Published Date Written by Edward Gleason
The "Alien Equation," more properly called the Drake Equation is described as the perfect blend of mathematics and dart throwing. Of all the myriad equations one encounters in astronomy, it is the one most reliant on pure guesswork. Developed by SETI founder Frank Drake, the Drake Equation's aim is to estimate how many communicative alien civilizations are lurking out there in our home galaxy. A 'communicative alien civilization' is one capable of transmitting information about itself to others through electromagnetic radiation or other means.
We humans have been a communicative race ever since Tesla and Marconi managed to manipulate radio waves in the latter 19th century. By galactic time scales, we've been announcing ourselves for a couple micro seconds. Of course, we don't know if we're the very first civilization in the galaxy capable of such transmission, or merely the most recent arrival to the Milky Way's vast communication network.
The Drake Equation tries to answer this question with a combination of stellar astrophysics and simple speculation. The equation states that the number of communicative civilizations is estimable when one multiples the rate of suitable star formation, the fraction of the aforementioned stars that have planets, the percentage of those planets suitable for life, the actual number of suitable worlds actually harboring life; the fraction of those life-bearing worlds that produce intelligent life and civilizations; the number of those civilizations that transmit "detectable" transmissions into space and the duration of time during which they emit these signals.
Ladies and gentlemen, most of those variables are currently unknown. We have to rely on our instincts, which are not always trustworthy, and our knowledge of ourselves, which is quite limiting, to address most factors within this equation. Whatever number the final estimation yields will be as conjectural as the values we selected for our calculations. However, we need less guesswork for the first two factors, the star formation rate and the percentage of stars harboring planets.
Astronomers believe that three "'solar masses" of stars form each year in the Milky Way Galaxy. A solar mass equals the mass of our Sun. This value, "three solar masses," refers to the combined sum of all stars born. That could equal three stars as massive as the Sun, or a greater number of less massive stars or even a more massive star. One should be quite careful with this statistic as it is an average. A year is negligible on astronomical time scales and star formation requires millions of those years. Moreover, we need to have "suitable" stars, those with lifespans long enough to allow life to develop.
We know that Earth needed every minute of four billion years to go through the painstakingly slow process of cooking simple molecules into complex life forms. The most massive stars are fleeting, with life cycles on the order of tens of millions of years: hardly enough time for a planet to cool, let alone labor through the biochemical processes life's development necessitates. Fortunately, most stars exist for billions of years (red dwarfs, the most common stars, live for a couple trillion years.).
The second factor, pertaining to planets, was all guesswork when Frank Drake introduced his equation in 1961. The only planets then known to us were those within our own solar system. Now, in 2013, astronomers have cataloged almost one thousand exo-planets, planets in orbit around other star systems. These outer worlds represent a small sampling of the myriad planets scattered around the Milky Way.
During the first decades after Drake developed this formula, humans didn't know if the galaxy contained only a smattering of planets or a proliferation. We're now confident that planets abound in the Milky Way, with a population perhaps exceeding that of the stars, themselves. Even amongst these 1000 worlds, astronomers have found a few "Earth-like" planets, those that are "rocky," not highly massive, and located within a narrow region where temperatures are neither excessively high (Mercury) nor low (Mars). Scientists have not detected life on these "super-Earths," of course, and they don't expect to find any. Simply because the worlds "could" be life-bearing doesn't mean they are.
One might think that this information would lend insight into the next factor: the percentage of other worlds that could harbor life. Based on present findings, one could be tempted to state that ratio is perhaps four out of a thousand, more or less. The problem is that our sample is far too small to allow for such conclusions.
Secondly, giant planets, those with masses equal to or exceeding that of Jupiter, are disproportionately represented. Astronomers have been finding exo-planets since 1992 and during most of the ensuing two decades, they employed detection methods capable of finding only the largest planets. Only recently, as with the launch of the Kepler spacecraft in 2009, have astronomers developed techniques enabling them to locate smaller, Earth-like worlds. The big boys take up most of the room in our exo-planet catalog, at least for now. We'll need many more years to ascertain a more accurate small world-big world ratio.
The remaining factors are anyone's guess. We know that life thrives on this planet and that a certain species developed a radio-transmitting civilization. This gives us a 1-1 correlation — life did produce a civilization in one instance. However, that is the only instance on record. How are we to know how long life persists on other worlds? Perhaps it starts, evolves, and then some catastrophe annihilates all life forms.
Our planet has sustained many powerful assaults throughout its history, not just the one 65 million years ago. The geological record is punctuated by mass extinctions. Maybe we're fortunate in that some life forms have always survived these widespread wipe-outs. Other planets might not be so lucky. As we have only one data point — Earth — we cannot make any conclusions at all about life's progression or resiliency on other worlds.
We know that the galaxy contains hundreds of billions of stars; we also suspect that the galaxy also harbors billions of planets. These two values -the first certain, the other inferred- bode well for humanity's search for extra terrestrials. Perhaps only by establishing contact with such aliens will we be in a better position to determine if life is rare in the galaxy or is commonplace.
(Edward Gleason is an astronomer and manager since 1999 of the Southworth Planetarium in Portland. He also was employed at the Maynard F. Jordan Planetarium in Orono. Gleason writes the daily e-mail article, “The Daily Astronomer.” Visit http://usm.maine.edu/planet for more about the planetarium.)