Just how many planets throughout our Galaxy are likely to harbor complex life? The first person to tackle this question quantitatively was Frank Drake. He proposed that the number of technologically advanced civilizations in the Galaxy (designated by the letter N) could be estimated by what is now called the Drake equation:

N = R*f_pn_ef_lf_if_cL

in which

The Drake equation expresses quantitatively the number of extraterrestrial civilizations as a product of terms, some of which can be estimated from what we know about stars and stellar evolution. For instance, thanks to new evidence for extrasolar planets, astronomers can now determine the first two terms, R* and f_p, by observation. We should probably exclude stars larger than about 1.5 M_⊙ because they have main-sequence lifetimes shorter than the time it took for intelligent life to develop on Earth—some 3.8 to 4.0 billion years. If that period is typical of the time needed to evolve higher life-forms, then a massive star would become a giant or even explode as a supernova before self-aware creatures could evolve on any of its planets.

Although low-mass stars have much longer lifetimes, they are less well-suited than stars in the range of 1 M_⊙ for supporting life on their planets because they are so cool. As noted earlier, only planets very near a low-mass star would be sufficiently warm for water to be a liquid. But, as also mentioned earlier, a planet that close would become tidally coupled to its star and develop synchronous rotation. One side would have continuous daylight, leading to the evaporation of oceans, while the other side would be in perpetual, frigid darkness. The only place that life could survive on such planets is in the narrow ring at the boundary between day and night. Such a small fraction of the land available greatly reduces the likelihood that life would be able to evolve to the complexity necessary for technological civilizations to develop.

As “ideal” life-supporting stars, this leaves main-sequence stars with masses near those of the Sun. These stars are of spectral types between F5 and M0. Based on statistical studies of star formation in the Milky Way, astronomers calculate that roughly one of these Sunlike stars forms in the Galaxy each year, yielding a value of R* = 1 per year.

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We learned in Sections 4-2 through 4-7 that the planets in our solar system formed in conjunction with the birth of the Sun. We have also seen evidence that similar processes of planetary formation may be commonplace around isolated stars. Many astronomers, therefore, assign f_p a value of 1, meaning they believe it is likely that most Sunlike stars have planets.

The chances that a planetary system has an Earthlike world are not yet known, although on-going observations of exoplanets should soon reveal this number. Let us hypothesize that one in five solar-type stars is orbited by a habitable planet, making n_e = 0.2.

Unfortunately, the rest of the factors in the Drake equation are very hypothetical. 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 value is, of course, an area of intense interest to biologists. For the sake of argument, we might also assume that evolution naturally leads 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, if we assume they all would, f_c would also be put at 1.

The last variable, L, the longevity of technological civilization, is the most uncertain of all—it cannot be tested! Looking at our own example, we see a planet whose atmosphere and oceans are increasingly polluted, potentially destroying the food chain. When we add in how close we have come to destroying ourselves with weapons of mass destruction, it may be that we humans are among the lucky few technological civilizations to squeak through its first years. In other words, L may be as short as 100 years. Let’s be a little more optimistic and assume that on average L = 500 years. Putting all of these numbers together, we arrive at

N = 1 × 1 × 0.2 × 1 × 1 × 1 × 500 = 100

Therefore, out of the hundreds of billions of stars in the Galaxy, there may be only 100 civilizations technologically advanced enough to communicate with us. Of course, the numbers used here are just estimates. A wide range of values has been proposed for the terms in the Drake equation, and these various numbers produce vastly different estimates of N, the total number. Increasing the average lifetime of advanced civilizations significantly increases 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 tens of millions of planets inhabited by intelligent creatures. Although we do not know yet, science enables us to home in on the number.