Source: Interesting Engineering

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 is often 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 is also highly-focused on searching for signs of technological activity (aka. technosignatures). The most time-honored method here involves surveys using radio telescopes, which search for signs of extraterrestrial transmissions.

Scientists have suggested that other activities should be searched for as well, including directed energy emissions (aka. laser). 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 and Fast Radio Bursts (FRBs) to gravitational waves. However, radio transmissions remain the only technosignature that scientists have monitored for.

A brief history of SETI

While it is impossible to pinpoint precisely when human beings looked up at the sky and wondered if there was life in the cosmos, some of the earliest recorded examples come to us from as early as 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 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 far more ancient tales of civilization beyond Earth existed.

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 narrative. 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 compendium the One Thousand and One Nights (aka. 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.

This trend would continue well into the early modern age and 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, and the efforts were largely focused on the planet Mars. At the time, astronomers and scientists believed that Mars was potentially habitable and even boasted its own indigenous “Martian” civilization.

Famed inventor and electrical engineer Nikola Tesla is credited for conducting the first SETI experiment. In 1896, he suggested how 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, his instruments did register an odd static signal that ceased when Mars set in the sky.

With the dawn of the Space Age in 1958, SETI research 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 SETI 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, also known as the “Big Ear” Observatory. Built in 1957, this flat-plane radio telescope would play a major role in multiple SETI surveys and would be responsible for one of the most significant possible detections ever made (see WOW! Signal, below).

From this point forward, SETI 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 SETI 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 their mandate to further 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 SETI. 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 nearest stars to Earth and 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 been (or continue to be) made by the SETI Institute’s Allen Telescope Array, the Very Large Array (VLA), and 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).

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Source: Interesting Engineering