The possibility of intelligent life elsewhere in the universe has intrigued humans since they first became aware of the vastness of space. Sadly, for science fiction romantics, it is unlikely we will ever have direct evidence we are not alone.
Surely advanced life exists somewhere. The number of places life might arise is unfathomable. There are at least 200 billion stars in our Milky Way Galaxy. Among these stars there are probably 50 billion planets, one percent of which are in what is called the habitable zone of their star.
Furthermore, there are well over 100 billion galaxies in the universe. Our sister galaxy, Andromeda, has more than a trillion stars.
The question, however, is whether we are likely to find evidence for intelligent life. Even if it exists, the distances are so great any search we conduct can only sample an infinitesimally small fraction of the universe.
Our galaxy is on the order of 100,000 light-years across. Had a signal been sent from the other side of the galaxy when modern humans were first beginning to populate Asia and Europe from their African homeland, it would only now be detectable.
On Earth, radio communications began roughly 100 years ago. For us to have heard back from another intelligent being, our earliest transmissions could only have traveled a distance of 50 light-years. That encompasses less than 2,000 stars; not very encouraging.
Meaningful dialogue is impossible. The nearest star, Alpha Centauri, is just beyond four light-years. Suppose intelligent beings exist there and you were to transmit a na?Øve question, "Do you speak English?" You will wait over eight years to hear the answer, "No."
Narrowing the discussion to the Milky Way Galaxy, Frank Drake proposed an equation in 1960 to facilitate thinking about such matters. To arrive at the number of civilizations in our galaxy with whom communications might be possible one must successively multiply a number of terms. These terms represent the fraction of instances where certain relevant conditions are met.
Start by multiplying the number of stars currently in the galaxy (call this A) by the fraction of those stars having planets (B). Multiply that by the fraction of those planets that also have conditions that could support life(C).
Continuing on, multiply the result by the fraction of those that actually do develop life (D), followed by the fraction where life develops from primitive to intelligent (E). Multiply that by the fraction that develop civilizations capable of transmitting detectable signals (F). Finally, multiply by the fraction of time since the galaxy formed that signals have been transmitted (G).
Any one of these factors can be expanded to include pertinent sub-conditions. Let's examine a few in more detail.
The concept of habitable zones, where conditions for life exist within a stellar system (condition C), must take into account some formidable hurdles. It must also address the question of what qualifies as life.
Almost certainly, all life forms are constructed of hydrocarbon molecules. The chemistry of anything else comes up short. The properties of liquid water must be considered as essential for life to get very far at all.
So, a candidate planet must have the atomic elements required by living things and a liquid water medium to facilitate the chemistry of life. Furthermore, temperatures must be relatively stable, allowing liquid water to have persisted for very long times.
Consequently, factor C is a product of the fraction of planets having the elements of life (call this c1), fraction having water (c2), and fraction having appropriate temperature ranges (c3).
Again, each of these terms can be broken down. The elemental composition of planets is determined by the history and properties of their star and neighborhood of stars. Planetary temperature is dependent upon such things as distance from the star, energetics (brightness) of the star and planetary atmospherics.
Not surprisingly, getting from the simplest life forms to intelligent life (condition E) has its own list of requirements.
If Earth was routinely pelted by extraterrestrial objects like asteroids or comets, development of complex life would be next to impossible. In its early history these events were routine. In fact, comets are thought to have been significant contributors of water to the Earth's oceans.
As complex life developed, such violent events caused massive extinctions. These include several large meteor strikes, one of which caused the extinction of large dinosaurs 65 million years ago.
Such catastrophic occurrences have diminished as the solar system aged. The large gaseous planets beyond Earth orbit tend to sweep out such nasty projectiles. Without our massive companion planets, conditions we find on Earth may never have developed.
Earth has a host of other properties making an existence like ours possible. From things as simple as its size (mass), to plate tectonics, composition of its atmosphere, tilt of its axis, and oceanic and atmospheric circulations, many factors play pivotal roles in nurturing and sustaining life.
Intelligent life is undoubtedly exceedingly rare. Civilizations have existed for less than 10,000 of the nearly 5 billion year history of Earth. One additional factor needing to be added to Drake's equation is the fraction of advanced civilizations that manage not to destroy themselves.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.