Rollerball

Moved here from a post in the In The News forum about UFO sightings and the possiblity of life elsewhere in the Galaxy.

Carl Sagan came up with an equation to estimate the number of technical civilizations in our galaxy alone.

N = S Fp Ne Fl Fi Fc FL

N is the number of planets where technological civilizations developed at some point in time.
S is the number of stars in the Milky Way Galaxy
Fp is the fraction of stars that have planetary systems
Ne is the number of planets in a given system that are ecologically suitable for life
Fl is the fraction of otherwise suitable planets on which life actually arises
Fi is the fraction of inhabited planets on which an intelligent form of life evolves
Fc is the fraction of planets inhabited by intelligent beings on which a communicative technical civilization develops
FL is the fraction of planetary lifetime graced by a technical civilization.

To derive N we must estimate each of these quantities. We know S, the number of stars in the Milky Way Galaxy, is a few hundred billion, recent estimates place it at 400,000,000,000.

We know from observation that most stars have planets. To be conservative he estimates the fraction of stars that have planetary systems, Fp, as roughly equal to 1/3. Then the total number of planetary systems in our Galaxy would be 130,000,000,000.

If each system were to have about ten planets, as ours does, the total number of worlds in the Galaxy would be more than a trillion.
In our own solar system there are several bodies that may be suitable for life of some sort: Earth, Mars and Jupiter’s moon, Titan. Once life originates it tends to be very adaptable and tenacious. There must be many different environments suitable for life in a given planetary system. But again being conservative he chooses Ne = 2, or two planets in each solar system that are suitable for life, so the number of planets in the Galaxy suitable for life becomes 300,000,000,000.

Now we come to less certain ground, choosing the number of planets that are suitable for life where life has arisen at least once. Experiments show that under the most common cosmic conditions the molecular basis of life is readily made, the building blocks of molecules able to make copies of themselves. Sagan once again chooses to be conservative at estimating Fl = 1/3, implying a total number of planets in the Milky Way on which life has arisen at least once as 100,000,000,000.

That in itself is remarkable, but we are not finished yet.

Considering the apparent difficulty in the evolution of large organisms represented by the Cambrian explosion, Sagan chooses Fi x Fc = 1/100, meaning that only 1 percent of planets on which life arises eventually produce a technical civilization. This estimate represents the middle ground among the varying scientific opinions. Some think that the equivalent of the step from the emergence of trilobites to the domestication of fire goes like a shot, others think that even given ten or fifteen billions years the evolution of technical civilizations is unlikely. Choosing that middle ground gives us 1,000,000,000 planets on which technical civilizations have arisen at least once. But that is very different from saying that there are a billion planets on which technical civilizations now exist, for that we must also estimate FL.

What percentage of the lifetime of a planet is marked by a technical civilization? The Earth has harbored a technical civilization, (which Sagan characterizes by the development of radio astronomy), for only a few decades out of a few billion years. So far for our planet’s FL is less than a millionth of a percent. And it is hardly out of the question that we might destroy ourselves within the next few hundred years. If civilizations tend to destroy themselves soon after reaching a technological phase there might be no one for us to talk to but ourselves. Civilizations would take billions of years of tortuous evolution to arise, and then snuff themselves out in an instant of unforgivable neglect.

But consider the alternative, the prospect that at least some civilizations learn to live with high technology; such societies might live to a prosperous old age, their lifetimes measured on geological or stellar evolutionary time scales. If only 1 percent of civilizations can survive technological adolescence and achieve maturity, then the number of civilizations in the Galaxy is in the millions. And that’s only talking about the Milky Way Galaxy, there are hundreds of billions of other Galaxies like ours.

If there are millions of civilizations distributed more or less randomly through our Galaxy, the distance to the nearest is about two hundred light years. Even at the speed of light it would take two centuries for a radio message to get from there to hear. Traveling that far for a visit is another matter.