The Copernican principle states the working assumption that Earth is not special—it is no more than an average planet, orbiting a medium-sized star, in an unremarkable part of an ordinary galaxy. If Earth is not unique, then there is little reason to think that other planets cannot also harbor life. Given the number of stars in the universe—in the order of 1023—this might be a statistical certainty. Over the centuries, many thinkers, such as American Carl Sagan, have pondered the possibility.

Is Earth alone?

In the 16th century, the Italian monk Giordano Bruno proposed that the stars were other suns, each of which could have its own solar system. Life could even populate these other earths. Believing that the universe was infinite, Bruno also insisted that it could have no center. Bruno was tried by the Roman Inquisition for these and other heretical beliefs and burned at the stake in 1600.

Throughout history, various astronomers have claimed to see evidence for life on other planets of the solar system. In the 1890s, American astronomer Percival Lowell claimed to have mapped artificial “canals” on Mars, while the dense clouds of Venus were imagined, by Swedish chemist Svante Arrhenius in 1918, to hide from view a lush surface blooming with life. It is now known that the clouds are acidic, while the surface of Venus is an inhospitable 864°F (462°C). However, these are just two planets out of potentially billions

The universe’s immensity and the apparent universality of its physical laws make it seem likely that microbial life exists elsewhere. Indeed, life may have arisen elsewhere and been transported to Earth. Greek philosopher Anaxagoras first suggested this idea of “panspermia” in the 5th century BCE. Naturalist Charles Darwin briefly turned to this idea while working on his theory of evolution by natural selection, troubled that the accepted figure for Earth’s age did not give enough time for complex organisms to evolve. Earth is now known to be vastly older than was believed in Darwin’s time, so panspermia is not needed to explain the origin of life on the planet.

Recent discoveries show that comets can carry many of the basic chemical components of life, but the exact mechanism by which life on Earth began remains a mystery. Solving that mystery should give a far better idea of how likely life is elsewhere.

“Do there exist many worlds, or is there but a single world? This is one of the most noble and exalted questions in the study of Nature.” Albertus Magnus

Computer simulations have shown that it is theoretically possible for simple single-cell life forms to exist inside comets or asteroids and to survive an impact like this with Earth.

Where is everyone?

One day, over lunch in Los Alamos in 1950, Italian scientist Enrico Fermi asked a simple question: “Where are they?” He reasoned that, even if only a small proportion of planets play host to intelligent life, given the unimaginable numbers of stars within the galaxy, one might expect a large number of civilizations to exist on other planets. At least some of them may have chosen to send messages or tried to visit Earth themselves. Earth has been producing electromagnetic signals for 90 years or so, since the dawn of radio and television broadcasting. These modulated radio waves—expanding and extending some 90 light-years in all directions—should be a giveaway of a technologically advanced society to any potential spacefaring intelligence.

In 1959, Giuseppe Cocconi and Philip Morrison suggested a bandwidth to search for alien radio messages. A year later, Frank Drake, at the National Radio Astronomy Observatory in Green Bank, West Virginia, set out to look for them. Drake founded Project Ozma, named for the queen of author L. Frank Baum’s imaginary Land of Oz—a place “difficult to reach and populated by exotic beings.” After a briefly exciting and noisy encounter with some top-secret military radio-jamming equipment, Drake and his team were met with silence. More than 50 years later, the silence has not yet been broken.

Order of the Dolphin

Drake drew together a diverse group of scientists to lay the foundations and protocols for the search for extraterrestrial intelligence (SETI). The group jokingly called itself the Order of the Dolphin, in reference to the work of neuroscientist John Lilly, who pioneered the science of speaking to dolphins. As one of the few people dealing with interspecies communication, Lilly was an important part of the group, which also included a young astronomer Carl Sagan, who was an expert on planetary atmospheres.

In preparation for the Order’s first meeting in 1961, Drake came up with a formula for the number of alien civilizations in the galaxy: N = R × fp × ne × fl × fi × fc × L The total (N) was reached by multiplying the factors necessary for intelligent extraterrestrials to evolve and be discovered. It depends on the rate at which stars suitable for intelligent life form (R*); the fraction of these stars that are orbited by planets (fp); the number of planets in any given planetary system that can support life (ne); the fraction of these planets upon which life actually appears (fl); the proportion
of life-bearing planets that go on to produce intelligent life (fi); the proportion of civilizations that develop technology that betrays detectable signs of their existence (fc); and, finally, the length of time such civilizations survive (L).

With these terms in place, bounding limits could theoretically be placed on each one. In 1961, however, not a single one was known with any confidence. Delegates at the meeting concluded that N was approximately equal to L, and a potential 1,000 to 100 million civilizations might exist in the galaxy. Although values for some of the variables in the Drake equation have been narrowed down over the intervening years, modern estimates of N still vary wildly. Some scientists argue that the figure may be zero.

The Pioneer plaque contains information about the location of Earth within the Milky Way.

“The search for extraterrestrial life is one of those few circumstances where both a success and a failure would be a success by all standards.” Carl Sagan

Message in a bottle

In 1966, Sagan cowrote Intelligent Life in the Universe, perhaps the first comprehensive discussion of planetary science and exobiology. The book was an expanded and revised version of an earlier edition, published in 1962 by the Soviet astronomer and astrophysicist Iosif Shklovsky. Although highly speculative, the book ignited discussion among scientists. It inspired NASA’s Project Cyclops report, an influential document now referred to as the “SETI Bible.”

In 1971, Sagan approached NASA with the idea of sending a message on its Pioneer spacecraft. Sagan and Drake worked on a design that would advertise Earth’s existence to alien civilizations and help them locate Earth in the cosmos. The graphics on the Pioneer plaque establish a unit of measurement using the 21-cm hydrogen emission line. Units defined by reference to Earthbased phenomena, such as meters and seconds, would be meaningless to extraterrestrial scientists. By choosing units from properties of nature, the hope was that they would be understood universally.

All the images on the plaque were scaled in terms of these units. A map of bright and distinctive pulsars points the direction to Earth, and Pioneer’s route is traced on a simple pictogram of the solar system. Images of a man and a woman were drawn by Sagan’s artist wife, Linda Salzmann Sagan.

Pioneer 10 and Pioneer 11, launched in 1972 and 1973, were fitted with Sagan’s plaque, etched on a 6- by 9-in (152 x 199 mm) gold-anodized aluminum plate. Critics warned that it would attract the unwanted attentions of power-hungry (or just hungry) aliens. Feminist groups were unhappy that the man waved in greeting, while the woman’s pose angled her body (they thought) submissively toward the male figure. Salzmann responded that women are smaller, on average; that having both figures waving might be interpreted as it being the natural arm position; and that she merely wished to show how the hips moved. Sagan had initially wanted the man and woman to be holding hands, but decided it might make the Earthlings look like a single creature with two heads.

“We began as wanderers, and we are wanderers still. We have lingered long enough on the shores of the cosmic ocean. We are ready at last to set sail for the stars.” Carl Sagan

The Arecibo message

While the search continued for beacons set up by intelligent beings and likely star systems, Drake and Sagan decided to send planet Earth’s own “we are here” signal. The 3-minute burst of 1,000 kW radio waves was designed to cross distances separating stars. Beamed out from the Arecibo radio dish in Puerto Rico in November 1974, the interstellar message was aimed at the globular cluster M13, a group of about 300,000 stars 25,000 light-years from Earth.

Instead of pictograms, the Arecibo message took the form of densely packed mathematical code, consisting of 1,679 binary digits (chosen because 1,679 is a product of two prime numbers, 73 and 23). The digital message contained the numbers 1 to 10 and information about the identity of the sender—details about DNA, the overall shape and dimensions of a human, and the position of planet Earth.

One of the hopes that accompanied the robotic explorers, as they were dispatched across the solar system from the 1960s onward, was that they might uncover some indication of extraterrestrial life within the solar system itself, even if it were only single-celled organisms. The spacecraft that touched down on planets, such as NASA’s Viking landers on Mars, carried experiments to test for signs of life. To date, no indication of life, either past or present, has been found, although some unexplored corners of the solar system remain candidates for life, such as the deep oceans thought to lie beneath the frozen surface of Jupiter’s moon Europa.

The Arecibo message was broadcast into space a single time in 1974. Coded in binary, its message is arranged in 73 rows of 23 columns.

Continued silence

No reply has ever been received to any of the 10 interstellar radio messages sent since 1962 and no communication has been detected. However, there have been false alarms. The most famous of these came in 1977, when an inexplicably powerful blast of radio signals was recorded coming from the direction of the Chi Sagittarii star system by Jerry Ehman at Ohio State University. He circled the signals on the readout and wrote “Wow!” next to them. The “Wow!” signal was never found again, however, and recent research suggests that it may have come from a hydrogen cloud surrounding a comet.

Given the vast distances between stars, it is still early days, however. The Arecibo message will not reach its target stars for another 25,000 years. Neither the Pioneer plaques nor the gold-plated disks carried by Voyagers 1 and 2—the Voyager Golden Record—are headed toward any particular star system. Unless they are intercepted, they are destined to wander the Milky Way eternally. Sagan, for his part, believed that to find life or to fail was a win-win scenario—either result would show something important about the nature of the universe.

Carl Sagan stands next to a model of the Viking 1 lander. The lander sent back signals from the surface of Mars from 1976 to 1982. Its instruments found no sign of life.

SETI in the modern age

NASA struggled to maintain its SETI funding, and today SETI is privately funded. Since the 1980s, the mantle has been taken up by the SETI Institute, based in Mountain View, California. UCal, Berkeley, through its SETI@home initiative, harnesses a network of volunteer computers to trawl Arecibo Observatory data for patterns that might indicate an unnatural radio source. Meanwhile, in 2016, China announced the completion of the largest ever radio telescope, the Five-hundred-meter Aperture Spherical Telescope (FAST). Among other things, FAST will search for extraterrestrial communications. It will eventually be made available to researchers from around the world.

In recent years, the focus of SETI has moved away from merely listening for messages. Efforts have been directed toward picking up biochemical signs of life or indications of advanced technology. Alien life should leave its signature in evolved planetary atmospheres, volatile molecules, or complex organic chemicals that could only be created by life processes. Highly technological societies may have learned how to harvest the energy of their star. A “Dyson sphere” megastructure completely or partially surrounding a star to capture its energy would affect the star’s observed output. It may also be possible to observe signs of asteroid mining or directly detect extraterrestrial spacecraft.

The SETI Institute’s Allen Telescope Array in California is used daily to search for possible alien communications, as well as for radio astronomy.

A cautious approach

In 2015, the Breakthrough Initiatives program was launched with the backing of Russian billionaire Yuri Milner. In addition to a $1,000,000 prize pool for SETI research and a plan to send a fleet of spacecraft to a nearby star, an open competition was announced to design a digital message to be sent to an extraterrestrial civilization. The Breakthrough Message project aims to accurately and artistically represent humanity and Earth, but pledges not to transmit any message until the risks and reward of contacting advanced civilizations have been debated.

Voyager 1 sent back this image of Earth from beyond the orbit of Pluto. The “pale blue dot” appears in a band of scattered sunlight.

Looking at ourselves

In 1990, Carl Sagan persuaded Voyager 1’s controllers to swivel its camera back toward Earth. From 4 billion miles (6 billion kilometers) away, the craft captured the “Pale Blue Dot” image. Sagan wrote: “Everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives on a mote of dust suspended in a sunbeam.” Sagan stressed the importance of looking at ourselves: “The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.”

SETI represents a series of questions whose answering would tell us about Earth’s place in the universe: whether the Copernican principle is correct, and if so, where else life has evolved. The answers could eventually provide humans with a way to transcend their origins and become a galactic species.

“We are almost certainly not the first intelligent species to undertake the search … Their perseverance will be our greatest asset in our beginning listening phase.” Project Cyclops Report


Carl Sagan is one of the most widely known 20thcentury scientists. His deep, honeyed tones are the instantly recognizable voice of the documentary series Cosmos. Sagan was raised in a workingclass Jewish area of New York and as a boy was an avid reader of science fiction. A talented pupil, he went to the University of Chicago in 1951 on a full scholarship. Sagan received his Ph.D. in 1960, showing that the high surface temperatures of Venus are due to a runaway greenhouse effect. Sagan conducted pioneering research in planetary science and exobiology (the biology of extraterrestrial life), which many in mainstream astronomy viewed with suspicion. In 1985, he wrote the sci-fi book Contact, which was later turned into a movie. With his visionary, positive, and humanist outlook, the Cornell University professor inspired a new generation of astronomers.

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