Perhaps the most asked question in all of history is the question of us being alone or not in the universe. For many years, we have been trying to make first contact with another civilization. There is actually a fundamental equation that describes this question created by Dr. Frank Drake in 1961. "The Drake equation is a probabilistic argument used to determine the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy" (Wikipedia). This equation summarizes the main concepts that scientists have to contemplate when searching for extraterrestrial life. This equation is thought of as more hypothetical than serious due to the uncertainty of many of the variables in it. It does not attempt to find a precise number. I don't even think that would even be possible.
Drake Equation [Credit to NASA]
Let's go through each of the variables individually:
N = the number of theoretically advanced civilizations in the Milky Way galaxy
R∗ = the rate of formation of stars in the galaxy
fp = the fraction of those stars with planetary systems
ne = the number of planets, per solar system, with an environment suitable for life
fe = the fraction of suitable planets on which life actually occurs
fi = the fraction of life-bearing planets on which intelligent life emerges
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
As evident from the variables in the equation, many of them are incalculable, at least at the technological level that the human civilization is currently at. This means that there need to be educated guesses in order to determine a theoretical value for N.
Here are the educated guesses that Drake and his colleagues in 1961 made (According to Wikipedia):
R∗ = 1 yr^-1 (1 star formed per year, on the average over the life of the galaxy; this was regarded as a conservative guess)
fp = 0.2-0.5 (one fifth to one half of the stars formed will have planets)
ne = 1-5 (stars with planets will have 1-5 of them that are capable of developing life)
fe = 1 (100% of these planets will develop life)
fi = 1 (100% of the planets with life will develop intelligent life)
fc = 0.1-0.2 (10-20% of the planets with intelligent life will be able to communicate)
L = 1000 -100,000,000 years (how long the communication from these intelligent civilizations will be able to last)
If the minimum of these values is inserted into the equation, then the value of N will be 20, but this value only comes from these hypothetical values. We really have no way of being bale to measure the likelihood of an intelligent species developing on another planet and then being able to communicate into interstellar space. These values may very well always remain a mystery to us.
In research by Adam Frank and Woodruff Sullivan have created a new equation which is in the bottom row of the original image above with the hope of answering a slightly different question: What is the number of advanced civilizations likely to have developed over the history of the observable universe? (NASA). This question makes sense to me due to the sheer age of the universe and the what we know. There could have very well been civilizations that have come before ours. There is even evidence for this in our own Solar System, with planets like Venus and Mars having drastically different climates now than in their past. This new equation uses elements from Drake's, but eliminates the need for L.
According to NASA, Frank and Sullivan's argument hinges upon the recent discovery of how many planets exist and how many of those lie in the habitable zone- which is where liquid water can exist on planets (and therefore there is a much greater chance of life developing). This allows Frank and Sullivan to create a variable called Nast. This is the product of R∗ (the total number of stars), fp (the fraction of those stars with planets), and ne (the average number of those planets that are in the habitable zones of their stars)."They call this the "Archaeological form" of the Drake Equation, which defines "A"as "the number of technological species that have ever formed over the history of the observable universe"" (NASA). The other variable in their equation is fbt, which is defined as "the likelihood of a technological species developing on one of those planets." This makes the variable more broad which means it can encompass just our galaxy alone, or the entire universe.
Given the age of the universe, which is roughly 13 billion years, it is very likely that there have been civilizations that are already fully extinct, even if they only lived for a short time. There is also validity on the other side of the argument as well, which means that there could be civilizations that come after ours is long gone. According to Woodruff Sullivan: "For us to have much chance of success in finding another "contemporary" active technological civilization, on average, they must last longer than our present lifetime." However, even if we were to make any sort of contact with another civilization, we have to look at the sheer distances between objects in the universe and its size. Sullivan makes the argument that even if they were only 20,000 light years away from us, then every exchange between us would take 40,000 years to travel back and forth, and those who sent the initial message would be long gone.
Moving away from the Drake Equation, there is another important dilemma that astronomers are faced with: the Fermi Paradox.
The Fermi Paradox seeks to answer the question that is on many people's minds: "Where is everyone?" Essentially, since our planetary system is fairly young compared to the rest of the universe, and that interstellar travel may be "easy" to achieve, this theory says that Earth should have been visited by aliens already (Space.com). Italian physicist Enrico Fermi realized that "any civilization with a modest amount of rocket technology and an immodest amount of imperial incentive could theoretically colonize the entire galaxy" The Search For Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, said on its website.
However, many years later, we still do not have any visits of extraterrestrial life to report on (other than the "alien abduction" stories that people famously like to tell). One would think that because of this, and how long humans have been around, that there are simply no other advanced civilizations in our galaxy. Micheal Hart wrote an article titled "An Explanation for the Absence of Extraterrestrials on Earth" in the Royal Astronomical Society (RAS) Quarterly Journal in 1975 in order to further explore the Fermi Paradox. In his article, he outlines four arguments that explore the paradox:
1) Aliens have not visited us because it is not possible for them. (This could be due to a number of reasons: astronomy, biology, or even engineering).
2) Aliens just chose not to come to Earth. (I honestly don't blame them).
3) Advanced civilizations are too young for aliens to reach us. This means that they may not have developed the technology to do so yet.
4) Aliens have visited Earth in the past, but we have not observed them.
Any of these arguments could be valid. Let's consider the age and size of the universe. "One estimate says that the universe spans 92 billion light-years in diameter (while growing faster and faster)" (Space.com). Most measurements indicate it being about 13.82 billion years old. After seeing this information, one would like to believe that aliens have had "plenty of time" to visit the Pale Blue Dot; however, we still have to consider the enormous cosmic distances that they would have to cross in order to reach us. The most interesting thing to note is that Fermi formed his theory long before exoplanets were first discovered. The number of confirmed exoplanet discoveries is growing ever so rapidly, which gives scientists hope that perhaps life could be abundant throughout the universe.
Space.com states that a study conducted in November of 2013 using data from the Kepler Space telescope suggested the one in five Sun-like stars has an Earth-sized planet orbiting in the habitable zone of its star. Of course, we do have to note that just because a planet is within the habitable zone of its star, it is not guaranteed to have life on it. Its atmospheric components still have to be suitable for life as well. We also have to remember that "life" could include bacteria and other microorganisms as well, since humans also arose from smaller lifeforms as well. Most people seem to think that all life has to be "intelligent life." As mentioned previously, even if intelligent life were to arise on another planet, it would need the technological capability to be able to communicate with us or visit Earth. This leads me to the next topic of brief discussion: The Kardeshev Scale.
The Kardeshev Scale is essentially a method of measuring a civilization's level of technological advancement based on the amount of energy they are able to use. The scale was created by Soviet astronomer Nikolai Kardashev in 1964. It has three designated categories:
Type I Civilization: Also called a Planetary Civilization, it can use and store all of the energy available on its planet.
Type II Civilization: Also called a Stellar Civilization, it can use and control energy at the level of its star.
Type III Civilization: Also called a Galactic Civilization, it can control energy at the scale of its entire host galaxy.
Humanity is not yet at a Type I civilization (Sagan suggested we are at a 0.7), but could reach it very soon. From there, depending on how long we are around, it will take humanity around a few thousand years to reach Type II and a few hundred thousand to a million years to reach Type III.
This was important to mention, because it is difficult to conceive the amount of energy and technology that it would take another civilization to create and harness in order to reach Earth.
We have only recently been figuring out the field of Astrobiology, which is the study of life in the universe. Our own technological limitations do not allow us to explore the exoplanets that we discover in order to find out more about them. We also have a limited knowledge of our own Solar System, because we are finding microbes living in very extreme conditions on Earth, which means that we cannot completely eliminate every other one of our planetary neighbors as life-bearing candidates.
Life itself could just be a rarity altogether. On our own planet it took about 3.5 billion years for life to arise. Perhaps we were just the lucky ones. As of right now, we don't have anyone else to communicate with, but that doesn't mean that other life isn't out there. It could even be searching for us, but we're too far away. There are many, many, many questions that arise from just this one simple question. It is what many astronomers and physicists hope to answer, or at least get closer to answering in their lifetimes.
I would like to think that we are not alone. For me personally, the universe is far too vast for us to be the only ever civilization (or life at all) to evolve on a planetary body. Based on the studies of exoplanets and habitable zones, it appears that many other planets are capable of holding life on their surfaces as well. Why haven't we found any yet? That question has yet to be answered. Maybe we simply don't have the technology yet. Maybe civilizations have already arisen and wiped themselves out. Maybe life doesn't last that long on a planet. Maybe interstellar communication and travel is just... too difficult. Human life could be rare and unique, but to me, that is just too frightening to think about. Hopefully one day we'll get even closer to figuring out our place within the cosmos. Hopefully we're not alone...
Want to learn more about the topics I discussed?
Here are some helpful links:
Drake Equation: https://www.seti.org/drake-equation-index
Fermi Paradox: https://www.space.com/25325-fermi-paradox.html
Kardeshev Scale: https://futurism.com/the-kardashev-scale-type-i-ii-iii-iv-v-civilization