The Search For ET, What Does The Drake Equation Say?

Charles Drake is a long time proponent for scientific research into the existence of extraterrestrial life. His inspirational work has certainly helped create the intellectual environment necessary to create the Search For Extraterrestrial Intelligence (SETI).

It was in 1960 when Charles Drake brought forth his famous equation designed to predict how many civilizations might exist in our Milky Way galaxy. It's a long equation, if you haven't seen it, multiplying a long sequence of probabilities in an effort to make a prediction. Included in the long list is

The average rate of star formation in our galaxy

The fraction of those that have planets

The fraction of those planets that can harbor life

The fraction of those that do produce life

The fraction of those that produce intelligent life

The fraction of those that produce technological civilizations that release detectable signals into space

The length of time that those civilizations release detectable signals into space

That's a lot of probabilities to multiply in order to get a final answer. Certainly in the 1960s the necessary values of many of those probabilities were either poorly known or not known at all. So the predictions of the number of concurrent technical civilizations varied from none to many thousands, depending upon the guesses of factors entered into the equation.

As astronomical research began to fill in some of these unknowns, a reasonable estimate seemed to be that the number of technical civilizations in the galaxy was about the same as the number of years an averate technical civilization existed. We, as a technical civilization, have been broadcasting radio signals into space for a little over a hundred years. So we might estimate that the number of civilizations in the galaxy is at least a hundred or so.

Some considerations about the probability of extraterrestrial life create optimism. Carl Sagan noted that primitive life started on our planet very early in earth's formation -- withing a few hundred million years. More recently, earth life forms have been found living in the harshest environments. In fact, a new term has been coined for these hearty life forms -- extremophiles.

Current theory is that earth-like life seems primarily to need a bit of energy and liquid water in order to get started. The mission has become: follow the water. That's what's driving the renewed interest in Mars exploration efforts. And it appears that in the past, perhaps as recent as a few millions or tens of millions of years ago, Mars had copious amounts of water.

In addition, Europa, a moon of Jupiter, is likely to be a large ice ball with a vast ocean beneath the ice. Gravitational stresses on Europa from its close proximity to Jupiter could provide much of the energy needed by primitive life forms. Even Ganymede and Callisto are suspected of having subterranean water.

It's also been confirmed that planet formation around stars is anything but rare. In fact, planet formation seems to be something that commonly happens around stars. So there are planets -- lots of them. And if our solar system is any indication, there's water on a very reasonable percentage of the rocky planets and large moons. So the ingredients for at least primitive live seem to exist throughout the galaxy.

However, there's a few pessimistic factors that also come into play. One is called the Fermi paradox, named after the great nuclear physicist Enrico Fermi. When discussions of alien life came up, and excitement about the possible numbers entered the conversation, he asked "If they are out there, where are they?" Meaning, why haven't they found us?

While we may not yet have the capability of detecting technological civilizations, surely some galaxy civilizations are much more advanced than ours, and should be able to detect us. So -- where are they?

During the cold war there developed a pessimism about the possibility of large numbers of technical civilizations because we, the one technical civilization we know of, seemed about to do ourselves in. It was a growing concern that technical civilizations don't last long because their social and political inertial lags so far behind their technical development. The worry was (and perhaps still is) that they eventually blow themselves up.

With that concern having waned a bit with the breakdown of the cold war, we now see another self imposed threat. We seem to be headed for a global warming catastrophe. Again there is sign for pessimism.

So here we are, still without an answer. We know that life seems to be able to exist in places we'd have previously assumed were too hostile, leaving hope that places like Mars, Europa, and perhaps even Titan might harbor primitive life. We know that on earth, even though early conditions were not very conducive to life, it began anyway. We know that planets do indeed seem to be around a large percentage of stars.

Yet the SETI program has so far found nothing. As Fermi asked: "Where are they?"