SETI (the Search for Extra-Terrestrial Intelligence)is mankind's attempt to reach out to the rest of the Universe.  We are not certain whether or not there are other life-bearing planets, or whether there are any civilizaztions close enough to us in both space and time for us to make contact with them. 

There are those who believe that there's "no one else out there," and there are those who believe that aliens have been visiting Earth for years now.  More than likely, the truth lies between these two extremes--it may be that there is, or was, life elsewhere in the Universe, but they are too distant for contact, or too far different from us to allow communication. 

None the less, the possiblity that there is life in the Universe that is similar to us is a compelling prospect.  The Universe is a huge space, and it would be so very lonely if we were the only intelligent life to exist in it.  Space just seems to huge for it to be ours, and ours alone.  We are creatures who like to exist in a group, and this may be one of the reasons we have embarked on the SETI project. 

This project generally involves the use of radio telescopes, which we use to listen for signals from another civilization.  We're not sure exactly what the chances are that there is another civilization out there, but we have a vague idea how we might figure that out--it's called the Drake Equation:

N = R* fp ne fl fi fc L

N = The number of communicative civilizations
The number of civilizations in the Milky Way Galaxy whose radio emissions are detectable.

R* = The rate of formation of suitable stars
The rate of formation of stars with a large enough "habitable zone" and long enough lifetime to be suitable for the development of intelligent life.

fp = The fraction of those stars with planets
The fraction of Sun-like stars with planets is currently unknown, but evidence indicates that planetary systems may be common for stars like the Sun.

ne = The number of "earths" per planetary system
All stars have a habitable zone where a planet would be able to maintain a temperature that would allow liquid water. A planet in the habitable zone could have the basic conditions for life as we know it.

fl = The fraction of those planets where life develops
Although a planet orbits in the habitable zone of a suitable star, other factors are necessary for life to arise. Thus, only a fraction of suitable planets will actually develop life.

fi = The fraction life sites where intelligence develops
Life on Earth began over 3.5 billion years ago. Intelligence took a long time to develop. On other life-bearing planets it may happen faster, it may take longer, or it may not develop at all.

fc = The fraction of planets where technology develops
The fraction of planets with intelligent life that develop technological civilizations, i.e., technology that releases detectable signs of their existence into space.

L = The "Lifetime" of communicating civilizations
The length of time such civilizations release detectable signals into space.

This information about the Drake Equation was obtained from the SETI Institue website.  We have the equation, but the trick is figuring out what all of those unknown vaues are equal to.  If we could accurately determine these values, we could determine how many civilizations are probably out there to be found.  (You can visit the
SETI Institute Website to learn more about the Drake Equation.) 

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