Why Earth Has Never Been Visited
Have alien intelligences from other parts of the universe visited the earth? This page is an attempt to rationally look at the issue of UFO’s and extraterrestrials. Within the last few years, scientists have gained considerable knowledge about the universe so that the question of extraterrestrial visitations can be addressed from a scientific, rational perspective.
Aliens visiting from elsewhere in the universe?
Have we been visited by extraterrestrial beings from elsewhere in the universe? First, I would like to eliminate the idea that we have been visited by beings located outside our own galaxy. Andromeda, the nearest galaxy to the Milky Way is 2 million light years distant. This means that if there were aliens in Andromeda, it would take them longer than 2 million years to come to earth.1 Another problem is why they would want to visit our galaxy. The Andromeda galaxy is considerably larger than our galaxy. If life were common in the universe, there should be many times more of it in Andromeda, than in our wimpy galaxy. Why would they even want to visit us? A third problem for potential aliens is detecting us. We have been sending radio waves for less than 100 years. It will be another 2 million years before those signals reach our closest neighboring galaxy. The light (and other electromagnetic signals) that they now see represent the way the earth looked 2 million years ago. Beings in other galaxies would have no way of knowing that advanced life forms existed in our galaxy.
SETI – aliens in our own galaxy?
Scientists have been actively searching for extraterrestrial intelligence in our galaxy for the last forty years. The search for extraterrestrial intelligence (SETI) has extended out to 40,000 light years from earth (in comparison, the galaxy is 100,000 light years across).2 To date, no signal from any extraterrestrial stellar system has ever been detected.
How many alien civilizations in our galaxy?
The Drake Equation (named for Frank Drake, the originator of the equation) is a way to estimate how many intelligent extraterrestrial civilizations might inhabit our galaxy. Below is the Drake Equation:
N = R* x fs x fp x nE x fl x fi x fc x L
- R* = average star formation rate
- fs = fraction of “suitable” stars capable of supporting a habitable planet.
- fp = fraction of suitable stars with planets.
- nE = average number of “Earth-like” planets.
- fl = average fraction of Earth-like planets with life.
- fi = average fraction of life-bearing planets evolving at least one intelligent species.
- fc = average fraction of planets with intelligent civilizations capable of interstellar communication.
- L = average lifetime that a civilization remains technologically active and will use radio communication.
These are the values based upon the most recent astronomical data:
R* 10 3 fs 0.02 0.01 fp 0.75 0.1 nE 0.01 0.00001 fl 1.0 0.00001 fi 0.0001 0.000001 fc 1.0 0.1 L 1,000,000,000 100,000,000
A most optimistic estimate for the number of intelligent civilizations in our galaxy is 150. This would mean that each intelligent civilization would be separated by an average of 2,000 light years. Such vast distances make contact unlikely and finding other advanced civilizations improbable. If these civilizations exist, they will not detect our radio signals for another 1900 years. How will they even know we are here? For information on how these results were calculated, see the appendix.
Fermi Paradox – Where are they?
Enrico Fermi, a prominent nuclear physicist of the last century, asked the question, “Where are they? Shouldn’t their presence be obvious?” Given at least 10 billion years for the existence of the Milky Way galaxy, one would think that intelligence would have developed before now – if life were common in the universe. Even at slow interstellar speeds, humans with advanced rocketry skills might be able to explore the galaxy in a few tens of million years. Some have suggested that aliens would choose not to reveal themselves to us. However, Frank Drake estimated that at least 10,000 advanced civilizations exist in the Milky Way. Carl Sagan raised the estimate to 1,000,000. Would all those civilizations stay at home or choose to conceal themselves from us? It seems highly unlikely. It would seem more likely that they do not exist.
Interstellar space travel – BIG problems
Obviously, our current rocket technology is incapable of providing the speed needed to make interstellar travel realistic. Matter/anti-matter engines might provide the power to accelerate near the speed of light. However, there is no way to contain or generate large amounts of anti-matter. The only way that we have produced anti-matter is through extremely large (miles across) particle accelerators. However, the meager few anti-matter particles generated are rapidly destroyed through interactions with ordinary matter. Nuclear power would provide a long-lived fuel supply for interstellar travel. Even so, such fuel would last only tens of years before being spent. Fusion power is yet to be harnessed, but, likewise, fuel would eventually run out. According to Frank Drake, “To send a spacecraft the size of a small airliner at one-tenth the speed of light requires as much energy as the US now produces in more than a hundred years.”3 In fact, the minimum amount of fuel required for such a spacecraft is 100 tons (assuming that a fusion reactor converts mass into kinetic energy at 100% efficiency).4 This does not sound practical to me!
Assuming that fuel and propulsion problems could be eventually solved, there are other, more serious, problems to contend with. Traveling near the speed of light is no simple problem. Running into small particles (like the size of a grain of sand) would punch major holes in any spacecraft, due to the high speed of impact. According to Frank Drake, “At relativistic speeds, even a collision with a particle of a few grams results in something close in energy to a nuclear bomb blast. Not good news for the space travelers.”3 A major biological problem seldom mentioned in the press is the blue shifting of the light from ordinary stars when traveling near the speed of light. The Doppler effect of traveling at such speeds would blue shift ordinary visible light all the way to the wavelength of gamma and x-rays. Shielding gamma rays is next to impossible (they can even travel through many feet of solid metal). When they do strike matter (like space traveler’s bodies), the results are devastating. This problem alone might restrict the speed of space travel to a small fraction of the speed of light.
Other significant problems would be involved in trying to keep biological organisms alive for many years of space travel. The lack of gravity would likely be fatal within a couple years (determined from the effects of prolonged weightlessness among the astronauts of the Space Station). Generating gravity would be possible through spinning, but might severely restrict the design of propulsion systems. In addition, it would be impossible to carry enough food and water for such a trip. Two solutions are possible – though not within the technology that we currently possess. One solution is to recycle all carbon and water. This process would have to involve capturing all biological waste (and dead bodies) and converting it back into food and water (doesn’t sound appetizing, does it?). The idea of making a self-contained bio-habitat is appealing, but impractical, due to the large amount of space required. A recent attempt to do this on earth was a miserable failure, since the designers failed to provide enough space to support all members of the small crew. A second possible solution to the food problem would be to put the travelers into suspended animation. Currently, we have no idea how to do this, and it does not seem possible to do so.
What about all the sightings, Roswell, abductions, etc.
UFO believers would ask about Roswell, UFO sightings and alien abductions. The problem I have with the whole Roswell/government conspiracy thing is that there is not one piece of physical evidence. The government has never been able to keep any kinds of secrets – much less over a period of 40 years. Regarding abductions, none of the people involved have been shown to have any signs of tampering, which would be readily apparent by MRI.
PBS has a good series on why scientists are skeptical about UFOs and abductions. See the comments of
This paper has shown that the probability of aliens visiting the earth is virtually zero. Potential aliens in other galaxies are too far away to detect our presence (since radio signals will not reach them for millions of years) and the travel times make intergalactic travel impractical. Recent scientific studies demonstrate that the universe is much less hospitable to life than it would seem from our unique Solar System and planet. A large proportion of our galaxy is uninhabitable. Parts of it would not even be expected to produce rocky planets. The highly unlikely collision that produced our large moon prevented the earth from being a waterworld.5 It also ejected the majority of our primordial atmosphere, which prevented the earth from going through a runaway greenhouse effect similar to what happened to Venus, our sister planet. Finally, our Solar System is unique in that it has large gas giants located only in the outer regions. Other systems discovered have gas giants located either near their star or in both inner and outer regions of their planetary system. The presence of gas giants near the star would eject any rocky planets from orbit. The presence of gas giants in the outer region of planetary systems is absolutely necessary for the survival of advanced life forms. Without Jupiter, the number of catastrophic collisions that the earth would experience would be at least 10,000 times greater. So instead of suffering massive species extinction events every 100 million years, the earth would experience these events every 10,000 years.6 Only bacteria and other simple life forms would be able to survive this kind of bombardment – no advanced life could ever form in the vast majority of planetary systems. These problems indicate that there would be no more than 150 advanced civilizations within our galaxy – and, more likely, we are completely alone in our galaxy.
Interstellar space travel is much more difficult than indicated in movies and television series, such as Star Trek and Star Wars and the like. First, it is not possible to travel at speeds greater than the speed of light – the physics of the universe prevent it. Second, traveling near the speed of light is impractical for biological organisms. Collisions with particles even the size of a grain of sand would be catastrophic. An even worse problem is that the light from ordinary stars would be blue-shifted all the way to the gamma end of the spectrum when traveling near the speed of light. These gamma rays would destroy all biological life – even if it were in suspended animation (if that were possible). In essence, these problems would restrict the speed of travel to well below the speed of light. The most optimistic estimate for the presence of extraterrestrial civilizations would put them 2000 light years apart. With no intermediate habitable stopping points, space travel over this distance would be impractical. So, even if we are not alone in this galaxy, it would be highly unlikely that any extraterrestrial civilization could have visited us. What about all the “evidence” for extraterrestrials and UFOs? See the links below for more information.
Related UFO Links
- Crop Circle Confession: How to get the wheat down in the dead of night By Matt Ridley
- Mysterious Circles Fact or Fiction? – Just a couple of “good ole boys” with an artistic bent having fun on Friday nights! Sorry, no mystery here.
- Circular Reasoning: The ‘Mystery’ of Crop Circles and Their ‘Orbs’ of Light (Skeptical Inquirer September 2002)
- Nickell, Joe, and John F. Fischer. 1992. The crop-circle phenomenon: an investigative report. Skeptical Inquirer 16:2 (Winter), 136-149.
- How to Make Your Own UFO (from NASA)
- Alien Autopsy: Table of Contents – Have a good laugh at their mistakes!
- The Truth Is, They Never Were ‘Saucers’ – Psychic Vibrations (Skeptical Inquirer September 1997) – The original 1947 sighting said they looked like boomerangs that “flew erratic, like a saucer if you skip it across the water,” but the reporter’s account called them “flying saucers.” The legend had begun…
Rare Earth: Why Complex Life is Uncommon in the Universe by Peter D. Ward and Donald Brownlee
A recent (2000) secular book that recognizes the improbable design of the earth. Paleontologist Peter D. Ward and astrobiologist Donald Brownlee examine the unusual characteristics of our galaxy, solar system, star, and Earth and conclude that ET may have no home to go to.
Appendix – how the results were calculated
- (average star formation rate) The current rate of star formation is ~3 stars/year. However, in the past, the rate was higher. The average rate over the history of our galaxy was ~10 stars/year.7
- (fraction of “suitable” stars capable of supporting a habitable planet) 95% of stars are smaller than the Sun.8 Small stars put out less energy, requiring potential life-containing planets to be closer to their star. The gravitational tidal effects result in synchronous rotation (where one side of the planet always faces the star), would affect atmospheric freeze-out due to the cold dark side. Large stars (more than twice the size of the Sun) burn erratically and rapidly (burn out in less than 1 billion years – too short to develop advanced life).8 Variable stars, neutron stars, and white dwarf systems are too unstable to support life. Only the area within 10,000 light years of the Sun are suitable stars (the galactic habitable zone).9 Radiation levels are too high near the center of our galaxy due to high densities. In addition, planetary orbits are likely to be less stable, due to stellar interactions. Stars in the outer region of our galaxy are unsuitable, since the rate of star formation is too low, resulting in low metallicity. Only about 20% of stars fall within the galactic habitable zone. Not only is the Sun within this zone, but it is between spiral arms, which puts it in a low density area. The Sun is at the co-rotation radius of the galaxy, which means that the orbits of surrounding stars are stable – proceeding at the same rate.10 In other regions of the galaxy, stars rotate at different rates around the galactic center, going in and out of the spiral arms as the galaxy rotates. The co-rotation radius is the only radius in the galaxy where stellar orbits do not interact. (0.05 * 0.2 = 0.01)
- fraction of suitable stars with planets. To date, about 10% of stars studied have planets orbiting them.11 Rocky planets cannot form unless the amount of metallicity is at least 60% of that of the Sun. The Sun is an unusually metal-rich star (richest out of 174 well-studied stars).8 The number of rocky planets is unknown, since none have been detected to date. Part of the problem is due to the limits of detection. This should change by 2009 when the Kepler Mission becomes operational.11 Optimistic estimates claim that rocky planets are common (at least 75% of systems will have them). However, if the abundance of rocky planets is similar to the percentage of stars that have gas giants, the estimate could go as low as 10% (and possibly much lower).
- average number of “Earth-like” planets. Not all rocky planets would be capable of supporting advanced life. Small rocky planets would loose their atmospheres (like Mars) shortly after formation. Large rocky planets would hold too much atmosphere, resulting in runaway greenhouse effect. Probably less than 10% of rocky planets would be right size to support advanced life. The habitable zone for planets is relatively small, representing less than 10% of the area where rocky planets might form. The planet must be able to support plate tectonics. Without plate tectonics, planets would be waterworlds (no dry land) and necessary nutrients would never be recycled. Intelligent life capable of communicating with us could not be exclusively aquatic, requiring the presence of land. Plate tectonics are a function of the thickness and composition of the crust and the presence of a large metallic core. None of the other rocky planets in our Solar System, other than earth, exhibit plate tectonics. Venus, which is nearly the same size as earth, does not have plate tectonics. Although driven by radioactive decay that keeps the mantle liquid, the ability of plate tectonics to function seems to be due to the removal of ~70% of the primordial crust of the Earth to a position in orbit overhead (during the collision that formed the moon).12 If that crust were returned and replaced on Earth it would fill the ocean basins with wall-to-wall continent. This would choke plate tectonics, as on Venus, and displace the oceans to flood the land to a depth of several miles. Nick Hoffman, Senior Research Scientist at La Trobe University, Melbourne Australia, claims that extraterrestrial “worlds will be, almost without exception, waterworlds.”13 Although collisions would have been common during the accretion phase of formation of the Solar System, a highly unlikely collision would be required to eject the earth’s crust into orbit and deposit the core of the collider into the earth’s core. The probability of such an event would likely be less than 1 in 10,000. Other problems involve the presence of other gas planets in the stellar system. The current information on extrasolar planets indicate that a large percentage of the giant planets tend to migrate inward towards their star after formation, which would eject any Earth-sized planets from the habitable zone. A very optimistic estimate of this value would be 0.01. It is more likely that this value would be less than 0.00001.
- average fraction of Earth-like planets with life. This value is quite disputable and subject to a wide latitude of possible values. Many scientists assume that since the earth developed life at a time when the conditions were inhospitable, that life emerges on virtually every planet that is capable of supporting it, There are some major flaws with this idea. First, it is now known that the “prebiotic conditions” assumed to have been present soon after the earth’s formation never existed. Oxidation of zircons over 4 billion years ago demonstrate that free oxygen was present on the earth before life emerged.14 None of the prebiotic chemistry works in the presence of even small amounts of free oxygen. Even with unrealistic “prebiotic conditions” the chemistry will not produce all the necessary biomolecules required for the first living system. In addition, hydrothermal sea vents, the current choice for the origin of life, would be unsuitable, since cell membranes cannot assemble in the presence of the salt of the oceans. Among all the 30,000 meteors collected on earth, none contain any evidence that life exists outside of the earth. The assumption that fl is nearly 1 is based upon the rapid appearance of life early in the history of the earth, which many claim indicates that it is easily produced abiotically in this universe. However, the science indicates that the biological precursors of living systems cannot be produced naturalistically, nor can they be assembled under conditions that existed on earth. Realistically this value would be very small – probably even zero.
- average fraction of life-bearing planets evolving at least one intelligent species. Intelligence is not something that would be expected to appear automatically. Taking the earth as an example, it took over 3 billion years for intelligent species to appear. Many worlds on which life might survive would be inhospitable to advanced life forms. For example, the rotation of the earth at its creation was complete in a scant 8 hours. At such rotation rates, a calm day would be characterized by 1,000 mph winds. Needless to say, intelligent beings would find it difficult surviving such conditions. The only reason the earth’s rotation period is now 24 hours is because of our large moon. The gravitational braking of the moon has slowed the earth’s rotation to a reasonable rate, while the earth’s gravity has slowed the moon’s rotation period to be in synchrony with its rate of revolution. Our large moon provides extraordinary stability to the inclination of the earth’s orbit (23.5°).15 Most planets exhibit up to 90° flips over periods of millions of years. Flips in which one pole faces the star would result in temperature instabilities of major proportion. The side facing the star would get very hot, while the side facing away from the star would be extremely cold. The length of the planet’s day would be the equivalent of the planet’s year, with many months of scorching temperatures followed by many months of frozen winter. Plants would be unable to survive such conditions, resulting in the collapse of the entire planet’s ecosystems. Our Solar System is unique in that it has large gas giants located only in its outer regions. Other systems discovered so far have gas giants located either near their star or in both inner and outer regions of their planetary system. The presence of gas giants near the star would eject any rocky planets from orbit. The presence of gas giants in the outer region of planetary systems is absolutely necessary for the survival of advanced life forms. Without Jupiter, the number of catastrophic collisions that the earth would experience would be at least 10,000 times greater. So instead of suffering massive species extinction events every 100 million years, the earth would experience these events every 10,000 years.5 Only bacteria and other simple life forms would be able to survive this kind of bombardment – no advanced life could ever form in the vast majority of planetary systems. Since we have discovered no other planetary systems (out of nearly 200 found to date) with large gas giants in a location to protect an inner planet from devastating impacts, we must put these odds at less than 0.01. The odds of a rocky planet having a collision to form a large stabilizing moon would also be much less than 0.01. Therefore, a very optimistic estimate of fi would be 0.0001 (0.01 x 0.01). It is much more likely that the value would closer to one in a million.
- average fraction of planets with intelligent civilizations capable of interstellar communication. It would seem that any intelligent civilization would eventually develop the ability to communicate through radio signals (unless they destroyed themselves with nuclear weapons). It would seem that this value would be close to one, although it would be difficult to estimate scientifically.
- average lifetime that a civilization remains technologically active and will use radio communication. The lifetime of civilization is difficult to estimate, but we can get an idea from our own planet. It took 4.5 billion years for advanced life to appear on the earth. The earth will be completely inhospitable to life within the next billion years due to increased solar luminosity (maybe sooner with human-caused global warming).16 According to Peter Ward, “The presence of complex life on the Earth will end in no more than a billion years (and perhaps much sooner), due to a sequentially predictable breakdown of habitable systems on our planet.”) Therefore, we can expect an average advanced civilization to exist for about one billion years. However, it is likely that additional factors may make this time much shorter. We are currently releasing carbon dioxide into the atmosphere at a rate that could cause a runaway greenhouse effect in a period of a few hundred years. Fortunately, it seems that the oceans have absorbed a large amount of that extra carbon, potentially saving us from our own self-induced doom.
- It is physically impossible to travel at the speed of light or faster. In addition, it takes near infinite energy to accelerate an object from stationary to the speed of light. For this reason, interstellar travel can only be a fraction of the speed of light. Realistically, it would probably take at least 20 million years.
- Relentless Evolution: Great Debates Part V
- The amount of energy the U.S.A. produces over 200 years is equal to 1.6 x 1022 joules. 1 kg of mass produces 9 x 1016 joules if all the mass is converted to energy (E=mc2), assuming nuclear fusion could be controlled. This means that the minimum amount of fuel required for such a trip is 180,000 kg (200 tons), which would be more than the weight of the spacecraft itself. This assumes 100% efficiency in converting mass to energy (which, of course is impossible). The amount of energy required to go even faster than 1/10 the speed of light goes up so that infinite energy would be required to travel at the speed of light. Obviously, infinite energy would require infinite mass to produce.
- The Moon And Plate Tectonics: Why We Are Alone
- Life on Earth: How Jupiter Helped and Hurt
- Lecture 35: Intelligent Life in the Universe from Terry Herter, Cornell University.
- Lecture 24: Is There Other Intelligent Life in the Universe?
- In Search of the Milky Way’s Habitable Zone
Forming Other Earths
- Mishurov, Y.N. and L. A. Zenina. 1999. Yes, the Sun is Located Near the Corotation Circle. Astronomy & Astrophysics 341: 81-85.
- The Kepler Mission: Looking for Earth-sized Worlds
In globular cluster 47 Tucanae, scientists found zero extrasolar planets out of 37,000 stars searched (Astronomers Ponder Lack of Planets in Globular Cluster from the Hubble Space Telescope).
- The Moon And Plate Tectonics: Why We Are Alone
- Venus – What the Earth would have been like
- Bortman, H. 2001. Life Under Bombardment from the NASA Astrobiology Insititute. – Alternating layers of oxidized iron in the so-called banded iron formation from Akilia Island in West Greenland demonstrates that free oxygen has been present on earth longer than 3.85 billion years.
Dimroth, E. and M. Kimberley. 1976. Precambrian atmospheric oxygen: Evidence in the sedimentary distributions of carbon, sulfur, uranium, and iron. Can. J. Earth Sci., 13:1161-1185.
Carver, J. H. 1981. Prebiotic atmospheric oxygen levels. Nature 292: 136-138.
Ohmoto H., Y. Watanabe, H. Ikemi, S.R. Poulson, B.E. Taylor. 2006. Sulphur isotope evidence for an oxic Archaean atmosphere. Nature 442:873-874.
- Greate Debates: Part VI: Encore, The Experts Respond
- Lifetime of Complex Life: Great Debates Part IV