8-4 The Drake equation helps scientists estimate how many civilizations may inhabit our Galaxy

We have seen that some notable locations within our solar system might have been suitable for life to exist. But what about life existing on countless other planets orbiting distant stars? Scientists have found and cataloged thousands of such distant planets and can infer that millions, if not billions, more exist, waiting to be discovered. There are, of course, two competing perspectives. One is that life on Earth is unique and only happened one time in the entire universe. The alternative perspective is that the fact that life exists on Earth means that extraterrestrial life, including intelligent species, might evolve on planets around distant stars, given sufficient time and hospitable conditions. How can we learn whether such worlds exist, given the tremendous distances that separate us from them? This is the great challenge facing the Search for Extraterrestrial Intelligence, or SETI.

Remote Communication Is More Likely than Close Encounters

A tenet of modern folklore is the belief that alien civilizations do exist, and that their spacecraft have visited Earth secretly, mostly avoiding detection. Indeed, public perception surveys show that between one-third and one-half of all Americans believe that unidentified flying objects (UFOs) continue to visit Earth from distant, alien worlds. Despite frequent themes in science-fiction adventure movies and television shows, there is no scientifically verifiable evidence of alien visitations. As an example, consider that many UFO proponents believe that the U.S. government is hiding evidence of an alien spacecraft that crashed near Roswell, New Mexico, in 1947. However, the bits of “spacecraft wreckage” found near Roswell turned out to be nothing more than remnants of an unmanned research balloon. To find compelling evidence of the presence or absence of intelligent civilizations on worlds orbiting other stars, we must look elsewhere.

With our present technology, sending even a small unmanned spacecraft to another star requires a flight time of tens of thousands of years. Speculative design studies have been made for unmanned probes that could reach other stars within a century or less, but these are prohibitively expensive. Instead, many astronomers hope to learn about extraterrestrial civilizations by detecting radio transmissions from them. Radio waves are a logical choice for interstellar communication, because they can travel immense distances without being significantly degraded by the interstellar medium, the thin gas and dust found between the stars. They also travel very fast—at light speed of 186,000 mi/s (300,000 km/s).

Over the past several decades, astronomers have proposed various ways to search for alien radio transmissions, and several searches have been undertaken. In 1960, Frank Drake first used a radio telescope at the National Radio Astronomy Observatory in West Virginia to listen to two Sunlike stars, Tau Ceti and Epsilon Eridani, without success. More than 60 more extensive SETI searches have taken place since then, using radio telescopes around the world. Occasionally, a search has detected an unusual or powerful signal. But none has ever repeated, as a signal of intelligent origin might be expected to do. To date, we have no confirmed evidence of radio transmissions from another world.

Are They Out There?

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Should we be discouraged by our failure to make contact? What are the chances that a radio astronomer might someday detect radio signals from an extraterrestrial civilization? The first person to tackle this issue was Frank Drake, who laid out all of the possible characteristics that one would have to consider in order to find another civilization. He proposed that the number of technologically advanced civilizations in the Galaxy could be estimated by combining all of the important variables into a single mathematical sentence. This is now known as the Drake equation:

Drake equation

Let’s consider this one aspect at a time. The first two factors, R* and f_p, can be determined by observation. In estimating R*, we should probably exclude stars with masses greater than about 1.5 times that of the Sun. These more massive stars use up the hydrogen in their cores in 3 billion (3 × 10⁹) years or less. On Earth, by contrast, human intelligence developed only within the last million years or so, some 4.56 billion years after the formation of the solar system. If that is typical of the time needed to evolve higher life-forms, then a star of 1.5 solar masses or more probably fades away or explodes into a supernova before creatures as intelligent as us can evolve on any of that star’s planets.

Although stars less massive than the Sun have much longer lifetimes, they, too, seem unsuited for life because they are so dim. Only planets very near a low-mass star would be sufficiently warm for life as we know it. More important, a planet sufficiently close to a star is subject to its strong gravitational forces, which impact the speed at which the planet can spin. This is important because a planet that orbits too close to its star would have one hemisphere heated to great temperatures because it would be gravitationally locked toward the star, while the other hemisphere would be in perpetual, frigid darkness (see Figure 1-23).

This leaves us to consider a small range of stars not too different from the Sun. (Like Goldilocks sampling the three bears’ porridge, we must have a star that is not too hot and not too cold, but just right.) Based on statistical studies of star formation in the Milky Way, some astronomers estimate that roughly one of these Sunlike stars forms each year in a galactic habitable zone where one might find such stars. This sets R* at 1 per year.

The planets in our solar system formed as a natural consequence of the birth of the Sun. We have also seen evidence suggesting that planetary formation may be commonplace around single stars. Many astronomers suspect that most Sunlike stars probably have planets, and so they give a f_p a value of 1.

Unfortunately, the rest of the terms in the Drake equation are extremely uncertain. Let’s consider some hypothetical values. The chances that a planetary system has an Earthlike world suitable for life are not known. Were we to consider our own solar system as representative, we could put n_e at 1. Let’s be more conservative, however, and suppose that one in ten solar-type stars is orbited by a habitable planet, making n_e = 0.1. The correct number is likely larger than this, but using a smaller number helps keep us from overestimating this unknown value. From what we know about the evolution of life on Earth, we might assume that, given appropriate conditions, the development of life is a certainty, which would make f_l = 1. This is an area of intense interest to astrobiologists.

For the sake of argument, we might also assume that evolution might naturally lead to the development of intelligence (a conjecture that is hotly debated) and also make f_i = 1. It is anyone’s guess as to whether these intelligent extraterrestrial beings would attempt communication with other civilizations in the Galaxy. Even within current political debates, it is an undecided question as to whether we should be trying to find other intelligent civilizations that are perhaps more technologically advanced, because our search might unintentionally alert their attention to our vulnerable existence. If we assume that other civilizations would try to communicate f_c, would be put at 1 also.

The last variable, L, involving the longevity of a civilization, is the most uncertain of all. Looking at our own example, we see a planet whose atmosphere and oceans are increasingly polluted by creatures that possess nuclear weapons. If we are typical, perhaps L is as short as 100 years. If so, there could have been countless civilizations existing elsewhere in our own Galaxy that have long since ceased to exist. Putting all these numbers together, we arrive at

In other words, out of the hundreds of billions of stars in the Galaxy, we would estimate that there are only 10 technologically advanced civilizations from which we might receive communications.

A wide range of values has been proposed for the many terms in the Drake equation, and these various guesses produce vastly different estimates of N. Some scientists argue that there is exactly one advanced civilization in the Galaxy and that we are it. Others speculate that there may be hundreds or thousands of planets inhabited by intelligent creatures, and extraterrestrial communication is just too difficult or too expensive in which to participate. If we wish to know whether our Galaxy is devoid of other intelligence, teeming with civilizations, or something in between, we must keep searching the skies.