10

According to astrophysicist Frank Drake, there should be at least 10 intelligent life forms in our galaxy alone.¹ What is most interesting about this number is the longer humans continue to live in an advanced society the number of possible life forms will continue to increase proportionately.

The number is derived from Drake's Equation which to some scientists is the second most famous equation in science. The equation with its variable definitions and approximations is as follows²: N = R_*f_pn_ef_lf_if_cL

* R_*: rate of star formation in the Galaxy--10/year
* f_p: fraction of stars with planets--0.5
* n_e: number of planets in each system that could potentially sustain life--2
* f_l: fraction of planets life creation occurs on--1
* f_i: fraction of planets life becomes intelligent
* f_c: fraction of intelligent life that evolves to produce advanced technology--this combined with the above is 0.01
* L: the lifetime of such advanced civilizations

These assumptions are based on the observations we make of our own planet, civilization, and the universe that we can observe. The derivation of this equation came in 1961 at what is considered to be the founding meeting on SETI (search for extraterrestrial life) in the United States.³ Since that meeting, where Frank Drake presented his equation to about a dozen people, scientific exploration of the universe has made near astronomical leaps. As a species we have sent countless satellites, telescopes, rockets, humans, and even a dog into the cosmos.⁴ Researchers have spent hours upon hours pouring over star emission spectra and telescope imagery. Much of this has been in the search for intelligent life or systems which might carry the right balance of ingredients to do so.

However, with all the time and advancement in the study of our universe, many scientists agree Drake's equation can be simplified down to the same form the researchers at the Green Bank Observatory agreed upon in 1961: N = L/10. Qualitatively, the number of potential intelligent life forms boils down to two questions, where are all the aliens and how long do advanced societies last?

These questions are encapsulated best by the Fermi Paradox, the inspiration for Drake's Equation. Enrico Fermi developed the following thought chain⁶:
* There are billions of stars similar to our Sun in the Milky Way.
* Probabilistically, these stars can have planets similar to Earth which would sustain life.
* Given that Earth has intelligent life, there statistically is a subset of planets that also have developed intelligent life.
* Of any intelligent life at least a portion should have developed interstellar travel as humans have begun this development.
* Even if interstellar travel is as slow as we predict, the galaxy could be traversed in a few million Earth years.
* Given that our Sun is much younger than a portion of the stars that could have met all of the proceeding logic, Earth should have been visited by extraterrestrial life.

Yet, as far as we are aware aliens have not been to Earth. There are a few quantitative hypotheses to help answer the questions⁷. When working with numbers based upon interstellar travel it can be a bit hard to put into perspective. While yes, statistically based on the number of stars in the galaxy there should be at least a handful of other life forms out there, Earth is only about 4.5 billion years old while the rest of the galaxy is 13.6 billion years old. There is a large possibility all the life forms lived and died in the nearly 10 billion years before we were around. Not to mention, Earth is about 30,000 light years from the center of the Milky Way.^8,9 It would be possible life is out there but is not old enough or advanced enough to make that journey yet, much like our own evolution. Another statistical explanation is the non-zero chance that humans are the only life forms to have formed, or the only ones to have advanced anywhere near interstellar travel.

Moving away from statistical reasoning there are some reasonable, albeit potentially harrowing explanations, based on psychological and sociological factors. The interesting and 'fun' options are those like the Zoo Hypothesis. This states aliens see us and are observing but not interacting.¹⁰ It quickly turns dark when you return to the simplification of Drake's Equation and look at the only remaining variable: L--the length of time an advanced civilization lasts.

Based on the current assumption of the equation mankind has been advanced enough to communicate with potential aliens since about 1974.¹¹ Given that we are currently on a steady track it is not absurd to assume mankind is still around in 2074.This is where the number 10 is solidified. There is the potential of at least 10 extraterrestrial lifeforms capable of saying hello. Unless, they all destroy themselves before they get the chance. If you're an optimist and think mankind could make it another 100, 1000, or even 10,000 years it only increases the odds aliens should be on your doorstep. Which would lend one to believe the opposite: something about the advancement of life to the point at which it could travel the universe causes it to cannibalize itself. We can only observe ourselves to estimate our count of potential neighbors, a measly 10 until we surpass 2074 without vanishing.

The only solution will come when we either become the first life form (known to us) to travel space and find other intelligent life, or we destroy ourselves and never reach an answer. With all hope, 10 is the smallest that number ever is and one day, even if by approximation, generations to come will see it slowly increase. For now I prefer to take Frank Drake's current assumption: mankind can last at least another 10,000 years. Which only leaves one question: where are the other 1000 life forms?

Sources
1. https://www.britannica.com/story/the-fermi-paradox-where-are-all-the-aliens
2. https://www.britannica.com/science/Drake-equation
3. https://www.seti.org/drake-equation-index
4. https://www.smithsonianmag.com/smithsonian-institution/sad-story-laika-space-dog-and-her-one-way-trip-orbit-1-180968728/
5. https://pages.uoregon.edu/jschombe/cosmo/lectures/lec28.html
6. https://en.wikipedia.org/wiki/Fermi_paradox
7. https://www.bbvaopenmind.com/en/science/physics/five-solutions-to-the-fermi-paradox/
8. https://www.space.com/19915-milky-way-galaxy.html
9. https://education.nationalgeographic.org/resource/resource-library-age-earth/#:~:text=Earth%20is%20estimated%20to%20be,about%204.03%20billion%20years%20old.
10. https://ui.adsabs.harvard.edu/abs/1973Icar...19..347B/abstract
11. https://www.celestis.com/blog/message-sent-into-space-and-what-we-heard-back/