Where are all the Aliens?: Habitable exoplanets and solutions to the Fermi paradox

First published on 27th January 2012. Last updated on 5th August 2017 by Dr Helen Klus

Are we really alone in the universe, or do other intelligent species know something that we don't?

1. Exoplanets and the Fermi paradox

In February 2011, NASA Space Scientists predicted that about half a billion planets in our galaxy might contain life[1]. Modern cosmology suggests that the Milky Way formed about 9 billion years ago[2], about a billion years before the formation of the first habitable planets[3]. It took about 4 billion years from the formation of the Earth to the evolution of intelligent life[4], so assuming this is not atypical, the first intelligent life forms in the Galaxy could have evolved about 4 billion years ago.

It would only take about half a million years for a self-replicating spacecraft, travelling at 10% the speed of light, to travel across the Milky Way*. This means that spacefaring intelligent life would only have to have evolved once, on any of the hundreds of millions of possible habitable worlds, during the last 4 billion years, for the Galaxy to be almost entirely inhabited.

This is not to mention the fact that the Milky Way is just one of about 50 billion galaxies in the observable universe[5], and that the NASA scientists were only considering life that requires liquid water to survive.

If the number of habitable worlds is so large, and the universe has existed for so long, then Where are all the Aliens?: Habitable exoplanets and solutions to the Fermi paradox

This question is known as the Fermi paradox after Italian physicist Enrico Fermi, who first discussed the idea in 1950, long before any exoplanets had been found[6]. In 1961, American astronomer and SETI (Search for Extraterrestrial Intelligence) founder Frank Drake reformed this query as the Drake equation[7].

If we assume that habitable worlds are common, we can conclude from the Fermi paradox that either:

1.1 Self-replicating spacecraft

Self-replicating spacecraft are probably the quickest and easiest way to explore the Galaxy. In theory, a self-replicating ship would travel to a star system where it uses resources - such as metals extracted from asteroids, or hydrocarbons, like those found on the moons of gas giants - to create replicas of itself. These then head off in different directions, repeating the process.

A self-replicating spacecraft, travelling at about 10% of the speed of light, would take 10 years to travel one light-year. In half a million years, they would be able to travel 50,000 light-years in any given direction, giving a diameter of 100,000 light-years, which is about the same as the diameter of the Milky Way. If our current predictions are correct, and there are at least half a billion habitable planets in the Galaxy, they will come across one about every 24,000 years*.

Self-replicating spacecraft are unlikely to be crewed with biological life forms, given the long timescales involved - they may even outlive the species that created them - but they could have artificial intelligences on board, or could be considered life forms in their own right. Different self-replicating ships may compete with each other for resources, 'mutating' with every replication in order to better fill their evolutionary niche.

Self-replicating spacecraft may be launched for a number of reasons:

Von Neumann probes would be designed to map the Galaxy, and transmit information about stellar systems back to its creators. These probes are named after Hungarian-American mathematician John von Neumann, who studied self-replicating machines[8].

Bracewell probes would be specifically designed to find, and communicate with, other alien species. Australian physicist Ronald Bracewell first proposed this idea in 1960[9]. Bracewell probes will presumably contain some kind of artificial intelligence that can provide information about its creators. If these probes find a habitable world that is devoid of intelligent life, they may wait and see if it evolves, perhaps hibernating until they are activated.

Seeder ships are designed to spread life throughout the Galaxy[10]. They may even contain the genetic information required to replicate their creators.

The most threatening self-replicating probes would set about wiping out life in the Galaxy. These might be launched by an aggressive or paranoid species. They might be created by accident, or they might be formed from benevolent self-replicating probes that have mutated, or made the conscious choice, to stop following their original orders.

Self-replicating probes that wipe out life are known as Berserkers. This name is taken from American science-fiction writer Fred Saberhagen's Berserker series. Saberhagen took the name from the Norse Warriors who were known to kill indiscriminately, giving rise to the English word 'berserk'.

a) Habitable worlds are common, but life is not.

There is currently no way of knowing how often life arises from inorganic matter, even in the 'perfect' environment. As far as we know, this has only happened once on Earth in the last four billion years. Although we might not yet have the capacity to detect microbial life that is not related to our own.

The term 'shadow biosphere' was coined by philosopher Carol Cleland and biologist Shelley Copley in 2005[11]. The term describes a potential second biosphere that has evolved on Earth. If any 'shadow' organisms were found then this would be a strong indication that life might exist elsewhere in the Galaxy.

Artist's impression of a habitable exoplanet

Artist's impression of newly confirmed habitable exoplanet Kepler 22-b. Image credit: NASA/Ames/JPL-Caltech/Public domain.

b) Life is common, but intelligent life is not.

Although life has been present on Earth for almost four billion years[12], only single-celled organisms existed for much of this time. Multi-cellular life may have evolved about 2 billion years ago[13]. Mammals evolved over 200 million years ago[14], and 'intelligent' life has only existed for about 200,000 years[15]. We have only been able to read and write for about 5000 years[16], and have only been industrialised for about 200 years[17]. This means that the vast majority of life on Earth has been 'un-intelligent'.

Intelligence is not an 'end point' of evolution, our intelligence is likely to be an evolutionary accident, and we have no way of knowing how likely it is that any given life form in the Galaxy will be intelligent.

c) Intelligent life is common, but they do not often make the technological advances required for space travel.

Even if intelligent life forms are common, they may not be able develop the technology needed for long distance space travel. We have not yet achieved this ourselves, and so do not know exactly what it will entail. Alien civilisations may not even reach our level of technology if their planet lacks resources, such as fossil fuels, or if they lack the ability to manipulate the external world.

Aliens like these are described as 'The Handicapped' in American science-fiction writer Larry Niven's Known Space series, and may include whales and dolphins on Earth. Aquatic life may have the added problem of not being able to create fire.

It's also possible that civilisations do not often reach the technological level needed for long distance space travel because life is usually destroyed before it can progress to this stage. Many have worried that it's in the nature of technologically advanced civilisations to destroy themselves through incompetence or aggression[18].

Life may also be periodically destroyed by naturally occurring events. There have been numerous mass extinctions on Earth[19], and we face threats from asteroids, gamma ray bursts[20], super-volcanoes[21], and even viruses and bacteria[22]. A more disturbing possibility is that life is periodically destroyed by other spacefaring life forms.

'Alien Parenthood' by Alex Ries.

Alien Parenthood. Image credit: Alex Ries, 2007/Copyrighted, used with permission.

'Pseudoraptor' by Alex Ries

Pseudoraptor. Image credit: Alex Ries, 2009/Copyrighted, used with permission.

d) Intelligent life is common, but they do not often desire space travel.

Perhaps technologically advanced alien civilisations are common, but most species simply don't desire space travel. This may be because it's common for life to evolve in a climate of competition, which gives rise to a fear of the unknown. Although this is evident in humans, it is also a fear we strive to overcome, and this is why we have explored the whole of our planet and travelled to the Moon.

Intelligent life forms may also avoid long distance space travel because it is not economical or sustainable. In 1983, astronomers Carl Sagan and William Newman suggested that self-replicating spacecraft would destroy most of the Galaxy, and so intelligent species would not expand so rapidly[23]. They may also destroy any other self-replicating probes they come across. This would be good for humanity, as any unsustainable rapidly expanding alien species would most likely destroy us before its civilisation collapsed.

It could also be the case that intelligent, technologically advanced aliens may just have different desires to our own. They may not feel a need to leave their planet, or they may spend their time exploring the universe in other ways, perhaps spiritually or through simulated realities[24]. Civilisations like these may be well defended and/or well hidden.

e) Intelligent life has travelled across most of the Galaxy, but they are still unaware of us.

It's possible that intelligent, spacefaring species have travelled throughout the Galaxy but are still unaware of us. This could be because life is extremely common, and we are not of great interest, or it may be due to the scale of the universe, and the fact that intelligent life has only existed on Earth for a relatively short time.

If aliens did once come to Earth and leave a Bracewell probe in the Solar System that is waiting to be activated, then it unlikely that we would have found it yet. A self-replicating probe would most likely stay somewhere with plenty of resources, like the asteroid belt, Kuiper belt, or Oort cloud, and we have scarcely explored any of these places.

Even if a probe was left on Earth, we may not yet have the ability to recognise it for what it is. Microscopic bio-engineered life forms, or molecular nanotechnology, could go completely unnoticed and - to paraphrase British author Arthur C. Clarke - 'any sufficiently advanced technology [may be] indistinguishable from magic'[25].

f) Intelligent life forms are hiding from us, either intentionally or unintentionally.

Perhaps aliens are aware of us, we're just not aware of them. It's possible that most intelligent spacefaring species are so different from us that we cannot detect them - any more than a tree knows when someone walks straight past it - and they cannot communicate with us.

It's also possible that aliens have directly communicated with humans before, during the 195,000 or so years when we did not keep written records.

In 1966, Sagan and Russian physicist Iosif Shklovsky suggested that the superhuman beings discussed in early myths and religions could really be aliens[26]. They highlight the example of the Babylonian fish-god Oannes, who is said to have come out of the sea during the day to teach humanity about writing, art, and science. The idea that aliens visited Earth in our early history has been extensively explored in science fiction.

Aliens may also be intentionally invisible, which means that they could make contact with us at any time. Any species that could master long distance space travel would almost certainly be capable of hiding from us. It's even possible that the 'empty' universe we observe is just a projection, and alien spacecraft are common.

There are many reasons why aliens may be hiding from us. We may be the subject of a scientific experiment, or being used for entertainment. Aliens may be treating the Earth like a wildlife preserve, or they may just be waiting until we reach a certain epoch. One reason for this is that it might help preserve our culture. They may also worry that people would become very afraid, and paranoid, if they revealed themselves, especially if their physical appearance is very different to our own. They are equally likely to avoid us because they find us disturbing to look at, or to communicate with.

Since it would only take one species, or fraction, to blow the cover of the others, these scenarios are more likely if there's only one dominant species of spacefaring aliens, or if different alien intelligences are united by tradition. This is possible; the first spacefaring life forms could be billions of years more advanced than subsequence civilisations, and so may affect their behaviour even after their demise. This is not dissimilar to life on Earth, where we are all born into set political and religious traditions that we have very little way of controlling.

g) Intelligent life forms are hiding from everyone, either intentionally or unintentionally.

Perhaps the most disturbing possibility is that we have not communicated with aliens yet because something is systematically destroying life in the Galaxy, either intentionally or unintentionally. In this scenario, any life forms that do remain will not want to make their presence known.

Aliens may be intentionally destroying life for selfish reasons. There is no definitive link between intelligence and ethics, and so no reason to believe that an intelligent species would be benevolent. Selfish spacefaring aliens may want to eat us, or enslave us for labour or entertainment, or destroy cultures through evangelism. They might attack us out of aggression, or because they consider us a threat. Many have criticised attempts to contact aliens for these reasons[27].

American physicist David Brin agrees compares us to children "in a strange and uncertain cosmos" and states that we should:

...listen quietly for a long time, patiently learning about the universe and comparing notes, before shouting into an unknown jungle that we do not understand[28].

If we're going to send information about ourselves to habitable worlds, then we should consider how much information we want to give away. If we tell a hostile alien species too much about our anatomy they may be better equipped to target us with biological weapons, or invasive species. If they know too much about our technology, they could attack us from a distance through electromagnetic transmissions[29a].

Aliens may also be destroying life in the Galaxy unintentionally. This could be due to the unforeseen result of a scientific experiment, or because of the creation of Berserker probes. A rapidly expanding civilisation may also inadvertently destroy other species through disease, or the introduction of invasive species, just as humans have done on Earth.

It's impossible to know how an alien disease, or species, would affect us. Unless they were specifically bio-engineered, they may have no affect at all, just as a 'shadow biosphere' may go unnoticed. On the other hand, if they are able to interact with us, then an invasive species could wipe out many native ones, and any disease could be extremely lethal and contagious, as our immune systems will not be equipped to deal with it. We may also suffer if species we rely on are destroyed, such as certain bacteria or grass.

h) None of the above.

It's possible that we cannot deduce anything from the Fermi paradox. When we imagine what aliens will be like, we are heavily influenced by what we think future humans will be like, and we are most probably wrong.

It's highly unlikely that anyone could have successfully predicted what life is like in 2012 as little as 50 years ago. The further you go back in time, the less likely it is that any predictions would be correct. It would be almost impossible for anyone born in the middle ages to successfully guess what life is like now, and we are even less likely to successfully predict what will happen to an advanced alien species.

2. How could future alien communication affect us?

Any aliens that communicate with us will almost certainly be more advanced, and so contact will most likely proceed on their terms. They could have the power to completely destroy life on Earth, but on the other hand, they might help us considerably, or provide a combination of good and bad effects[29b].

The idea that alien life forms are destroying all other life in the Galaxy has been discussed above, but it's also possible that we could be specifically targeted for an attack. This may be for reasons beyond our control, perhaps we have a specific talent or resource that they wish to exploit, or are just in their way, but we may also appear to be a threat to other life forms. We have changed the atmosphere in ways that are visible from space, and will be recognised as the product of an intelligent species that is both rapidly expanding and unsustainable[29c].

Aliens may wish to destroy us before we destroy more species on Earth, or before we discover how to build self-replicating ships of our own and become a threat to them. In 1981, British physicist Edward Harrison suggested that advanced aliens might view us in an analogous way to how we view viruses[30].

'Invasion 1' by Nicolas Ferrand

Image credit: Nicolas Ferrand/Copyrighted, used with permission.

Aliens may also damage life on Earth by accident. They may lead us to destroy ourselves if they provide us with information that we misunderstand, they may physically crash into us, or they may not realise the affect that an experiment or brief visit to Earth will have on us.

Russian science fiction writers Arkady and Boris Strugatsky explore this idea in the novel Roadside Picnic, which describes the aftermath of an alien visitation. The aliens do not appear to notice us, and do not stay long, but they leave behind a number of strange artefacts that appear magical to us, and can be dangerous. The visit is described as akin to a human picnic on the side of the road while on a long car journey, the devices are like the rubbish we sometimes leave behind.

Although the knowledge that we are not alone in the universe will have scientific, philosophical, and religious implications that could be both good and bad, alien contact may not have much over-all effect on the future of humanity. This could happen if the aliens do not want to communicate further with us, if communication is too difficult, or if we're unable to learn much from them because they are less advanced than us, or too different. This is more likely to happen if we discover aliens before they discover us. It's also possible that if alien contact is not direct, many will not accept that it has happened[29d].

Alien contact may also be highly beneficial for humanity. A benevolent advanced race may be able to solve many of Earth's problems at little cost. They could teach us their philosophy, mathematics, and sciences, and provide us with new art, music, and literature.

Diameter of the Milky Way = 100,000 light-years = 9.5×1017 (950 million billion) km.
Speed of light = 299,792 km/s,
10% speed of light = 29,979.2 km/s.

Time =
Time =
= 3.2×1013 s = 1,000,000 years.

Assuming they are roughly in the centre, and send ships in each direction, the time is cut in half.

Number of stars in Milky Way = 300,000,000,000.

Number of stars containing habitable planets in Milky Way (assuming each planet is attached to a different star) = 500,000,000.

Number of stars/Number of stars containing habitable planets
= 600.

With an average distance of 4 light-years between stars, it will come across a habitable planet every 4 × 600 = 2400 light-years.

Travelling 10% the speed of light, this will take 24,000 years.

3. References

  1. Borucki, W. J., 2011, 'Kepler Mission Overview and Planet Discoveries', American Association for the Advancement of Science Annual Meeting.

  2. Del Peloso, E. F., Da Silva, L., de Mello, G. P., and Arany-Prado, L.I., 2005, 'The age of the Galactic thin disk from Th/Eu nucleocosmochronology - III. Extended sample', Astronomy & Astrophysics, 440, pp.1153-1159.

  3. Lineweaver, C. H., Fenner, Y. and Gibson, B. K., 2004, 'The galactic habitable zone and the age distribution of complex life in the Milky Way', Science, 303, pp.59-62.

  4. Patterson, C. C., 1953, 'The isotopic composition of meteoric, basaltic and oceanic leads, and the age of the Earth', Proceedings of the Conference on Nuclear Processes in Geologic Settings, 1, pp.36-44.

  5. NASA/Glenn Learning Technologies Project, 'Of Stars and Drops of Water', last accessed 01-06-17.

  6. Jones, E. M., 1985, 'Where is everybody? An account of Fermi's question', NASA STI/Recon Technical Report N, 85, pp.30988.

  7. SETI Institute, 'The Drake Equation', last accessed 01-06-17.

  8. Von Neumann, J. and Burks, A. W., 1966, 'Theory of self-reproducing automata', IEEE Transactions on Neural Networks, 5, pp.3-14.

  9. Bracewell, R. N., 1960, 'Communications from superior galactic communities', Nature, 186, pp.670-671.

  10. Freitas Jr, R. A., 1980, 'A self-reproducing interstellar probe', Journal of the British Interplanetary Society, 33, pp.251-264.

  11. Cleland, C. E. and Copley, S. D., 2005, 'The possibility of alternative microbial life on Earth', International Journal of Astrobiology, 4, pp.165-173.

  12. Moorbath, S., 2005, 'Palaeobiology: Dating earliest life', Nature, 434, pp.155.

  13. El Albani, A., et al, 2010, 'Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago', Nature, 466, pp.100-104.

  14. Fraser, N. C., Walkden, G. M. and Stewart, V., 1985, 'The first pre-Rhaetic therian mammal', Nature, 314, pp.161-163.

  15. McDougall, I., Brown, F. H. and Fleagle, J. G., 2005, 'Stratigraphic placement and age of modern humans from Kibish, Ethiopia', Nature, 433, pp.733-736.

  16. Schmandt-Besserat, D., 1992, 'From Counting to Cuneiform', University of Texas Press.

  17. McNeese, T., 2000, 'The Industrial Revolution Era: 1760-1830', Milliken Publishing Company.

  18. Sagan, C., 1973, 'Carl Sagan's Cosmic Connection: An Extraterrestrial Perspective', Cambridge University Press.

  19. Barnosky, A. D., et al, 2011, 'Has the Earth's sixth mass extinction already arrived?', Nature, 471, pp.51-57.

  20. Melott, A. L., et al, 2004, 'Did a gamma-ray burst initiate the late Ordovician mass extinction?', International Journal of Astrobiology, 3, pp.55-61.

  21. Timmreck, C., et al, 2010, 'Aerosol size confines climate response to volcanic super-eruptions', Geophysical Research Letters, 37, pp.24705.

  22. Pedersen, A. B., Jones, K. E., Nunn, C. L., and Altizer, S., 2007, 'Infectious diseases and extinction risk in wild mammals', Conservation Biology, 21, pp.1269-1279.

  23. Sagan, C. and Newman, W. I., 1983, 'The solipsist approach to extraterrestrial intelligence', Quarterly Journal of the Royal Astronomical Society, 24, pp.113.

  24. Bostrom, N., 2003, 'Are we living in a computer simulation?', The Philosophical Quarterly, 53, pp.243-255.

  25. Clarke, A. C., 2000, 'Profiles Of The Future: An Inquiry into the Limits of the Possible', Phoenix.

  26. Shklovskiĭ, I. S. and Sagan, C., 1998 (1966), 'Intelligent Life in the Universe', Emerson-Adams Press.

  27. Diamond, J., 1999, 'To Whom It May Concern', The New York Times.

  28. Brin, D., 2006, 'Shouting at the Cosmos', Lifeboat Foundation.

  29. (a, b, c, d) Baum, S. D., Haqq-Misra, J. D. and Domagal-Goldman, S. D., 2011, 'Would contact with extraterrestrials benefit or harm humanity? A scenario analysis', Acta Astronautica, 68, pp.2114-2129.

  30. Soter, S., 2005, 'SETI and the cosmic quarantine hypothesis', Astrobiology Magazine, 17.

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