It's no secret that searching for life beyond Earth, whether it is within our own Solar System or beyond, is very challenging work. For decades, scientists have sent robotic and crewed missions to other celestial bodies to search for signs of past or present life.

In fact, with the Voyager probes, the Cassini-Huygens mission and the New Horizons' spacecraft, all of the major bodies of our Solar System beyond Earth have effectively been explored. That's Mercury, Venus, the Moon, Mars, Ceres and Vesta, and Jupiter, Saturn, Uranus, Neptune, and Pluto (as well as their largest moons).

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And yet, all our best efforts have managed to turn up nothing - or at least, nothing conclusive. Now, why is that? Could it be that intelligent life is rare, hard to find, or not looking to be found? Or could it be humanity is alone in the Universe, staring out into a great black abyss with no one looking back?

The Search at Home:

So far, the majority of our efforts to find extra-terrestrial life have been focused on Mars. The earliest efforts were the Viking 1 and 2 missions, which sent landers to the surface in 1976 (a few months apart). Unfortunately, the results of these surveys were inconclusive, which is why Opportunity, Curiosity and the Mars 2020 rover are continuing the search.

In the coming decade, NASA and the European Space Agency (ESA) are planning to send robotic missions to Europa to see if there really is life locked away beneath the icy moon's surface. Similar efforts are being proposed to explore other "Ocean Worlds" like Ceres, Callisto, Ganymede, Titan, Enceladus, Mimas, Triton, Pluto, and others.

While these efforts might reveal that life exists elsewhere in our Solar System (most likely in the form of microbes) it does very little to help in the Search for Extra-Terrestrial Intelligence (SETI). Here, scientists have spent decades monitoring deep space and other planets for indications of biological processes ("biosignatures") and technological activity ("technosignatures").

So far, the results have been equally unencouraging, which has led many scientists and theorists to come up with various explanations for "the Great Silence".

Enrico Fermi’s Big Question:

In 1950, while working at Los Alamos National Laboratory, physicist Enrico Fermi is said to have asked the question that launched a thousand possible answers. While having lunch with his colleagues and speaking on the subject of SETI, he famously asked: "Where is everybody?"

This became the basis of the Fermi Paradox, which addresses the discrepancy between the (supposed) statistical likelihood of there being life beyond Earth with the dearth of evidence for it. This question reflected the state of SETI in Fermi's time and, unfortunately, things have not changed much since.

And the fact of the matter is this is surprising when you consider that based on even the most conservative estimates, there should be at least some intelligent life out there. And given how long the Universe has been around (13.8 billion years), some of that life should have reached a very high level of technical development by now.

The Drake Equation:

The principles and theories behind the Drake Equation are closely related to those of the Fermi Paradox. Named for American astronomer Francis Drake, the equation was his attempt to formalize the theoretical parameters that SETI researchers had been operating within for decades.

In essence, the equation is a means for calculating the number of extraterrestrial civilizations in our galaxy that we would be able to communicate with at any given time. The equation is expressed as N = R* x fp x ne x fl x fi x fc x L, where:

N is the number of ETIs that we might be able to communicate with

R* is the average rate of star formation in our galaxy

fp is the number of stars that have a system of planets

ne is the number of planets that will be able to support life

fl is the number of planets that will develop life

fi is the number of planets that will develop sentient (aka. intelligent) life

fc is the number of civilizations that will develop advanced technologies

L is the length of time that these civilizations will have to transmit radio or other communications signals into space

The Drake Equation, a means of calculating the likelihood of ETIs in our galaxy at any given time. Credit: University of Rochester

Granted, many of the parameters Drake specified in his equation were subject to a significant degree of uncertainty. Even today, we still have no idea how to assign values to most of them. For example, astronomers have a pretty good idea how many stars there are in our galaxy and the average rate of star formation - between 100 and 200 billion stars, with a handful being added every year.

Thanks to recent advances in extrasolar planet research, scientists are able to place constraints on the number of stars that have a system of planets (most will have at least 1) and the number of these that will be able to support life (aka. those that are "potentially habitable"). So it's fair to say we have a pretty good idea what the values for the first, second and third parameter are.

Beyond that, however, we don't have the slightest idea. We have no idea how many potentially habitable planets will actually give rise to life, let alone how many of those will develop life capable of communicating with us, or how long such a civilization could be expected to live before some cataclysmic event or other fate them caused their communications to cease.

And how could we? Right now, we know of only one planet where life exists (Earth) and only one species that is capable of communicating with radio waves or other parts of the electromagnetic spectrum (humanity). But that's not really the point of the Drake Equation.

In the end, Drake proposed this equation as a sort of statistical exercise, designed to show that even by the most conservative estimates, there ought to be at least a few civilizations out there right now that humanity could be hearing from.

Moreover, given the age of the Universe, at least a few of those civilizations should have been able to develop extremely advanced technology by now. Which brings up another important concept known as...

The Kardashev Scale:

In 1964, Soviet astronomer and SETI researcher Nikolai Kardashev proposed a classification method for grouping species based on their level of technological development. The resulting scale had three levels (or types) which classified species based on the amount of energy they could harness.

By definition, Type I civilization (aka. "planetary civilizations") are those that have developed the means to harness and store all of the energy of their home planet. According to Kardashev, this would amount to the consumption of 4 x 1019 erg/sec which would likely be in the forms of fusion power, antimatter, and renewable energy on a global scale.

Next up are Type II civilizations ("stellar civilizations"), which evolved to the point where they could harvest all the energy emitted by their star - which Kardashev speculated would likely involve a structure like a Dyson Sphere. In this case, this would work out to a consumption of 4 x 10³³ erg/sec.

Type III civilizations ("galactic civilizations") are those that would be able to harness the energy of an entire galaxy, which would work out to energy consumption on the order of 4 x 1044 erg/sec.

Based on the fact that the Universe has been around for 13.8 billion years, and the fact that our Solar System has only existed for the last 4.6 billion years of that, it would seem likely that at least a few civilizations would have been able to achieve a Type III level of development. Even with our modest means, it would be very difficult for humans to miss the signs of such a civilization

So again, we are forced to ask why we have found no signs of intelligent life in the cosmos. How it is that the odds of intelligent life seems so likely, but the evidence is so lacking? Here's where things get particularly interesting, frightening, and more than a little mind-blowing.

The Hart Tipler Conjecture and "Great Filter" Hypothesis:

There's the obvious answer: that extra-terrestrial intelligence simply doesn't exist. This was the conclusion argued by Michael Hart, an American astrophysicist in a paper he published in 1975 - titled "Explanation for the Absence of Extraterrestrials on Earth".

This argument was elucidated further by mathematician Frank J. Tipler in his 1979 study, "Extraterrestrial Intelligent Beings do not exist". In what has come to be named the Hart-Tipler Conjecture, they argue that if any ETIs had developed the means for interstellar travel, they would have visited the Solar System by now.

Another possibility was suggested by economic Robin Hanson in an online essay, "The Great Filter - Are We Almost Past It?", which was published in 1998. As he summarized his argument:

"Humanity seems to have a bright future, i.e., a non-trivial chance of expanding to fill the universe with lasting life. But the fact that space near us seems dead now tells us that any given piece of dead matter faces an astronomically low chance of begating such a future. There thus exists a great filter between death and expanding lasting life, and humanity faces the ominous question: how far along this filter are we?"

In Hanson's view, this "filter" must lie somewhere between life's starting point (abiogenesis) and the proliferation of advanced life beyond its home planet and star system. Using humanity as a template, he also outlined a nine-step process that life would need to follow in order to produce a complex and space-faring species. These included:

Habitable star system (organics and habitable planets)

Reproductive molecules (e.g. RNA) Prokaryotic single-cell life Eukaryotic single-cell life Sexual reproduction Multi-cell life Animals capable of using tools

Industrial civilization

Wide-scale colonization



According to the Great Filter hypothesis, at least one of these steps must be improbable. If it is an early step then humanity's existence is a statistical rarity and our future prospects would seem bleak. If it is a later step then there would be many civilizations (past and present) that have reached our current level of development, but not progressed further.

In any case, no species has reached the ninth step in our galaxy, or it would be teeming with evidence of their existence. So it is entirely possible that intelligent species don't survive the transition from step eight to step nine, which would coincide with a Type I to Type II level civilization.

As you might suspect, this is not good news for humanity. Given the environmental issues that have become evident since the latter half of the century - air and water pollution, waste, drought, ozone depletion, global warming, etc. - it is entirely possible that no species survives becoming advanced.

And with the threat of nuclear war still a possibility, it is also possible that intelligent species are destined to wipe themselves out. In this respect, the fact that we have not found evidence of any ETIs can be seen as a good sign. As Hanson indicated in his essay, there is a bright side to the fact that humanity has not found evidence of extra-terrestrial life yet:

"But contrary to common expectations, evidence of extraterrestrials is likely bad (though valuable) news. The easier it was for life to evolve to our stage, the bleaker our future chances probably are."

Planetarium Hypothesis:

Beyond the Hart-Tipler Conjecture and the Great Filter, there are many other possible reasons for why we haven't found evidence of intelligent life yet. Another popular explanation is that the reason we haven't found any evidence of ETis is that they don't want to be found!

In 2001, famed science fiction author Stephen Baxter asserted as much in his seminal essay, "The Planetarium Hypothesis - A Resolution of the Fermi Paradox". In an attempt to resolve the Fermi Paradox, Baxter postulated that humanity' astronomical observations are actually an illusion created by a Type III Civilization who are keeping humanity in a giant "planetarium". As he put it:

"A possible resolution to the Fermi Paradox is that we are living in an artificial universe, perhaps a form of virtual- reality `planetarium', designed to give us the illusion that the universe is empty. Quantum-physical and thermo-dynamic considerations inform estimates of the energy required to generate such simulations of varying sizes and quality. The perfect simulation of a world containing our present civilisation is within the scope of a Type K3 extraterrestrial culture. However the containment of a coherent human culture spanning ~100 light years within a perfect simulation would exceed the capacities of any conceivable virtual-reality generator."

This concept is similar to the Simulation Hypothesis, which posits that the observable Universe is actually a massive holographic simulation. This idea has deep roots in mystic and empirical philosophy, which included the practice of questioning whether or not reality is in fact real.

In this case, however, it is suggested that the purpose of keeping humanity in a simulation is to protect ourselves (and our hosts) from the dangers associated with "first contact". Variations on this hypothesis generally state that ETIs are employing other forms of advanced technology to remain undetected (e.g. cloaking devices or other such things).

Other Possibilities:

In an attempt to answer Fermi's challenge, other possibilities have been suggested that too numerous to count. However, some of the more popular suggestions include the following:

Intelligent life is very rare:

It could be that our searches for evidence of ETIs have not yet succeeded because we haven't been looking for long enough. This certainly fits in with more conservative estimates using the Drake Equation.

Intelligent life is too far apart:

Addressing our failure to find evidence of radio signals and other transmissions technology is due to distance. Simply put, ETIs may be too distant in terms of space and time since transmissions will only be discernible within a limited volume of space.

Similarly, it's possible that civilizations are not around long enough to pick up on alien transmissions. In fact, in a recent study that was co-authored by Frank Drake, a team of scientists argued that any alien signals picked up by human observers will most likely have come from a civilization that went extinct a long time ago.

Intelligent life is hibernating:

This possibility was suggested by Oxford research associate Anders Samberg and colleagues from the Future of Humanity Institute (FHI). In their 2017 study titled, “That is Not Dead Which Can Eternal Lie: the Aestivation Hypothesis for Resolving Fermi’s Paradox“, they suggest that ETIs are engaged in "aestivation" - a prolonged state of torpor organisms enter into during a particularly hot or dry period - and waiting for better conditions.

We don’t know what to look for:

As it stands, we know of only one planet that supports life (Earth) and only one example of technologically-advanced life (our own). For this reason, all of our searches for biosignatures and technosignatures are based entirely on what we are familiar with.

Perhaps that is the problem, and perhaps we should be casting a wider net. Unfortunately, that just isn't possible because our scientists wouldn't know where to begin. Given the limits of our technology, we are forced us to look for "signatures", which makes looking for life "as we don't know it" impossible.

We haven't been looking long enough:

In cosmological terms, humanity has been an "advanced" species for a very short time. Radio communications have only existed on Earth since the end of the 19th century, and radio telescopes have only existed since the 1930s. As such, it could be that not enough time has passed for aliens to pick up on our radio transmissions, or for us to pick up on theirs.

Intelligent life is already here!:

Here's a possibility that no fan of science fiction will fail to recognize! Perhaps aliens not only exist but are moving among us and gathering information as we speak. You have to admit, if we ever found out an ETI and were capable of making contact, wouldn't we want to do a little investigating first to prevent any "cultural misunderstandings"?

Intelligent life destroys itselfor others: Here we have the extension of the Great Filter hypothesis. In this scenario, it could be that no intelligent species survives climate change, nuclear war, etc., or that more advanced species wipe out less advanced species - creating the illusion that intelligent life is rare.

Humanity is early to the party:

Another sobering suggestion is that humanity is actually one of the first intelligent species to emerge in our Universe and hasn't found any intelligent species because they haven't achieved our level of development yet. Harvard Professor Abraham Loeb and colleagues suggested this possibility in a 2016 study, titled "Relative Likelihood for Life as a Function of Cosmic Time".

Artist's impression of a habitable planet orbiting a red dwarf star. Credit: cfa.harvard.edu

Exploring the possibility of life emerging in a star system as a function of time, they found that long-lived stars (such as low-mass, M-type red dwarfs) have the best odds of producing life-bearing planets. In this respect, it could be argued that humanity is actually an early arrival to the party, rather than a late one (as has generally been assumed).

Alas, all of these possibilities are informed by the same basic problem: we just don't know. Until we find examples of extra-terrestrial life and ETIs, we won't know with any confidence under what conditions life is able to emerge and evolve.

Possible Detections of ETIs:

In the meantime, there is the possibility that humanity has found evidence of ETIs and simply didn't realize it. There have also been numerous instances where potential signals were detected and we simply haven't been able to prove that they came from an extra-terrestrial source yet.

WOW! Signal:

On August 15th, 1977, astronomers using the Big Ear radio telescope at Ohio State University detected a 72-second radio signal coming from the direction of the Sagittarius Constellation. This powerful signal, which quickly earned the nickname the “WOW! Signal”, was thought by some to be extra-terrestrial in origin.

Since then, the WOW! Signal has been an ongoing source of controversy among SETI researchers and astronomers. This is because all attempts to date to find a natural cause - which include asteroids, exoplanets, stars, signals from Earth, hydrogen clouds and comets – have been inconclusive. To date, it remains the strongest candidate for a possible alien transmission.

Tabby’s Star:

On September of 2015, citizen scientists with the Planet Hunters project noticed that the star KIC 8462852 (aka. Tabby’s Star) was experiencing a mysterious dip in luminosity. Located in the constellation Cygnus, roughly 1,470 light years from Earth, this star experienced fluctuations and experience a drop of up to 22% in brightness.

Since then, observatories around the world have noted further incidents of dimming and multiple studies have been conducted to try and offer a natural explanation for this behavior. These have ranged from a circumstellar debris disk, shattered comets and asteroids to the presence of a giant planet, a planet with rings, or a planet that had been consumed in the past.

However, it was the proposal that the irregular dimming could be caused by the presence of alien megastructures that attracted the most attention. While no evidence has been produced to reinforce this idea, the fact that no natural explanation has been able to account for the star's behavior has kept it in the public mind.

Fast Radio Bursts (FRBs) on Repeat:

Here is yet another example of astronomical phenomena that appears to defy natural explanation. Basically, FRBs are short-lived radio pulses that last only a few milliseconds. Since the first was discovered in 2007 (known as the Lorimer Burst), only about two dozen have been detected (mostly in archival data) and only a handful have been found to be repeating.



In the case of one-off events, several theories have been offered for what causes them - ranging from exploding stars and black holes to pulsars and magnetars. However, no viable explanation has been offered to date for repeating FBRs, leading some to suggest that they might be evidence of alien radio transmissions.

‘Oumuamua:

On October 19th, 2017, the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) in Hawaii announced the detection of an object named 1I/2017 U1 (aka. ‘Oumuamua). Unlike the many Near-Earth Objects (NEOs) that periodically pass near to Earth, 'Oumuamua was the first to known object to have come from interstellar space.

After multiple follow-up observations were conducted, scientists were still unable to determine whether 'Oumuamua was an asteroid or a comet. On the one hand, its composition data indicated that it was likely to be icy, but it didn't form a tail like a comet. However, it then accelerated out of the Solar System like a comet would when experiencing outgassing.

Based on its behavior, two scientists from the Harvard-Smithsonian Center for Astrophysics - Shmuel Bialy and Prof. Abraham Loeb - speculated that 'Oumuamua might actually be an interstellar light sail or the remains of an interstellar spacecraft. This not only would explain why it sped up as a result of radiation pressure from our Sun, it would also explain 'Oumuamua's orbit.

For starters, after entering the Solar System, 'Oumuamua passed within 0.25 AU of our Sun, which is a good orbit for intercepting Earth without experiencing too much solar irradiation. In addition, it came to within 0.15 AU of Earth, which could have been the result of orbital corrections designed to facilitate a flyby.

If this was indeed the case, then 'Oumuamua could be transmitting images of Earth to its home system as we speak! It's also possible that our Solar System is littered with the remains of many interstellar probes right now, since astronomers have deduced that objects like 'Oumuamua enter our Solar System on a regular basis.

****

Decades later, Fermi's Paradox continues to haunt us. What's more, the Drake Equation continues to serve as a thought experiment where most of the parameters are still subject to major uncertainty. Until we find evidence of life beyond Earth, we won't know if life is capable of emerging and thriving under conditions that are not "Earth-like". And until we find evidence of intelligent life beyond Earth, we won't know for certain if any exists.

But that's the cool thing about the Fermi Paradox: you only need to solve it once! The moment we find evidence of an extra-terrestrial civilization (assuming we ever do), the Paradox will be solved for all time. And it really doesn't matter if the civilization is still alive or not. One signal, one glimpse of a megastructure, or one confirmed spaceship sighting, and we will know with certainty that humanity is not alone in the Universe.

In the meantime, all we can do is wait and get better at searching for it!

Further Reading: