fp = the fraction of suitable stars that have planets
In 1961 astronomers had not yet discovered a single planet orbiting a star other than the sun. Nonetheless, the Green Bank conferees assumed on the basis of theory that planets were as common as flies and pegged the fraction of planet-endowed stars at about one-half. This was a Quasimodo-size hunch.
Today we can do better. Since 1995, several teams have found more than 170 worlds around other stars. About 5 to 10 percent of the studied stars have planets, but that's a lower limit: Current instruments cannot find bodies much smaller than Neptune, and they tend to miss ones in slow orbits. Meanwhile, planets keep showing up in places scientists once considered unlikely, such as around double stars. According to planet hunter Geoff Marcy of the University of California at Berkeley, "the fraction of stars having planets is probably as high as 90 percent."
ne = the number of habitable planets around each star
In days gone by, scientists would speak solemnly about our solar system's "habitable zone"—a theoretical region extending from Venus to Mars, but perhaps not encompassing either, where a planet would be the right temperature to have liquid water on its surface. Our solar system has exactly one planet (Earth) that makes the cut, leading people at the time to estimate ne as 1.
A major surprise of the past few decades has been the creeping realization that life is tougher than that, and several worlds in our solar system are gentler than once believed. Certain extremophiles (microbes that tolerate harsh conditions) have been found in seawater well above the boiling point, in polar ice, and even deep underground. Some of these microscopic tough guys do not need oxygen to survive and could do just fine in subterranean aquifers on Mars, in the seas that might slosh beneath the frozen surfaces of three of Jupiter's moons, or under the hydrocarbon-coated landscape of Saturn's moon Titan, where a trickle of heat from below might maintain pools of liquid water.
The old habitable zone idea is far too restrictive. The latest thinking says that each planetary system might include two, three, or more worlds that could support life.
fl = the fraction of habitable planets that have life
As we move to the right in Drake's equation, the values of the terms become increasingly uncertain. We haven't yet found a speck of evidence for biology on another world, so we have no objective way to judge whether life is a onetime fluke or a near-inevitable phenomenon.
More and more, the evidence points toward the latter. The basic ingredients for terrestrial life—a couple dozen simple molecules, including water—are now known to be abundant in the galactic clouds where stars are born. Recent experiments show that carbon dust grains, the kind that condense into planets, foster the creation of amino acids, the building blocks of proteins. Just about any planet anywhere would be pummeled and dusted with life's ingredients as it forms.
Drake's group conservatively estimated fl at 10 percent, a value that still seems reasonable. Obviously, that number would leap much higher if we found something as simple as alien pond scum (or its remains) on Mars or another nearby world.
fi = the fraction of inhabited worlds that develop intelligent life
Stephen Jay Gould famously described humans as an evolutionary accident and argued that had Earth's history been slightly different, we would never have appeared. But if we weren't here, would evolution eventually have produced some other species of thinking beings instead? In other words, is intelligence sufficiently useful that it will sooner or later appear?
One way to answer the question is to see if other terrestrial species have tended toward intelligence. Studies of fossil dolphin skulls show an inclination toward greater brainpower over the last 50 million years. Numerous other creatures, especially some whales and birds, also show a pattern of increasing brain size relative to their bodies over the past tens of millions of years. Four decades ago, Drake and company guessed that fi must be close to 1. Today there's evidence that the path of evolution may often lead to the development of intelligence.
fc = fraction of intelligent species that develop radio technology
Just because you are smart, does that mean you will end up broadcasting your news and views into space? The answer depends as much on sociology as on evolutionary biology and is as speculative today as it was in 1961. Very few societies on Earth developed science as we know it today. On the other hand, the number is not zero—the Greeks, the Chinese, and the Maya did, among others. Once invented, science proved so useful that it spread like mold on a petri dish.
It seems obvious that if a species has the brainpower for speech, along with the sort of appendages that can manipulate a pair of pliers, it will eventually blunder into science, technology, and radio. Some societies may get stuck using nothing more than the wheel or the abacus, but it takes only one scientist to figure out how electricity and magnetism work, as James Clerk Maxwell did in 1864. Within a generation, hordes of tinkerers were fiddling with crude radio sets; two generations after that, Drake had access to a radio dish that could pick up signals from light-years away. So fc was, and still is, assumed to be close to 100 percent.
L = the average time span during which civilizations transmit detectable signals
Even if technological societies are born all the time, we're never going to find them if they quickly destroy themselves. During the cold war, many researchers worried that the lifetime of a technological civilization might be distressingly short, maybe a couple of centuries or less. The level of expertise needed to build a radio is just a little less than that required to build a nuclear weapon, they argued, so just as a civilization starts tuning up its transmitters, some internal spat causes it to go off the air in a storm of mushroom clouds.
In the last decade this dystopian attitude has eased somewhat, replaced by a different concern. Television transmitters are quickly giving way to fiber-optic cables and direct-broadcast satellites. Earth could soon become radio quiet—a technological evolution that might occur on an alien's world too.
It sounds as if L is destined to be short, no matter what: Either technically adept societies self-destruct, or they go mute. But this sobering view is based on human experience of only about 60 years. Things could change, and change drastically. When a technological civilization develops rockets, the colonization of nearby space will most likely follow. Perhaps aliens will build a solar-system-wide GPS that belches radio signals willy-nilly. Maybe they broadcast an asteroid weather report. At any rate, it's reckless to assume they won't have powerful transmitters.
Drake set the value of L at 10,000 years. Others have made estimates both drastically shorter and longer, but this is the one term of the equation where our guesses are truly guesses.
N: And the answer is . . .
When Frank Drake and his compatriots plugged their best guesses into the equation, they came up with an answer in the thousands—meaning that intelligent life is common enough that there should be a civilization within about 1,000 light-years of Earth.
The distinguished dozen in 1961 were both prescient and plain lucky. With all the additional information available today, Drake's estimate remains convincing. In fact, as Drake says, "many of our assumptions of those days have become established facts."
Things might easily have turned out otherwise. Astronomers could have discovered that planets are scarce or that potential habitats are rare. Instead, our growing understanding of the universe justifies Drake's optimism and occasionally goes it one better. That's why, despite 45 years of null results, many of us are more convinced than ever that we are not alone.