As Enrico Fermi famously once said, "Where is everybody"?" These words were uttered in the summer of 1950 when the search for extraterrestrial intelligence (SETI) was heating up. They also captured the frustrations and unresolved questions surrounding the existence of extraterrestrial life.
At the time, many scientists were of the belief that, statistically speaking, life had to be plentiful in our Universe. This is still a common belief, seeing as how the Universe is so very, very big and there are countless planets out there for life to take root on.
And yet, all efforts to find that life have so far produced nothing - at least, nothing definitive. What these efforts have done is given our greatest scientific minds the opportunity to test various methods, theoretical frameworks, and the purpose of the search itself.
So to build on the words of the late and great Enrico Fermi, let's take a look at the history of SETI so far and ask the question, "What have we learned?"
Why are we looking?
Humanity has been looking for signs of extraterrestrial intelligence for more than a century. However, our species has been contemplating the existence of intelligent life beyond Earth for much, much longer.
One could easily make the argument that looking for life beyond our planet is the result of us wanting to push our physical and intellectual boundaries. As long as humans have been around, we have been preoccupied with what we don't know.
In this sense, searching for signs of life beyond Earth is no different than contemplating the origins of the Universe, the meaning of life, or the possible existence of the divine. Into that great unknown, we've always hurled our hopes, fears, and most vivid imaginings.
Call it curiosity, call it anthropocentrism, call it arrogance, call it destiny, but there has been a drive to look out into the cosmos and ponder whether or not there are any species like us out there (aka. intelligent).
However, our search efforts are limited because our technology and frame of reference are similarly limited. So perhaps a quick tutorial on the relevant terminology and strictures is in order.
Ready? Here we go!
What are we looking for?
Are far as we are currently aware, the existence of life is dependent upon the availability of certain elements and the presence of certain conditions. One of the most important of these is what scientists refer to as a "circumsolar habitable zone" (CHZ).
Also known as a "Goldilocks zone" or "habitable zone" (HZ), this term refers to the range of distances around a star where water can exist in liquid form on the surface of a planet. Planets that are too close will have lost their water to evaporation, while planets that are too far will only have water in the form of ice.
These ranges depend on the type of star being studied. Blue/white stars (O-, B-, and A-type) are significantly larger and hotter than other types, so their habitable zones are likely to be wider and farther away. M-type red dwarf stars, which are the smallest and coolest stars in the Universe, are likely to have smaller habitable zones that are much closer to the star.
Those that are in between - such as G-type yellow dwarfs (like our Sun) - are likely to have habitable zones similar to that of our Sun. This corresponds to a distance of about 150 million km (93 million miles), or one Astronomical Unit (AU), which is the average distance between Earth and the Sun.
Planets within this zone can be examined for signs of chemical elements that we associate with life (biosignatures). These include carbon dioxide, which is essential for photosynthesis, is emitted by complex organisms, and allows for the stabilization of temperatures through the Greenhouse Effect.
Oxygen is another indicator, since it is indicative of plant life and photosynthetic organisms, and is also essential for complex life. Since nitrogen is also an important buffer gas (making up over 78% percent of Earth's atmosphere), it is also considered important for life.
Methane is an organic molecule that is often the result of biological processes - such as the decay of organic tissue or digestion in some animals (such as cows). Therefore, its presence in a planet's atmosphere is considered a potential sign of life.
Hydrogen gas is also considered a biosignature by some scientists for three reasons. First, the presence of hydrogen in an atmosphere can have a warming effect similar to carbon dioxide and therefore extend the range of a star's habitable zone.
Second, it is a possible indication of volcanic (and geological) activity on a planet's surface, which is considered essential for life as we know it. Third, hydrogen gas can be the result of chemical disassociation of water due to exposure to UV radiation. In this process, water is broken down into oxygen and hydrogen gas, the latter of which is lost to space. Hydrogen gas is therefore seen as a possible indication of water on a planet's surface.
Aside from biological indications, SETI research also focuses on searching for signs of technological activity (technosignatures). One method involves surveys using radio telescopes, which search for signs of extraterrestrial transmissions.
Some scientists have suggested that other activities should be searched for as well, including directed energy emissions (lasers). Assuming that extraterrestrials use lasers for communications and other purposes, astronomers could observe nearby stars and exoplanets for errant flashes of laser light or laser beacons.
Other means of communication that astronomers have recommended searching for include neutrinos, Fast Radio Bursts (FRBs), and gravitational waves. However, radio transmissions remain the only technosignature that scientists have monitored for thus far.
A brief history of SETI
While it is impossible to pinpoint precisely when human beings first looked up at the sky and wondered if there was other life in the cosmos, some of the earliest recorded examples come to us from Classical Antiquity.
For example, in the time of Greek philosopher Anaximander (ca. 610 - 546 BCE), the existence of life on other worlds was the subject of metaphysical philosophical debate. By the time of Democritus (ca. 460 - 370 BCE), the idea was formalized with the term "cosmic pluralism".
In the 2nd century CE, Assyrian satirist and rhetorician Lucian of Samasota wrote A True History, which contained a tale about an inhabited Moon. While intended as a humorous tale, this story indicated that people were already thinking about the possability of civilization beyond Earth.
Similarly, extraterrestrial life is depicted in ancient works like The Tale of the Bamboo Cutter (aka. The Tale of Princess Kaguya), a 10th century CE Japanese story. The protagonist of this story, Princess Kaguya, is a celestial being who was sent from the Moon, and her people eventually return to reclaim her.
Another example is the medieval Arabic tale, The Adventures of Bulukiya, which is part of the One Thousand and One Nights (also known as The Arabian Nights). The story centers on a protagonist whose quest for the herb of immortality takes him to Heaven and Hell and across the cosmos to various populated worlds.
Works describing life on other worlds continued well into the 20th century, with writers like Johann Kepler, H.G. Wells, Edgar Rice Burroughs, and Olaf Stapleton speculating about the existence of civilizations on other planets in the Solar System or beyond.
However, it was not until the late 19th and early 20th centuries that the first efforts were made to confirm the existence of life beyond Earth. These efforts were at first largely focused on the planet Mars. At the time, some astronomers and scientists believed that Mars was potentially habitable and even boasted its own indigenous "Martian" civilization.
Famed inventor and engineer Nikola Tesla is credited for conducting the first experiment to search for extraterrestrial life. In 1896, he suggested a way in which a scaled-up version of his wireless electrical transmission system could be used to contact a civilization on Mars.
In 1899, Tesla was conducting experiments at his laboratory in Colorado Springs. While working with electrical transmissions in a low-pressure environment, he reported the possible detection of a signal from Mars. Though never confirmed, he claimed that his instruments registered an odd static signal that ceased when Mars set in the sky.
With the dawn of the Space Age, the search for extraterrestrial life received far more attention and investment. Between the 1950s and 1960s, the first projects that targeted other star systems were mounted.
In 1960, Francis Drake conducted the first modern search for extraterrestrial life in an experiment known as Project Ozma, using the Green Bank Telescope in West Virginia. This project consisted of a radio survey of Tau Ceti and Epsilon Eridani but found nothing of concrete value.
There are also the efforts of the Ohio State Radio Observatory, known as the “Big Ear” Observatory. Built in 1957, this flat-plane radio telescope would play a major role in multiple surveys in search for extraterrestrial life, and would be responsible for one of the most significant possible detections ever made (see WOW! Signal, below).
From this point forward, these surveys became far more common. In 1971, NASA greenlighted a study known as Project Cyclops, which called for the construction of a 1,500 radio antenna array to search for extraterrestrial signals. While it was never built, the report informed much of the work that followed.
In 1979, the Berkeley SETI Research Center launched an initiative known as the Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations (SERENDIP).
This program consisted of analyzing deep-space radio telescope data obtained by large radio telescopes - like those located at the Green Bank and Arecibo Observatory. It has also led to the development of improved spectrometers for the sake of conducting SETI research.
In 1980, Carl Sagan, Bruce Murray and Louis Friedman (from NASA’s Jet Propulsion Laboratory) came together to create the U.S. Planetary Society. As part of its purpose of furthering SETI research, this society has played a significant role in the development of SETI-related programs and software.
These include Sentinel, a project that ran from 1983 to 1985 and relied on the Harvard/Smithsonian radio telescope at Oak Ridge Observatory. These efforts were followed in 1985 and 1995 with the Megachannel Extra-Terrestrial Assay (META) and the Billion-channel Extraterrestrial Assay (BETA), respectively.
In 1992, NASA launched the Microwave Observing Program (MOP), a long-term effort to survey 800 stars that are relatively close to the Solar System. This project relied on NASA's Deep Space Network (DSN), the Green Bank Telescope and Arecibo Observatory's 300 m (1000 ft) radio telescope.
Congress canceled the program in 1993, forcing the MOP team to continue without government funding. By 1995, the SETI Institute resurrected the program under the name Project Phoenix. By 2004, the project had observed no less than 800 stars within a 200 light-year radius of Earth.
In 2016, Russian-Israeli billionaire Yuri Milner founded Breakthrough Initiatives, a non-profit organization dedicated to interstellar exploration and the search for extraterrestrial life. A major effort launched by this organization is the project known as Breakthrough Listen - a ten-year, $100 million effort that constitutes the largest SETI program mounted to date.
This project relies on radio wave observations from the Green Bank and Parkes Observatory, as well as optical surveys made by the Automated Planet Finder (APF). Combined with innovative software and data analysis techniques, this program will survey 1 million of the stars nearest to Earth, along with the 100 closest galaxies, for signs of radio and laser transmissions.
That same year, China finished work on the Five-hundred-meter Aperture Spherical radio Telescope (FAST) (aka Tianyan, or the “Eye of Heaven”). This antenna dish is currently the largest filled-aperture radio telescope in the world (previously, it was Arecibo) and much of its operations in the near future will consist of SETI research.
In 2017, the Dominion Radio Astrophysical Observatory (DRAO) finished construction on a dedicated interferometric radio telescope. Known as the Canadian Hydrogen Intensity Mapping Experiment (CHIME), this telescope will be intrinsic to the study of FRBs (see the Lorimar Burst, below).
Radio surveys have also been (or continue to be) made by the SETI Institute’s Allen Telescope Array, the Very Large Array (VLA), and through the SETI@home project. There have also been multiple near optical and near-infrared light (NIL) surveys of the Milky Way and other galaxies. These have been performed using instruments like the Near-Earth Object Wide-field Survey Explorer (NEOWISE), and the Keck/High-Resolution Echelle Spectrometer (HIRES), the Wide-field Infrared Survey Explorer (WISE) and Two Micron All-Sky Survey (2MASS).
Possible alien signals
Okay, so with all these surveys and all these dedicated facilities, what exactly what we found? As it turns out, there have actually been a few instances where potential signals were detected.
And while follow-up surveys have been unable to offer confirmation in any of these cases, scientists have not been able to rule out the possibility of an extraterrestrial intelligence being involved. So here they are in chronological order...
This event, which is considered by many to be the best candidate for an extraterrestrial radio signal, occurred on August 15th, 1977. On this date, the Ohio State Radio Observatory received a very strong radio signal that appeared to be coming from the direction of the Sagittarius constellation.
On the following day, volunteer astronomer Jerry Ehman circled the indicated signal on a printout and wrote “WOW!” next to it in the margin. This event came to be known as the “WOW! Signal”, but follow-up surveys of the Sagittarius constellation have failed to provide confirmation.
In 2007, Duncan Lorimer (a professor of astrophysics at the University of West Virginia) assigned his student (David Narkevic) the task of examining archival data obtained by the Parkes radio dish in 2001. Analysis of the data found a radio burst that lasted just 5 milliseconds and was coming from a location near the Small Magellanic Cloud (SMC).
This event, which came to be known as the Lorimer Burst, was the first-ever detected. By 2013, several more events were detected, which would henceforth be designated as Fast Radio Bursts (FRB). By 2016, evidence of a repeating FRB was found in archival data obtained by the Arecibo radio telescope.
To date, the exact cause of FRBs remains unknown. While many scientists have theorized that they could be the result of natural phenomena like a rapidly rotating neutron star or a black hole, some have gone as far as to suggest that they may be evidence of extraterrestrial transmissions.
In 2015, citizen astronomers with the Planet Hunters project published a paper detailing their observations of KIC 8462852 - a star located 1,470 light-years from Earth. Using data obtained by the Kepler Space Telescope, the team detected a 22% drop in brightness that could not be explained.
The star would come to be nicknamed Tabby’s Star (aka. Boyajian’s Star) after team leader Tabetha S. Boyajian. From 2015 to 2018, the star experienced additional dimming events of varying magnitudes that deepened the mystery.
Multiple attempts were made to explain these patterns, which included transiting comets, a consumed planet, a debris disk, a ring system, and others. However, some suggested that the dimming possibly the result of an alien megastructure orbiting the star.
On October 26th, 2016, Breakthrough Listen observed Tabby’s Star for eight hours for signs of radio signals. In the months that followed, follow-ups observations were made, but no signals have been detected.
In December 2018, a search for laser light emissions was carried out using the APF. While a number of candidates were identified, further analysis showed that they were terrestrial in origin and all came from the Earth.
In 2016, a team of astrophysicists from Uppsala University suggested that SETI researchers should look for signs of "physically impossible effects caused by highly advanced technology," by looking for stars and galaxies that suddenly became undetectable.
To illustrate their point, the team examined the positions, motions, and magnitudes of 10 million celestial objects that had been observed as part of the Sloan Digital Sky Survey (SDSS). Specifically, they were looking for objects that did not appear to be in their expected positions.
In the end, the team found one star that was visible in one image but significantly dimmer in the next. The team advised that this star be the target of follow-up observations to determine if this was the result of a natural phenomenon.
In April 2017, this project released its first set of results, which included 11 events that passed the threshold for significance. However, it was concluded that they were all consistent with radio frequency interference.
On August 30, 2017, Breakthrough Listen said it picked up a series of 15 FRBs coming from a dwarf galaxy about 3 billion light-years away. Breakthrough Listen researchers said the possibility of the source being extraterrestrial life can't be ruled out as yet.
On October 19th, 2017, the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) announced the detection of an interstellar object as it made a flyby of Earth. This object, designated 1I/2017 U1 (aka. ‘Oumuamua), was the first object of its kind to ever be detected.
Multiple follow-up observations were conducted as the object left the Solar System to determine its true nature and origin. Initially, astronomers believed that the object was a comet, but then concluded that it must be an asteroid since it failed to form a tail when it made its closest approach to the Sun.
However, 'Oumuamua sped up as it was leaving the Solar System, which was more consistent with a comet. Multiple explanations were offered and a paper, by postdoctoral researcher Shmuel Baily and Harvard professor Abraham Loeb, suggested it could be an interstellar probe.
Loeb and Baily argued that 'Oumuamua's behavior was consistent with a solar sail. Breakthrough Listen even dedicated survey time to monitor 'Oumuamua for signs of radio transmissions in December of 2017, but found nothing.
Nevertheless, the behavior and trajectory of the object (which took it on direct course past Earth) were seen by some as possible indications that 'Oumuamua could have actually been a survey probe sent from another star system.
This has led astronomers to recommend that observations be conducted with future interstellar objects - such as C/2019 Q4 (Borisov). It has also been recommended that spacecraft (like Project Lyra or the ESA's "Comet Interceptor") be sent to study one of these objects up close.
This is an especially tantalizing possibility considering that astronomers have concluded that interstellar objects pass through our Solar System on a regular basis.
Messages sent from Earth
In addition to listening to the Universe for signs of extraterrestrial signals, there is also the approach known as "Active SETI". This consists of crafting messages to send to space in the hopes that another species might pick up on the transmission and eventually answer.
The term Messaging Extraterrestrial Intelligence (METI) is also applied to this approach. It was coined by Russian scientist Alexander Zaitsev, who sought to draw a distinction between the passive and active methods of searching for alien intelligence in a 2006 paper.
So what have we had to say to our celestial brethren?
In 1962, scientists from the Evpatoria Planetary Radar (EPR) center in the Crimea sent out a radio message in Morse Code to the planet Venus. The first word was three letters - M-I-R - the Russian word for "peace", which was subsequently followed by "Lenin" and "SSSR".
This message was the first radio broadcast in the history of humanity intended for an extraterrestrial civilization. The signals bounced off the surface off Venus and returned about four and a half minutes later.
In 1974, the most powerful METI broadcast ever sent into space was made from the Arecibo Observatory. Known as the Arecibo Message, it consisted of a simple visual message composed by Francis Drake and Carl Sagan.
The message consisted of 1,679 binary digits that were arranged in a rectangle measuring 73 rows by 23 columns (both prime numbers). The message contained a number of easy-to-understand elements in different colors that would presumably be discernible to an intelligent species.
These included the numbers one to ten, the atomic numbers of the atoms that make up in DNA (hydrogen, carbon, nitrogen, oxygen, and phosphorus), the formulas for the sugars and bases in the nucleotides of DNA, the number of nucleotides in DNA, a graphic of the double helix structure of DNA, and a stick-figure depicting the profile of a human.
Also included was the human population of Earth, a graphic of the Solar System (with Earth indicated), a graphic of the Arecibo radio telescope and the dimensions of the antenna dish. The message was aimed at the globular star cluster M13, located 21,000 light-years from Earth.
The Pioneer Plaque was the first "message in a bottle" sent by humanity into space. The brainchild of Carl Sagan, this plaque was included on the Pioneer 10 and 11 missions and depicts the location of Earth in the Galaxy, as well as a naked man and woman drawn in relation to the spacecraft.
These spacecraft are heading generally towards the star Aldebaran in the Taurus constellation, and to the constellation of Aquila, respectively, and will take millions of years to get there. Nevertheless, both plaques could be intercepted in interstellar space at some point along their journey.
Voyager Golden Record:
The “Golden Record” of the Voyager 1 and 2 space probes (both which are now in interstellar space) took things a step further. Whereas the Pioneer Plaque was intended as a message in a bottle, the Golden Record was more of a "time capsule".
This attempt to communicate with extraterrestrials was also crafted by Carl Sagan, along with many of his colleagues from Cornell University. In addition to the cover (which depicted instructions on how to play it), the record contained sounds and images selected to portray life and culture on Earth.
Countless messages have been composed and transmitted by space agencies, non-profit groups, or as part of privately-funded projects. These include the Cosmic Call (1/2), the Teen Age Message, A Message From Earth, the WOW! Reply, the Lone Signal, and the ASREM message.
All were conducted between 1999 to 2016 and targeted stars between 17 and 69 light-years from the Earth. In the coming years, Breakthrough Message plans to mount an international competition to create messages that would be broadcast using their participating institutes. A total prize pool of $1,000,000 will be awarded to the winning entries.
So what have we learned? For starters, we've learned that the search for extraterrestrail life is very challenging work, and that Fermi's question cannot be answered so easily. Researchers have also amassed a small catalog of possible signals and detections that - while they are not yet confirmed - cannot be ruled out.
In the end, all we can do is count on the process of continuous discovery and look to future developments and more sophisticated instruments. In the coming years, several next-generation space and ground-based observatories will be coming online that could be game-changers.
Between better instruments, improved methods of data-mining and sharing, and more precise techniques for discerning the presence of biological and/or technological activity, humanity is prepared to make some impressive discoveries in the near future... if in fact they are there to be discovered.
For as Arthur C. Clarke famously declared: "Two possibilities exist: Either we are alone in the universe or we are not. Both are equally terrifying." And, as always, there is the possibility that we will never learn which is true.
Only time will tell if humanity achieves "First Contact", or is forced to endure the "Great Silence".
- METI - Is Active SETI Really Dangerous?
- ESA - Vital signs of life on distant worlds
- NASA - What in the World is an Exoplanet?
- Breakthrough Initiatives - Breakthrough Listen
- SETI Institute - Search for Extraterrestrial Intelligence
- NASA - SETI: The Search for ExtraTerrestrial Intelligence
- PSU - The Penn State Extraterrestrial Intelligence Center (PSETI)
- NASA - The Search for Good Science: The Cancellation of NASA's SETI Program