For those interested in the probability of intelligent life outside the earth (at least in The Milky Way Galaxy) Frank Drake postulated an interesting method of guesstimating the number of intelligent civilizations out there, intelligence being defined as the ability to use radio signals.
There is an argument to be made, that from the probabilistic angle there is probably some form of intelligent lifeform out there, but most people tackling this question are stuck on a carbon based lifeform whose life evolves from some sort of vitamins, some even entertain the hubristic naivete of a bipedal man like creature. I am not past thinking that there ought to be something so different from life out there that it is going to be as different from life as non-life is different from life, maybe something beyond even the realm of hyperlife.This line of speculation would make for a very interesting Science Fiction plot.
See more about Drake' equation from the wiki
Drake equation - Wikipedia, the free encyclopedia
Or glimpse it's essence below, excerpts from the same wiki.
The Drake equation (sometimes called the Green Bank equation or the Green Bank Formula) is an equation used to estimate the potential number of extraterrestrial civilizations in the Milky Way galaxy. It is used in the fields of exobiology and the search for extraterrestrial intelligence (SETI). The equation was devised by Frank Drake in 1961.
he Drake equation states that:
where:
N = the number of civilizations in our galaxy with which communication might be possible;
and
R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time such civilizations release detectable signals into space.[3]
[edit]Alternative expression
The number of stars in the galaxy now, N*, is related to the star formation rate R* by
where Tg = the age of the galaxy. Assuming for simplicity that R* is constant, then and the Drake equation can be rewritten into an alternate form phrased in terms of the more easily observable value, N*.[4]
Alternative expression
The number of stars in the galaxy now, N*, is related to the star formation rate R* by
where Tg = the age of the galaxy. Assuming for simplicity that R* is constant, then and the Drake equation can be rewritten into an alternate form phrased in terms of the more easily observable value, N*.[4]
R factor
One can question why the number of civilizations should be proportional to the star formation rate, though this makes technical sense. (The product of all the terms except L tells how many new communicating civilizations are born each year. Then you multiply by the lifetime to get the expected number. For example, if an average of 0.01 new civilizations are born each year, and they each last 500 years on the average, then on the average 5 will exist at any time.) The original Drake Equation can be extended to a more realistic model, where the equation uses not the number of stars that are forming now, but those that were forming several billion years ago. The alternate formulation, in terms of the number of stars in the galaxy, is easier to explain and understand, but implicitly assumes the star formation rate is constant over the life of the galaxy.