Does life exist outside of our solar system? What are the odds that it does?
These two questions are at the heart of an equation that tries to bring some perspective about the likelihood that intelligent life exists elsewhere in the Universe.
The Drake Equation
In 1961, Frank Drake developed the Drake equation to focus on the factors, which determine how many intelligent, communicating civilizations there are in our galaxy. It is a formula for determining the number of intelligent and communicating civilizations exist. Remember, even if there is life on other planets, and even if there is intelligent life, the intelligent life must be sophisticated enough to create the conditions for communication. If not, we would not know that it exists. So, keep that in mind when you investigate the Drake Equation. Even though it is in math form, the equation is easy to understand. It goes from a general to specific formulation of likely civilizations to exist.
The Formula
N = R* fp ne fl fi fc L
Here is what the symbols stand for.
N = The number of communicative civilizations found in the Universe R* = The rate of formation of suitable stars (stars such as our Sun) fp = The fraction of those stars with planets. (Current evidence indicates that planetary systems may be common for stars like the Sun.) ne = The number of Earth-like worlds per planetary system fl = The fraction of those Earth-like planets where life actually develops fi = The fraction of life sites where intelligence develops fc = The fraction of communicative planets (those on which electromagnetic communications technology develops) L = The “lifetime” of communicating civilizations
Two things are important here. The numbers are guestimates at best, and they reflect the knowledge of some astronomers and physicists. But over time, they can be changed and modified.
The equation shows a combination of factors that are necessary to have a communicating civilization; one where you know that they exist.
The equation states that the number of communicating civilizations in our galaxy depends on several factors that must combine to yield a habitable planet where life has the chance to develop to a certain level of technological know-how. These factors include the rate of formation of stars like the Sun, followed by the fraction of those stars with planets. Next, what is the likelihood that those planets are Earth-like? So you are looking at the fraction of Earth-like planets that exist. However, just because there is an Earth-like planet, it does not mean that life will automatically develop, so there must be some fraction of such planets where life does indeed develop. Then the true question becomes how many of those planets with life actually develop intelligent life? There will just be a fraction of the life becomes intelligent, even if eventually intelligent life will develop far into the future. But if it is not present now, that does not count.
Then once the fraction of intelligent species develops, how many can communicate in a way we would detect? How long will it actually take to communicate with those civilizations? If they are close by, maybe it will take between five and 10 years. If not it may be more than a hundred.
Example:
N = R* fp ne fl fi fc L
R=2.00; f=.45, ne=.50,fl=.20,fi=.05,fc=.50,L=500
Then N= 1.125 civilizations
What the Numeric Values Mean
To understand how to derive N, look at what the individual values represent.
R = 2: this is double the estimated rate of formation of Sun-like stars; the current maximum estimate is 20 new stars per year in the galaxy. (This may be low.)
fp = 0.45 (45 percent) represents 1/2 of the high estimate of 90 percent of these stars that have planets.
ne = 0.50 (50 perent) represents an educated guess which is that for every two stars with planets, there will be one habitable planet.
fl = 0.2 (20 percent) another educated guess representing the notion that life will emerge on only one in five habitable planets.
fi = 0.05 (5 percent) another estimate which is that intelligent life will develop on only five out of every 100 planets with life.
fc = 0.5 (50 percent) if there is intelligent life, there is at least a 50-50 chance that they will develop the technology necessary for interstellar communication. (Yes, it will develop or no, it will not.)
L = 500 represents the number of years for which an intelligent civilization will be broadcasting its presence through radio transmissions.
Fluctuations and Assumptions
These numbers can and have changed over time as more astronomical observations occur. The significant feature is that astronomers are now looking for evidence of exoplanets that are in the habitable zone. Once that number starts to become available, the values for the percents can change.
The most important assumption is that intelligent life will look like ours and have a carbon base. The equation variables have this built in. For instance, stars with habitable life must have a similar mass structure to the Sun, and life must form in a planet within a habitable zone. However, if the alternate life form is not carbon based, who knows what the intelligent life will look like?
At this time though, it does not look very promising that there will be intelligent life out there that we will be able to communicate with, at least not nearby. Yet, at the same time the work is not complete and the equation shows how much work must still be done to get a realistic and viable answer.