27-4 The Drake equation helps scientists estimate how many civilizations may inhabit our Galaxy

We have seen that only a few locations in our solar system may have been suitable for the origin of life. But what about planets and moons around other stars? The existence of life on Earth seems to suggest that extraterrestrial life, possibly including intelligent species, might evolve on terrestrial planets around other 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.

Close Encounters Versus Remote Communication

A common belief is that alien civilizations do exist and that their spacecraft have visited Earth. Indeed, surveys show that between one-third and one-half of all Americans believe in unidentified flying objects (UFOs) of alien origin. A somewhat smaller percentage believes that aliens have landed on Earth. But, in fact, there is absolutely no scientifically verifiable evidence of alien visitations. As an example, 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. Atomic isotopes are one way to tell if a material comes from Earth or not, because we know that the abundances of many isotopes vary in our solar system and beyond, yet no alien spacecraft debris with a nonEarth isotope signature has been found.

While there is no evidence of alien visits to Earth, alien civilizations could exist around some of the billions of stars in our Galaxy. To find real evidence for intelligent civilizations on other worlds, we must look to the stars.

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 probes are prohibitively expensive. Instead, many astronomers hope to discover 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 (see Section 8-1).

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. Since then, many SETI searches have taken place 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.

Is There Anybody Out There?

Should we be discouraged by this 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 proposed that the number of technologically advanced civilizations in the Galaxy could be estimated by a simple equation. This is now called the Drake equation:

Drake equation

• N = number of technologically advanced civilizations in the Galaxy whose messages we might be able to detect
• R_* = the rate at which solar-type stars form in the Galaxy
• f_p = the fraction of stars that have planets
• n_e = the number of planets per solar system that are Earthlike (that is, suitable for life)
• f_l = the fraction of those Earthlike planets on which life actually arises
• f_i = the fraction of those life-forms that evolve into intelligent species
• f_c = the fraction of those species that develop adequate technology and then choose to send messages out into space
• L = the lifetime of a technologically advanced civilization

The Drake equation is enlightening because it expresses the number of extraterrestrial civilizations in a simple series of terms. We can estimate some of these terms from what we know about stars, stellar evolution, and planetary orbits. The Cosmic Connections figure depicts some of these considerations.

For example, 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 intelligent creatures like us can evolve on any of that star’s planets.

Although stars less massive than the Sun have much longer life-times, they also 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, and a planet that close is subject to strong tidal forces from its star. We saw in Section 4-8 how Earth’s tidal forces keep the Moon locked in synchronous rotation, with one face continually facing Earth. In the same way, a planet that orbits too close to its star would have one hemisphere that always faced the star, while the other hemisphere would be in perpetual, frigid darkness.

These considerations leave us with 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 in the Galaxy each year in the galactic habitable zone (see the Cosmic Connections figure). This result sets R_* at 1 per year.

As we saw in Section 8-4 and Section 8-5, 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 (see Figure 8-8). Many astronomers suspect that most Sunlike stars probably have planets, and so they give f_p a value of 1.

Unfortunately, the rest of the terms in the Drake equation are very uncertain. Let’s play with some hypothetical values. The chances that a planetary system has an Earthlike world suitable for life are not known. (But observations of planets around other stars made by the Kepler spacecraft will help determine this number in the coming years.) 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 1 in 10 solar-type stars is orbited by a habitable planet, making n_e = 0.1.

What about the fraction of Earthlike planets that develop life? Given the existence of life on Earth, we might assume that, given appropriate conditions, the development of life is quite likely, and we could set f_l = 1. However, it is also possible that life on Earthlike planets is rare. With only one Earthlike planet (our own Earth) to observe, there is no way to deduce the probability of Earthlike planets in general to form life. Furthermore, biochemists do not understand the detailed steps of life’s origins and therefore cannot predict the probability for life to form. This is a topic of intense interest to astrobiologists, but we will be optimistic and assume here that f_l = 1.

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, but were we to assume that they would, f_c would also be put at 1.

The last variable, L, involving the longevity of an advanced civilization, might be 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. 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 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. 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.