The chances of detecting signs of extraterrestrial beings are small, but there is a deep human curiosity about whether we are alone in the Universe and this drives a continuing search.
Even in April, the temperature in Green Bank, West Virginia can be bitterly cold, especially at four o’clock in the morning. Retired professor Frank Drake knows this only too well, as back in 1960, when he was 29 years old, that was the time he started work at the Green Bank Telescope. He was the first human being to conduct a search for extraterrestrial intelligence. He tuned the receiver of the radio telescope to pick up radio waves given out by hydrogen atoms. Given the importance of water (comprising hydrogen and oxygen) to life on Earth, Drake hoped that this fundamental frequency would be a natural carrier for extraterrestrials to transmit signals. He then turned the telescope to the first target: a sun-like star called Tau Ceti just 12 light years away. In the hope that an extraterrestrial signal would come through loud and clear, he rigged a tape recorder and a speaker in the room. But for Tau Ceti, he recorded nothing, so he turned to Epsilon Eridani, the next star on the list. Within minutes the room filled with powerful radio fuzz; he could hardly believe it was this easy. But then the signal disappeared. After days of searching, the transmission returned but this time it was clearly terrestrial interference. Undeterred, Drake and his colleagues continued the search, and are still searching today.
Half a century since the search began, there is still no evidence for other intelligent races in our own Galaxy. Some have dubbed this lack of signals “the great silence.” Yet this is by no means proof of the nonexistence of extraterrestrials. The Galaxy is so large, the radio spectrum so wide, and the technology we have available so limited (compared to what we can imagine building if we had the money), that we have barely scratched the surface in this search.
“Absence of evidence is not evidence of absence.”
CARL SAGAN 20TH CENTURY ASTRONOMER
The best way to understand the magnitude of the undertaking, and the huge uncertainties involved, is to do what Frank Drake himself did in 1960 and try to estimate the number of extraterrestrial civilizations there should be in the Galaxy. Drake attempted this by writing down a long string of factors which multiplied together would give this number:
1: The average number of stars to form per year in the Galaxy.
2: The fraction of those stars that form planets.
3: The fraction of those planets that could support life.
4: The fraction of life-supporting planets that do indeed form life.
5: The fraction of those living planets that develop intelligent life forms.
6: The fraction of those intelligent life forms that develop technology.
7: The average lifetime of a communicating species; in other words how long a civilization will use radio technology, leaking signals into space for us to hear.
Dishearteningly, the only factor that is known is the first one. Astronomers have shown that the Galaxy gives birth to about seven new stars per year. They are now working on an estimate of the second term, the fraction of stars that form planets. It has always been assumed by astrophysicists that our Solar System is typical and that most stars should form planets. However, testing this assumption has proved tricky because it is incredibly difficult to see a planet around another star. A planet does not emit light and so it is hidden in the glare of its parent star. Therefore taking an image is the equivalent of trying to discern a pinhead held next to a searchlight. Nevertheless, during the last 15 years astronomers have inferred the existence of more than 400 planets around other stars, terming them “exoplanets.” They have found them by detecting the way each planet’s gravity induces a wobble in its parent star (see Why Do the Planets Stay in Orbit?).
“The Sun, with all the planets revolving around it, and depending on it, can still ripen a bunch of grapes as though it had nothing else in the Universe to do.”
GALILEO GALILEI 17TH CENTURY ASTRONOMER
This method is not sensitive enough to detect smaller planets; it is only suited to revealing planets larger than Earth. For a more complete picture of the number and variety of planets, astronomers are now using a space telescope called Kepler to monitor 100,000 stars for the drop in light caused when a planet slips across in front of it, a celestial alignment known as a “transit.” Far above the distorting effects of the Earth’s atmosphere, Kepler is accurate enough to detect the slight reduction caused by transiting Earth-sized planets. Astronomers believe that this will give them a census of how many stars form planets and so allow them to provide an answer for the second term in the Drake equation. Kepler’s survey will also help them determine the third term—how many of those planets might be suitable for life.
Of the more than 400 planets currently inferred to exist around other stars, only one is thought to be habitable. It is called Gliese 581c, and orbits a dim red dwarf star just 20 light years from Earth. The planet is 1.5 times larger than Earth, holds between five and ten times more mass, and generates a gravitational field twice as strong. The planet is either a large rocky world, dubbed a “super-Earth,” or an oceanic planet with some similarity to Uranus or Neptune in our own Solar System. Uranus and Neptune have similar masses to the upper estimate for Gliese 581c and their ices would melt close to a star, transforming the planet into a world with nothing but ocean on its surface. Regardless of whether it is rocky or oceanic, astronomers believe it is habitable because it lies just within its star’s “habitable zone.”
This is the region surrounding a star in which a planet could be warm enough (but not too warm) to support liquid water on its surface. The zone’s distance from the star is determined by the star’s temperature, which is a signpost of how much energy it pumps into space. Obviously, the Earth lies within the Sun’s habitable zone, which extends from between 0.95 and 1.5 times the distance of the Earth from the Sun.
THE HABITABLE ZONE
Gliese 581c orbits a red dwarf star, which is significantly cooler than the Sun and so creates a much smaller habitable zone lying much closer to the surface of the star. By comparison, the Earth is 14 times further away from the Sun than Gliese 581c is from its star. Consequently this exoplanet completes an orbit so quickly that its year lasts just 13 Earth-days. At this proximity, the gravity of the star is so strong that Gliese 581c is trapped into showing just one face to the star, with the result that one side of Gliese 581c is in permanent daylight and the other in permanent night. Calculations show that its surface temperature would allow liquid water and so it must be thought of as habitable. The same cannot be said for any of the other known exoplanets, either because they lie outside their star’s habitable zone or because they are thought to be gas giants with no solid surface.
In our own Solar System, Venus, at 0.75 times the Earth’s distance from the Sun, is well outside the inner boundary of the habitable zone, but Mars is right on the outer boundary at 1.5 times Earth’s distance. These are not encouraging statistics for the Drake equation because it means that out of our eight planets, only Earth and possibly Mars can be considered habitable. If the same pattern is repeated across the Galaxy, then it means that only between one quarter and one eighth of the planets detected will be habitable. The Kepler mission will help us to refine this figure by being sensitive to planets of many sizes and orbital configurations, and astronomers are particularly on the lookout for Earth-sized worlds within the habitable zone of their star.
Some astronomers have pointed out that by focusing exclusively on the habitable zone we may be blinding ourselves to exotic possibilities of life, ones that do not require Earth-like conditions. Also, there may be unexpected places where the temperature is suitable for liquid water. Jupiter’s moon, Europa, for example, is well outside the traditional habitable zone and yet tidal effects from Jupiter’s powerful gravitational field supply it with enough energy to maintain a global subsurface ocean of water. And, according to the astrobiologists, wherever there is water, there may also be life (see Is There Life on Mars?). Nevertheless, until we understand more about such exotic locations, most researchers take a conservative approach, preferring to underestimate the number of habitable planets than overestimate them.
The next term in the Drake equation is the proportion of those habitable planets that go on to create life. Astronomers around the world are currently designing missions that could reveal living planets to us, not by the radio signals emitted into space but by the way life forms alter the chemical composition of their planet’s atmosphere. The widespread presence of life on Earth puts methane and oxygen into the atmosphere. Without constant replenishment from plant and animal metabolisms, the gases would disappear because they react with each other making water and carbon dioxide. So, a clear signal of a living planet would be the presence of oxygen and methane. Indeed, the recent discovery of localized pockets of methane on Mars has ignited the hope that the planet may harbor some life (see Is There Life on Mars?).
Astronomers need equipment capable of collecting enough light from the feebly dim exoplanets to perform a spectral analysis (see What Are Stars Made From?) of their atmospheres. It is a difficult undertaking; only four exoplanets have ever been imaged from Earth: three around a star called HR8799 and one around the bright star Fomalhaut. Once the glare of each central star was blocked out, the exoplanets appeared as tiny points of light, and to stand any chance of chemically analyzing these worlds a telescope would have to collect their light for weeks or even months. This is impractical when so much other science could be achieved in that time, so astronomers and engineers are planning to build a dedicated space telescope.
Until this plan is put into action and data is collected, scientists find themselves able to do little more than guess where life may be. It is impossible to say how easily life could have formed elsewhere when we are still unclear about the steps taken for life to form on Earth (see Are We Made From Stardust?).
TIMELINE OF LIFE ON EARTH
The values of the remaining terms in Drake’s equation are similarly little more than guesswork, because the only example we have of a living planet is the Earth and no scientist would draw a conclusion from a sample of one. Nevertheless, it is all we have to go on for the moment. The next two terms are the number of living planets that develop intelligent life, and the number of those that develop technology capable of sending radio messages into space. If Earth’s history is typical, those chances might be rather small as there have been many evolutionary hurdles for life to jump before arriving at intelligence.
The fossil record shows that evolution has been a slow but accelerating process on our planet. For more than half the Earth’s age, life was confined to the simplest form of cell, known as a “prokaryote,” which lacked a nucleus. Everything that the cell needed to function was simply jumbled together inside the cell membrane. It was only around two billion years ago that cells evolved nuclei in which to keep their genetic material. This leap in complexity was followed by the cells evolving other specialized compartments to better organize their contents and make more efficient use of the resources around them.
Eventually, cells somehow evolved to work together and this led to the development of multicellular organisms but, again, only after a vast period of time. Complex life became successful on Earth only half a billion years ago, in an evolutionary episode called the “Cambrian Explosion.” This remarkable era saw the development of most major groups of animals during a period of just 70 to 80 million years. The extraordinary rate of evolution may have been stimulated by the build-up of oxygen in the atmosphere, providing more energy to power more complex life forms. The oxygen build-up was actually the worst case of planetary pollution in the history of the Earth. Oxygen is the waste gas given out by photosynthesizing cells, which turn sunlight into their energy for life; it is also the waste product of certain microbial metabolisms. It is a highly reactive gas, and in some circumstances it is toxic because of the aggressive way it attacks biological molecules. As oxygen accumulated in the atmosphere, it triggered a mass extinction of Earth’s early microbes, ironically destroyed by their own pollution.
“If the Eiffel Tower were now representing the world’s age, the skin of paint on the pinnacle-knob at its summit would represent Man’s share of that age.”
MARK TWAIN 19TH CENTURY WRITER
Some ancient microbes survived by finding niches away from the oxygen, for example underground; other species evolved coping strategies. These included binding the oxygen into molecules such as collagen, which then provided structural support so that organisms could grow larger. They also learned how to use the oxygen to generate energy and this proved to be highly successful, leading to all animals breathing oxygen. It is often said that the availability of energy-laden oxygen led to the development of our power-hungry brains. Even so, humans with their intelligent brains did not arrive on the scene until just 200 thousand years ago, and the technological ability to send messages into space not until just 70 years ago. In other words, when taken over the entirety of Earth’s lifespan, humans have been around for the merest blink of an eye, and the ability to send radio signals into space for a fraction of that.
It has to be considered that just because a planet can support intelligent life does not mean that those life forms are guaranteed to develop the necessary communication technology. Take, for example, Gliese 581c, the only habitable planet so far known around another star. It is possible that this is a water world, with an all-encompassing ocean and no continents. If so, then it could evolve an intelligent aquatic species but it would seem unlikely that such an underwater civilization would develop electrical technology.
The final term in the Drake equation is the average time for which a communicating civilization exists. It has to be noted that the abilities to transmit signals and to destroy ourselves with atomic weapons arrived at about the same time in human history. This has led some to speculate that intelligent races will not live for long after developing technology, perhaps only a century or two. It does not have to be war that silences us; our technology is undoubtedly contributing to climate change, which could have catastrophic repercussions. Others of a more optimistic outlook believe that we will overcome these problems. If so, then our species and our technology could exist for a very long time. It is estimated that the Earth will remain habitable for some billion more years before the Sun heats up so much that it boils away our oceans (see What Will Be the Fate of the Universe?). So potentially, humankind could be a transmitting into space for a long time. The “bottom line” for the Drake equation is that the final term is usually the one that dominates all the rest. The longer we estimate for the lifetime of a technologically advanced civilization, the more of them we should expect to be populating the Galaxy, increasing our chances of eavesdropping on them.
Following on from Drake’s early attempts, the current era of search for extraterrestrial intelligence (SETI) was provisionally launched in 1971 when NASA commissioned a study into the design for the ultimate SETI telescope. Called “Project Cyclops,” this gargantuan field of interlinked telescopes would have rendered its metaphorical single eye capable of detecting any stray radio communications from planets within 1000 light years of Earth. (This would be the same kind of radio radiation that spills from Earth into space as a result of our radio and television broadcasts and military radar signals.) Traditional radio telescopes are simply incapable of picking up such weak transmissions.
Cyclops was, however, never built and without it SETI scientists have to rely on extraterrestrials targeting Earth with a concentrated radio signal designed to attract attention; this assumption has drawn criticism. Skeptics ask why an advanced alien culture would want to talk to us; it is, they say, the equivalent of our trying to talk to an amoeba. Notwithstanding the skepticism, the widespread interest shown by people in whether we have alien counterparts is underlined by the popularity of the “SETI@home” program. Released in 1999, this is computer software that uses home computers’ idle time to analyze data from Berkeley’s SERENDIP project, which stands for the Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations. The SERENDIP receiver sits on a radio telescope and collects data from whatever the telescope happens to be observing. The data is then distributed across the Internet to people running the SETI@home software, which analyzes it for possible signals and sends the results back to Berkeley. To date some curious signals have been discovered but nothing that has stood up to further scrutiny.
The SETI scientists may find an extraterrestrial signal tomorrow, or next year, or it may be in the next decade or the next century, or never. We currently do not possess the costly technology required to make a definitive search. Given the factors involved, especially the long time it took for intelligent life to develop on the Earth, many astronomers believe that, while there may be many planets that develop life in some form, only a very small proportion will go on to evolve intelligent beings with interstellar communication technology. Without any evidence to the contrary, it remains possible that we are the only intelligent species in the Galaxy—or even in the Universe.