Ever since astronomers first learned that our planet is not the center of the Universe and that Solar System is merely a drop in the cosmic bucket, people have wondered if there could be intelligent life out there. Over time, our focus has shifted away from neighboring planets and onto neighboring stars and galaxies.
And yet, all attempts to find evidence of intelligent life have come up short. Or, at least, they've failed to provide definitive evidence that intelligent life is out there and looking to communicate with us. But considering how our technology (and our frame of reference) has been limited over time, it's possible that we simply weren't listening properly.
In the history of the Search for Extraterrestrial Intelligence (SETI), there have been a number of "candidates" (potential extraterrestrial signals), or unexplained phenomena that merited follow-up investigation. While most were attributable to natural cosmological events, there were a few that were never fully explained.
Years later, scientists still puzzle over mysterious faraway signals, dimming stars, fast radio bursts, and even an interstellar object that defied classification. Recently, scientists also discovered a signal that appeared to be coming from the neighboring star system of Proxima Centauri (and even appeared to be coming from one of its planets!)
Could it be that extraterrestrial intelligence (ETI) has attempted to communicate with us, and we simply didn't know it? Perhaps! All we know for sure is that when a "candidate" arises, scientists must eliminate all other possibilities before making any claims.
Is Anybody Out There?
While efforts to find extraterrestrial life only go back about a century, the idea that life could exist on other planets began to be treated as a scientific possibility as early as the "Age of Reason." For instance, during the 17th century, several luminaries speculated about the possibility that the Moon (or other celestial bodies) could be inhabited by native civilizations.
These included famed astronomer Johannes Kepler (1571 - 1630), English historian and clergyman Francis Godwin (1562 - 1633), French writer and playwright Savinien Cyrano de Bergerac (1619 - 1655), and Dutch physicist, astronomer, and inventor Christiaan Huygens (1629 - 1695).
In his treatise Cosmotheoros (1698), Huygens even went as far as to argue that life could exist on other planets within the Solar System. He based this conclusion on dark and bright spots that he observed on the surfaces of Mars and Jupiter, which he interpreted to be evidence of water and ice.
In the 18th century, famed French critic François-Marie Arouet (aka. Voltaire) dealt with the subject of aliens and space travel in his story Micromegas (1752). The story follows the titular character, who is a member of a giant species inhabiting a planet around the star Sirius, as he travels to other planets (Saturn, Mars, and Earth) to debate philosophies with smaller life forms.
Two other 19th-century French authors also predicted the existence of alien civilizations - Charlemagne Ischir Defontenay, in his story Star ou Psi de Cassiopee (1854) and astronomer Camille Flammarion, in his 1862 treatise La Plurality des Mondes Habités (“The Many Inhabited Worlds”).
Selenites, Martians, and Venusians
As time went on and astronomy became more sophisticated, scholars began to contemplate that there could be life on the nearest astronomical bodies - and that we might be able to communicate with them! This included the Moon, Mars, and Venus, bodies whose surface conditions and environments were not yet well-understood.
A good example is Jules Verne's famed novel, From the Earth to the Moon (1865). Not only does Verne raise the possibility that there is intelligent life on the Moon (“Selenites”), he also suggests in one passage that humans might be able to communicate with them:
“[S]ome practical geniuses have attempted to establish actual communication with her. Thus, a few days ago, a German geometrician proposed to send a scientific expedition to the steppes of Siberia. There, on those vast plains, they were to describe enormous geometric figures, drawn in characters of reflecting luminosity, among which was the proposition regarding the ‘square of the hypothenuse,’ commonly called the ‘Ass’s Bridge’ by the French.
“‘Every intelligent being,’ said the geometrician, ‘must understand the scientific meaning of that figure. The Selenites, do they exist, will respond by a similar figure; and, a communication being thus once established, it will be easy to form an alphabet which shall enable us to converse with the inhabitants of the moon.’”
H.G. Wells also explored the idea of life on the Moon in his book The First Men in the Moon (1901). The story is told from the point of view of two men who build a spaceship and travel to the Moon, where they are met by a subterranean culture of insectoids (also called “Selenites”) that have developed a complex and advanced society.
During this same time, speculation about other celestial bodies (like Mars and Venus) became far more widespread. With his War of the Worlds (1897), Wells created the familiar trope about "Martian invaders," which lasted for many decades.
The same was true of Venus, whose dense and opaque atmosphere allowed many to speculate freely about what conditions were like on the surface. In Olaf Stapledon’s 1930 novel Last and First Men, he described Venus as a tropical planet that is mostly covered in oceans. He also tells of how human efforts to terraform the planet resulted in the genocide of its native marine inhabitants.
Also, in the 1930s, Konstantin Tsiolkovsky - the “father of astronautic theory" - predicted many of the theories that went into the early SETI efforts. In a 1932 essay titled, “Is there a God?” he stated that :
“Millions of milliards of planets have existed for a long time, and therefore their animals have reached a maturity which we will reach in millions of years of our future life on Earth. This maturity is manifest by perfect intelligence, by a deep understanding of nature, and by technical power which makes other heavenly bodies accessible to the inhabitants of the cosmos.”
This was followed by an essay he published in 1933, titled "Planets are Inhabited by Living Creatures," in which he listed six reasons why it was scientifically sound to believe in extraterrestrial life:
- All these trillions of suns and all rarefied gaseous masses consist of the same substance as the Earth is composed of
- All planets are separated from the Sun. Therefore, they are also composed of the same matter as the matter which our planet is formed of
- All celestial bodies are subject to gravity. Therefore, weight is found on all planets
- There are liquids and gases on all major planets
- All planets are exposed to the same rays of their suns
- Almost all planets have day-and-night cycles and seasons
Here too, Tsiolkovsky would prove to be predictive, as his reasons involved some of the same lines of thinking that went into astronomer Frank Drake's famous equation for estimating the likelihood of extraterrestrial intelligence (the Drake Equation) roughly three decades later.
In the 1950s, Ray Bradbury envisioned Venus as an ocean planet in his short story The Long Rain (1950), where humans must live in "Sun Domes" that protect from the constant rains, and are periodically attacked by the indigenous inhabitants. Isaac Asimov's 1954 novel Lucky Starr and the Oceans of Venus similarly describes Venus as an ocean planet with native inhabitants.
By this decade, even before the Space Age began, the idea that there could be life on the Moon had been dispelled. But many science fiction authors still wrote stories involving native Martians, like Ray Bradbury’s The Martian Chronicles (1950), Lester del Ray’s Marooned on Mars (1952), and many works by Robert A. Heinlein (The Green Hills of Earth, Red Planet, and The Rolling Stones, among others).
The Search Begins!
Concordant with these speculations, the first SETI experiments were aimed at Mars and Venus. In the former's case, it was the famed inventor and engineer Nikola Tesla who conducted what is considered the first SETI experiment.
In 1896, he suggested that a scaled-up version of his wireless electrical system could be used to contact civilization on Mars. In 1899, while conducting experiments with electrical transmissions, he reported the possible detection of a static signal from Mars that ceased when Mars set in the sky.
Meanwhile, the first attempt to send a message was directed at Venus in 1962, by scientists from the Evpatoria Planetary Radar (EPR) center in the Crimea. This was known as The Morse Message, which consisted of a radio transmission sent in Morse Code - the letters M-I-R (Mir, the Russian word for “peace”) followed by “Lenin” and “SSSR.”
By the 1960s and 70s, speculation about Martians and Venusians disappeared as missions like the Mariner, Viking, and the Soviet Venera programs sent craft to study these planets and found no signs of civilizations. In the age of space exploration, efforts to find extraterrestrial life would be focused on faraway stars.
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 were also the efforts made by the Ohio State Radio Observatory (aka. the “Big Ear” Observatory), which would play a major role in multiple SETI surveys from the late 1950s onwards. In fact, the Big Ear was responsible for picking up the most significant candidate signal ever received (see WOW! Signal, below).
In 1971, NASA greenlighted a study known as Project Cyclops, which recommended the creation of a massive array of radio antennae (1,500 in total) to search for extraterrestrial signals. While the array was never built, the report itself informed many future surveys.
This would include the 1979 initiative launched by the Berkeley SETI Research Center, known as the Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations (SERENDIP). This program consisted of analyzing deep-space radio data obtained by large radio telescopes like Big Ear and the Arecibo Observatory.
In 1980, Carl Sagan and NASA scientist Bruce Murray and astronautics engineer Louis Friedman created the U.S. Planetary Society, which would play a significant role in future SETI projects. These included Suitcase SETI (1981-82), Sentinel (1983-85), the META (1985-1994), and BETA (1995 onward).
In 1992, NASA launched the Microwave Observing Program (MOP), a long-term effort that used NASA's Deep Space Network (DSN), the Green Bank Telescope, and Arecibo Observatory. The program was canceled in 1993 and resurrected in 1995 by the SETI Institute. Between 1995 and 2004, Project Phoenix (as it was renamed) would fulfill the MOP's goal of observing 800 stars within a 200 light-year radius of Earth.
In 2016, Yuri Milner founded Breakthrough Initiatives, a non-profit dedicated to SETI and interstellar exploration. One of their flagship programs is Breakthrough Listen, a ten-year, $100 million dollar project that will use the Parkes Observatory and Automated Planet Finder to survey the 1 million stars closest to Earth, along with 100 galaxies - the greatest SETI effort to date.
In 2016, China finished work on its Five-hundred-meter Aperture Spherical radio Telescope (FAST), which replaced Arecibo as the world's largest radio telescope.
In 2017, the Dominion Radio Astrophysical Observatory (DRAO) finished construction on its interferometric radio telescope, the Canadian Hydrogen Intensity Mapping Experiment (CHIME). Both telescopes will be intrinsic to SETI research and the study of Fast Radio Bursts (FRBs).
These and many other observatories have made a significant contribution to SETI research (and some still do). In the time that we've been looking, a few discoveries have left scientists scratching their heads.
It's not an easy thing, looking for alien life when you're not even sure what you should be looking for. The best we can do is to look for things we are familiar with and hope that leads us to discoveries that will expand our horizons. It was Arthur Wellesley, 1st Duke of Wellington (leader of the British forces at the Battle of Waterloo) who perhaps said it best:
"All the business of war, and indeed all the business of life, is to endeavor to find out what you don't know by what you do; that's what I called 'guessing what was at the other side of the hill.'"
When it comes to SETI, this amounts to using Earth as a template when deciding if exoplanets are "potentially-habitable" or not. It also means looking for technologies that we ourselves have tested and validated, or at least those that are scientifically sound.
Using this framework, there have been a number of times when we may have heard from extraterrestrials and simply can't prove it yet. The most recent was the detection of the BLC1 signal from Proxima Centauri (more on that below). But it was hardly the first.
The Wow! Signal:
On August 15th, 1977, astronomers at Big Ear Observatory detected a narrowband radio signal that lasted for 72 seconds and appeared to be coming from the direction of the Sagittarius Constellation. A few days later, Jerry R. Ehman, an astronomer at the observatory, noticed the anomaly while reviewing a printout of the data.
So impressed was he by the results that he circled the reading on the printout - denoted by alphanumeric code: "6EQUJ5" - and wrote "Wow!" next to it. This powerful signal would henceforth be nicknamed the “WOW! Signal” and was thought by many to be a signal from an extraterrestrial civilization.
The signal had no modulation (which is used to transmit information via radio), and follow-up attempts to find it revealed nothing. Nevertheless, the WOW! Signal remained the strongest candidate for a possible alien signal. In 2017, new evidence suggested that the signal could have been generated by a hydrogen cloud accompanying a comet. Then, in 2020, astronomers believed they had narrowed down the source of the Wow! signal to a star very much like our own, 1,800 light-years away, in the constellation of Sagittarius. But nothing definitive has been proven.
As Seth Shostak, the senior astronomer at the SETI Institute, characterized it:
“Was that E.T., or was it not E.T.? Nobody knows. Nobody has ever found another explanation for what that might have been. It's like you hear chains rattling in your attic, and you think, ‘My God, ghosts are real.’ But then you never hear them again, so what do you think?”
Bursts on Repeat (Alien Morse Code?):
In 2007, astronomer Duncan Lorimer and his student David Narkevic were looking through archival pulsar data and noticed something very interesting: a transient radio pulse that lasted mere milliseconds. This became known as the "Lorimer Burst" and was the first of several fast-radio-bursts (FRBs) detected by astronomers.
Since then, over a hundred and fifty FRBs have been detected (mostly in archival data), most of which were extragalactic in origin. The first Milky Way FRB was detected by the CHIME radio telescope in April of 2020, and the data indicated that they were closely linked to neutron stars with powerful magnetic fields (magnetars).
However, in a few instances, FRB sources were found that were repeating. To date, no natural explanation has been given for this phenomenon, which has led some to suggest that they might be evidence of extraterrestrial transmissions. Possibly a cosmic version of Morse Code, where the sender is tapping out a message on repeat to indicate a pattern.
In 1960, Freeman Dyson popularized the concepts of space-based megastructures in a paper titled "Search for Artificial Stellar Sources of Infrared Radiation." In this paper, he hypothesized that extraterrestrial civilizations could reach a level of development where they would be able to build a spherical structure that could contain their entire solar system (and harness all their sun's energy).
According to Dyson, these megastructures (aka. "Dyson Spheres") could be found by searching for point sources of infrared radiation. Since then, Dyson's work has inspired speculation about other types of megastructures and how they could be detected by looking for stars that experience periodic dimming (similar to how transmitting exoplanets are detected).
In September of 2015, a team of citizen scientists with the Planet Hunters project announced a large dip in luminosity while observing KIC 8462852 (nicknamed "Tabby's Star" or "Boyajian's Star") - a main sequence F-type star located 1,470 light-years from Earth in the Cygnus constellation.
Their observations indicated that a major dip occurred on March 5th, 2011, when the star dimmed sharply and abruptly by 15 percent. This was followed by an incident on February 28th, 2013, where the star dimmed by up to 22 percent before brightening again. Follow-up observations noted new fluctuations that lasted from mid-May 2017 to July 2018.
Further studies using Kepler data have shown that Tabby's Star experiences frequent, small, and non-periodic dips in brightness lasting about a day at a time. Whereas large dimming events are consistent with a large object passing between the star and observers here on Earth, the day-long dimming events are consistent with many small masses orbiting the star in tight formation.
Attempts to explain this behavior have ranged from a circumstellar debris disk, shattered comets and asteroids, a giant planet (or a planet with rings), and a planet or exomoon that broke apart around the star. However, none of these explanations fully fit the observational data.
A 2019 study by researchers from the University of Nebraska-Lincoln identified multiple analogs for Tabby's Star. Using data from the Northern Sky Variability Survey (NSVS) and the All-Sky Automated Survey for SuperNovae (ASAS-SN), they identified twenty-one stars that also experience infrequent small dips in brightness and a long-term decline between dips.
In addition, a team of astrophysicists from Uppsala University proposed a study in 2016 that suggested SETI researchers look for stars and galaxies that suddenly became undetectable. To illustrate, the team examined the positions, motions, and magnitudes of 10 million celestial objects from the Sloan Digital Sky Survey (SDSS), for objects that no longer appeared in their expected positions.
From this, the team found one star that was visible in one image but significantly dimmer in another. They advised that this star should be the subject of follow-up observations to determine if this was the result of a natural phenomenon.
In June of 2020, a team of astronomers using the ESOs Very Large Telescope (VLT) noted the sudden absence of an unstable massive star in the Kinman Dwarf galaxy and offered a few possible explanations for it in an accompanying study.
These included that it was partially obscured by dust or collapsed into a black hole without producing a supernova. The team from Uppsala University, however, suggested it might be evidence of a "disappearing star."
The possibility that our Solar System has been visited by interstellar probes has become rather popular (and controversial) in recent years. It began in 2017 when astronomers at the Panoramic Survey Telescope and Rapid Response System-1 (Pan-STARRS-1) announced the detection of an unusual object making a flyby of Earth.
Unlike Near-Earth Asteroids (NEAs) that periodically pass close to Earth, this object - designated 1I/2017 U1 (aka. Oumuamua, Hawaiian for "scout") - was the first known object to have come from interstellar space. Multiple follow-up observations were made of the object as it left the Solar System.
However, astronomers were unable to make sense of the spectra, some of which were consistent with a comet and others an asteroid. For starters, the readings obtained indicated that 'Oumuamua had a flattened shape and was oddly-proportioned. 'Oumuamua also accelerated as it made its way out of the Solar System, which was consistent with outgassing caused by solar radiation.
However, astronomers had already ruled out it being a comet when it failed to develop a tail as it made its closest pass to the Sun. It also didn't tumble violently as a result of its acceleration, which is what happens with comets. This led Dr. Shmuel Baily and Harvard Professor Abraham Loeb to suggested in a paper that 'Oumuamua might be an artificial object - like a solar sail.
Aside from its mysterious configuration and the way it accelerated, its trajectory allowed it to slingshot around the Sun and then make a close flyby of Earth. This was seen as a possible indication that 'Oumuamua could have actually been a survey probe sent from another star system.
Multiple studies were conducted after 'Oumuamua's departure that confirmed interstellar objects are likely to enter the Solar System on a regular basis and that many of them have stayed. This was bolstered by the arrival of another interstellar object known as C/2019 Q4 (Borisov) - clearly identified as a comet - less than two years later (in August of 2019).
If these findings are correct, then the Solar System could be littered with the remains of past interstellar probes. Next-generation observatories like Vera Rubin will be able to detect these objects as they enter our Solar System. Multiple proposals have even been made to build spacecraft that could rendezvous with them and even return samples to Earth.
Message from Next Door?
On Dec. 18th, 2020, astronomers announced that the Parkes radio telescope had picked up a radio signal between April and May of 2019 coming from Proxima Centauri (the closest star to the Solar System). This signal was detected by Breakthrough Listen, which was conducting observations with Parkes, was therefore designated Breakthrough Listen Candidate 1 (BLC1).
The signal was rather intriguing since it was a very sharp narrowband emission (982 MHz) that lasted for about 30 hours. In addition, it was undergoing a shift in frequency (aka. Doppler shift), which was said to be consistent with a moving radio source, like a planet orbiting the star.
This raised the possibility that the source could be Proxima b, an Earth-like planet orbiting within Proxima Centauri's Habitable Zone (HZ) whose existence was confirmed in 2016. An anonymous source involved in the study of the signal claimed that this might be the strongest candidate since the “Wow! Signal.”
However, multiple astrophysicists and scientific institutes have felt the need to remind people that this signal is a "candidate," not a confirmed extraterrestrial transmission. For starters, a probability assessment performed by Harvard astronomer Amir Siraj and Prof. Loeb found that the odds of BLC1 being a radio signal from Proxima Centauri were about 10-8.
The SETI Institute also indicated that while the detection of BLC1 is exciting news, nothing conclusive has been determined yet. For one, follow-up observations failed to detect the signal again. Second, there was the way the signal was only detected during a 30-hour period between April and May.
According to the SETI Institute, there are also numerous possible explanations that are more likely than the signal being an alien transmission. These include telemetry signals from one of the many satellites in Low Earth Orbit (LEO), which routinely jam up the radio spectrum and make it necessary to tease signals from the background noise.
Another possibility is that BLC1 was an emission from a planet with a strong magnetic field. This could Proxima c, a second planet orbiting Proxima Centauri (a Super-Earth or a Mini-Neptune) that was announced on Jan. 15th, 2020, and confirmed between April and June.
Then there's the possibility that it was a naturally-occurring radio signal from a distant cosmic source, which just happened to be behind Proxima Centauri at the time. Many of these possibilities present their own problems, but the point is that scientists have not yet begun to exhaust all other possible explanations - a prerequisite in SETI research.
As Franck Marchis, the Senior Planetary Astronomer with the SETI Institute, summarized:
"Of the 300 million exoplanets that could be habitable in our galaxy, which is 200,000 light-years across, it would be an astonishing coincidence for two civilizations (ours and one on Proxima b or c) to be using the same technology at the same time. Although I love the idea, it seems highly improbable—which is why I suspect we will quickly find a more down-to-earth explanation for the signal’s origin."
Unfortunately, what is true for BLC1 is true for all of the other "candidates" as well. Between the Wow! Signal, FRBs, dimming stars, interstellar objects, and BLC1, we are plagued by a high degree of uncertainty. We cannot rule out the possibility of extraterrestrials (yet) but can't confirm it either.
In a way, though, this is what makes the efforts so far all the more exciting. Knowing that there could be other intelligent lifeforms in our Universe (and that we might have already heard from it) is just the motivation needed to keep looking. After all, the main reason for searching and questing is the desire to satisfy our curiosity.
What secrets does the Universe hold? What is the meaning of it all? What lies beyond the realm of our understanding? What is on the other side of that hill? We continue to press forward in the hopes of answering these questions. Until we do, we have a handful of potential answers to keep us warm!
- Big Ear Radio Observatory - The "Wow!" Signal
- Shostack, S. "A Signal From Proxima Centauri." - SETI Institute (Dec. 19th, 2020)
- Frank, M. "Did Proxima Centauri Just Call to Say Hello? Not Really!" - SETI Institute (Dec. 21st, 2020)
- NPR - "Will E.T. Hear Us? Reply To 'WOW! Signal' Gets Beamed Into Space Today." (Aug. 15th, 2012)