Life in our Galaxy: How Many Other Intelligent Species Are There?

     It seems pretty unlikely that there is other life in the galaxy from our perspective right now. As a human race, we have been communicating radio waves for 78 years giving extra-terrestrials a chance to respond to our inadvertent hails. We have projects like SETI that have been actively searching for and attempting to communicate with potential visitors of the Milky Way. We even have powerful telescopes that have accurately mapped thousands of nearby stars and extrasolar planets with no traces of life, intelligent or otherwise. But what if I told you right now that there is almost certainly at least 126 advanced, technological civilizations that have the willingness and ability to communicate with Earth right now in our own stellar neighborhood, the Milky Way? It would probably sound a lot like science fiction. Maybe so, but it is a real possibility. Let me present you to the famed Drake Equation:

     This equation is literally the key to telling us exactly how many alien civilizations are out there right now, alive and well, and able to communicate. So, what's the problem? why haven't we found extra-terrestrial life in our galaxy if we know where to look? The truth is, we don't. This is because four of these variables, f_l, f_i, f_c, and L, we simply don't know the values to. If the Drake equation is broken down into its variables, we find that:

N = The Number of communicating civilizations in our galaxy right now.
R = The Rate at which stars are formed each year.
f_p = The Fraction of those stars which have Planets.
n_e = The Number of Earth like planets that can potentially support life.
f_l = The Fraction of these planets which actually forms Life.
f_i = The Fraction of these planets with life that develops Intelligent life.
f_c = The Fraction of these civilizations that develop a means to Communicate effectively.
L = The Length of time that these intelligent species communicate for.

     Many of these variables are very clear, and have been accurately measured and tested. R, for example, has been calculated and known for many years. NASA has clocked the average star birth rate to be approximately 7 stars per year based on patches of radioactive aluminum throughout our galaxy. so, for our equation, R = 7. Now, with the launch of the Kepler space telescope, f_p has also been determined relatively accurately over the past decade or so. Of the 153,000 stars that were accurately measured for planets using multiple means, they discovered that approximately 34% of stars have some form of planets around them. However, this does not even account for the number of planets that were missed or not observed long enough to discover a planet. Recent studies have now estimated f_p = .80, meaning about 80% of stars form planets. But of these planets, how many are in the "Goldilocks" zone for the potential formation of life or habitability? Studies show that approximately 22% of stars have Earth-sized planets within the habitable zone. Out of the 34% that had planets, this means that about 65% of the planets are habitable. This percentage seems to be quite accurate closer to home. Even in our own solar system, there are 2 or 3 roughly Earth-sized planets within our habitable zone (depending of whether or not you count Venus or Mars), and only one of them supports life. So, averaging the 65% with the low estimate of 33% in our solar system, then multiplying by the 22% of stars with planets in the habitable zone, we can equate n_e = .11 or about 11%.

Goldilocks Zones of Sun-Like and Other Stars

     Alright, about half way done. Now here's the real problem. The last four variables in the Drake equation are impossible to determine precisely with our current knowledge of the galaxy. Since we have never discovered another example of life in our own Milky Way, the estimation range of these last four numbers is quite vast. So now we must start using ranges. Of the Earth-sized planets in habitable zones of their sun, how many of these will develop life? According to some, the answer to this is 100%, meaning if a planet has the potential to support life, it will develop life. This is not an unreasonable suggestion. Geological records of the Earth show that once our home planet developed conditions stable enough for life, it developed rather quickly. However, we do not know if our creation was a chance accident or a common occurrence. This number could literally be anywhere between 0% and 100%. Averaging these two will assign a probability of approximately f_l = .5, or 50% of planets will develop life at some point after they become habitable. Once life is created, what are the chances that this life becomes intelligent? So, lets step back a moment and take a look at what humanity had to overcome to become an intelligent community. Our journey from a puddle of primordial goo to walking, talking, bipedal beings had a few speed bumps along the way. We endured billions of years of evolution, a total of 5 mass extinctions, and factors that we can't even begin to imagine! However, if humanity is indeed "average", then over our 3 billion years of evolution on planet Earth, there were 5 chances that could have extinct life on this planet. This puts the value of f_i = .2, at around 20%. This is assuming, of course, that if life exists on a favorable planet, that it will "strive" to become intelligent on its own. This is, however, still reasonable due to the effects of evolution that can be seen across the globe.

Some Extrasolar Planets Discovered by Kepler Project that may Host Life

     So now we have an intelligent species that has overcome all odds on an Earth-sized planet around a star in its Goldilocks zone. Will they develop a means to communicate. If they are truly intelligent, then yes, of course they will. Think about it. There were civilizations on opposite ends of the globe, like the Mayans and the Egyptians, that both developed independent, effective languages without having any prior knowledge of each other. An intelligent species strives to share their intelligence with others. Assuming that in the 10,000 years from intelligence to when they develop some means of interplanetary communicative means, like radio waves, they don't kill themselves or are wiped out by a natural disaster (the latter of which is unlikely due to the fact that an average mass extinction only occurs once every 700 million years), the civilization will communicate. This puts the value of f_c = .9, allowing for a 10% chance that they will either never develop communications or they will be wiped out somehow. Good. So the final variable remains; how long will the civilization communicate for before they either are wiped out by natural disaster, destroy themselves, or move on to a more advanced form of communication that we cannot detect? Once the civilization has become intelligent, they will almost immediately become space-faring and colonize and terraform many other worlds. This makes destruction by means of natural disaster unlikely. What is more likely is that they will destroy themselves.

     Since we have developed radio communication in 1936 (and yes, it was a radio broadcast by Hitler), we have sustained 2 world wars, a cold war, and multiple terrorist groups that threaten to rid of all others who don't agree with their beliefs. The first world war was mostly waged in Europe and had little chance of wiping out the entire human population, but with the invention of the atom bomb in the 1940's, this threat is all too real. There have been two circumstances in the last 70 years where the world almost destroyed itself with nuclear warfare, the first of these being World War II in 1945, and the second being the Cold War nuclear scare of 1983. Both of these events could have ended humanity, making our radio transmissions cut short at a mere 9 or 37 years. however, it has been 31 years since the nuclear scare in 1983 and no such event has occurred since then this means that tensions of humanity (though far from gone) are generally decreasing as time moves onward. I will give a civilization a 60% chance of surviving their own destruction, leaving us with just one final element; how long will this civilization communicate for with simple radio waves? History has shown that technology advances exponentially. However, we are still using radio waves after 78 years. I would estimate that we will sacrifice this open source of communication to a more specific, private form of communication like lasers or some form of telepathic enhancement within the next 500 years. averaging that with the civilizations that kill themselves within 20 years, we can calculate 20*(.4)+500*(.6) 300 to give us an approximate value of 300 for L. So, lets put it together:

R = 7 stars per year.
f_p = .8 ratio of stars with planets.
n_e = .11 ratio of these planets which are habitable.
f_l = .5 ratio of these habitable planets which develops life.
f_i = .2 ratio of these life forms that become intelligent.
f_c = .9 ratio of these intelligent life forms that will develop effective communication.
L = 300 years of active, effective communication.

N = (7)*(.8)*(.11)*(.5)*(.2)*(.9)*(300) = 16.63

     So what exactly does this mean? There are currently about 17 civilizations in our Milky Way galaxy right now that are intelligent and willing and able to communicate. This, however, does not mean that there are only 17 intelligent civilizations within the milky way. This is only the number of them that we are likely to find and communicate with in the next 200 years or so. If we assume that species are more friendly at communicating, the value of L may shoot up to however long that civilization has been alive for, which, in a 13 billion year old galaxy, may be close to 10 billion years. The chances that an alien civilization that advanced would ever have the motive to talk to us would be astronomically small, however, as they have probably already discovered millions of other civilizations just like us in galaxies all across the universe. So the true question for "L" is how long will a civilization be interested in contacting other life. Some studies report that it is humanity's drive to discover other life in the galaxy. But if there really is 30,000 civilizations in the galaxy right now, that would get pretty boring after a while. life would no longer be unique. so how many civilizations would it take for an intelligent species to consider life "common"? Lets say a civilization discovers the nearest 15 civilizations before either becoming bored with communicating with lesser beings or conjures laws against tampering with the development of these civilizations.Well, the galaxy has approximately 5,500,000,000,000 (5.5 trillion) cubic light years of area and about 200,000,000,000 (200 billion) stars. This means that there is an average of 27.5 cubic light years per star, or, if you prefer, a stars nearest average neighbor is about 1.87 light years away. This means our civilization will have communicated with every other civilization within 65450 cubic light years, or about 25 light years in all directions. Taking into account that it takes them about 50 years to go from a communication era to a space era, then another 100 years from space era to interplanetary era, then another 150 years from an interplanetary to an interstellar era, then another 200 years to explore all of their nearby star systems before becoming bored (with the speed of light as a barrier), the value of L can now be expanded to L = 500, then:

N = (7)*(.8)*(.11)*(.5)*(.2)*(.9)*(500) = 27.72

     This is almost 28 potential intelligent life forms, willing to communicate just within our galaxy alone right now! But this still doesn't answer the question as to how many intelligent species have lived in our galaxy ever. To find this, we simply eliminate the first and last factors of the Drake equation to find out what percentage of stars in the milky way have had an intelligent species living on a habitable planet at one point in time.

N% =  (.8)*(.11)*(.5)*(.2)*(.9) = .0079 or 0.79%

Multiplied by the number of stars in our galaxy:

N = (.0079)*(200,000,000,000) = 1,580,000,000 life forms

     1.58 billion intelligent life forms have existed in the galaxy at one point or another. Since that number is approximately the length of time in years that life could have been erupting in our galaxy, we can assume that these civilizations would have spawned proportionally with time. This means that every six years, a new intelligent species is born in our galaxy on average! This leaves us with about 80 alien civilizations within just 500 years of our own technology that may be willing to communicate with us! Now, since the galaxy is so vast, the closest of these civilizations would be on average 1800 light years away (as can be seen from the graph above), but still, there is a chance! So, in conclusion there is approximately:

• 17 intelligent civilizations in our galaxy that are not too much more advanced and are within the technological realm of communicating with us.
• 28 intelligent civilizations in our galaxy that may or may not be more advanced than us, but still are willing to communicate with us.
• 1.58 billion intelligent species that have ever existed in the Milky Way.
• A new intelligent species is born every 10 years on average.
• About 80 intelligent civilizations in our galaxy right now that are within 500 years of our technology.
• The nearest intelligent civilization that is willing to communicate with us is most likely over 1800 light years away from us.

     And what about humanity? Where do we fit into the Drake equation? Well, if we consider ourselves an intelligent life form, we just entered into the vast network of communicating aliens just 78 years ago, and we are still alive. This means we know for a fact that there is no willing or able species within 39 light years that will communicate with us (given that they have to respond to our radio waves). Given time and technology advances in the coming years, there is a high chance that we will discover extra-terrestrial life within the next 1000 years. It is only a matter of time.