The Drake equation is a method of estimating the number of alien civilisations (note: not alien life forms; alien civilisations) that we may be able to find, by breaking it down into components.

The Drake equation amounts to a summary of the factors affecting the likelihood that we might detect radio-communication from intelligent extraterrestrial life. The last four parameters, fl, fi, fc, and L, are not known and are very hard to estimate, with values ranging over many orders of magnitude. Therefore, the usefulness of the Drake equation is not in the solving, but rather in the contemplation of all the various concepts which scientists must incorporate when considering the question of life elsewhere, and gives the question of life elsewhere a basis for scientific analysis.

• Wikipedia, Drake equation

There is a kind of built in assumption that extraterrestrial life, under the right conditions probabilistically determined, will follow more or less the same path. This may be a fundamentally incorrect assumption, but it is another one of those things that we just don't know.

Take the jump between life going from the unicellular to the multicellular for instance. How can we be certain that there was anything fundamental that prompted this niche invention? Most life on earth is still unicellular, and it does very very well, covering almost every surface of the planet. Perhaps the evolution of multicellular organisms (on which all conceivable intelligence and civilisation relies) was no more inevitable than the evolution of an individual species - which must be near 0. In other words, even if a form of life is capable of moving to the next cutoff in the equation, it will not necessarily do so.

However, it is difficult for us to sketch generalised paths for the rise of other spacefaring civilisations with any hard scientific basis since we have a sample size of only one. As such, we can only use the data we have available and just be aware of its limitations.

Back in 1961, Frank Drake proposed a probabilistic formula to help estimate the number of active, radio-capable extraterrestrial civilizations in the Milky Way Galaxy. It goes like this:

Where:

• N is the number of civilizations in our galaxy with which we might hope to be able to communicate
• R* is the average rate of star formation in our galaxy
• fp is the fraction of those stars that have planets
• ne is the average number of planets that can potentially support life per star that has planets
• fl is the fraction of the above that actually go on to develop life at some point
• fi is the fraction of the above that actually go on to develop intelligent life
• fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
• L is the length of time such civilizations release detectable signals into space

People have plugged in a variety of values over the past 50 years — all of them purely speculative. Values for N have ranged anywhere from one (i.e. here's looking at you kid) up to the millions.

George Dvorsky, A New Equation Reveals Our Exact Odds of Finding Alien Life