived (Edwards 2010).


If we take all of the components I’ve talked about into consideration, for single-cell organisms to evolve into clever lifeforms able to constructing civilisations, all the following should occur.

  • Single cell creatures should have developed in into complicated cells, one thing akin to eukaryotes.
  • To scale back the frequency of mass extinctions, the planetary system should have a big big planet in its outer reaches.
  • To make sure the planet stays at a liveable temperature, the mum or dad star should be a single star in a much less crowded area of the galaxy.
  • To make sure the planet stays at a liveable temperature over an extended time frame, the mum or dad star’s power output should not differ an excessive amount of.  This constraint guidelines out superior life round the most typical sort of star within the galaxy, crimson dwarfs .
  • To scale back the frequency of dramatic modifications in local weather resulting in mass extinctions, the planet should have a big satellite tv for pc.
  • The planet should rotate quickly sufficient to generate a magnetic subject, as with out this complicated life wouldn’t be attainable on its floor.
  • The planet should be massive sufficient to carry an environment.
  • Multi mobile organisms should have developed.
  • The planet should have a big fraction of its floor as dry land. The idea is that though aquatic creature comparable to dolphins are undoubtedly clever, solely land primarily based creatures might construct civilisations.
  • A land-based clever species has emerged throughout a survival of the fittest.
  • The clever species has managed to outlive main environmental modifications comparable to ice ages and has constructed a civilisation.

Placing all these collectively, followers of the Uncommon Earth speculation imagine that the chance of all of the above taking place on a planet on which easy life has developed might be as little as a billion to 1.

If it have been this low, then the anticipated variety of clever speaking civilisations in a galaxy comparable to ours could be very low.  If the disputed figures inside the Drake equation have been guessed to be on the low facet, N might be as little as zero.000016.  This is able to imply that not solely are we the one clever speaking civilisation in our personal Milky Approach galaxy, but in addition that there are none in any galaxies inside tens of tens of millions of sunshine years of us.

If so then the Earth wouldn’t be simply be an peculiar planet orbiting an peculiar star in an peculiar galaxy.  It could be a really particular place certainly!



Edwards, L. (2010) Human have been as soon as an endangered species, Accessible at: https://phys.org/news/2010-01-humans-endangered-species.html#jCp (Accessed: 7 April 2018).



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