Introduction
One of the frequently asked questions in astronomy is, “are we alone in the universe?” It is hard to believe that we are alone in this universe that is home to billions of stars and innumerable planets and satellites. The astronomical community is divided into two groups in the matter of opinions on this. According to one group of astrophysicists, there are many earth-like objects existing in the universe, and it is only a matter of time before we find out our neighbors. A few astrophysicists, on the other hand, cherish the notion that life on the earth is unique and rare in the space of the universe. Therefore, it is highly unlikely to spot another earth like system in the universe. Even if a planet similar to the earth exists in the universe, it is very difficult to find that out. There are many theories and many empirical calculations in support of both the assumptions. This essay will discuss the Drake equation and calculate the probability of the existence of intellectual life using Fermi-like thinking, touching upon how “Fermi paradox” contradicts the results and “Rare Earth” idea supports the “Fermi Paradox”. The Second half of the paper will discuss the recent findings and personal opinion on the theories and scope of further research.
The Drake Equation
The search for life beyond the earth is something that has eluded the astrophysicists for generations. For sustaining life, it is important to have water on the planet, among other factors. Frank Drake first came up with an equation to calculate how many stars can have earth like planets in their orbits to sustain life (Nadis 2010).
N is the number of civilizations in Milky Way galaxy where radio communication may be possible, or in other words, which lies in our past light cone.
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 support life per star that has plants.
Fl is the number of planets that actually develops life.
Fi is the fraction of planets that actually goes on to develop intellectual form of life.
Fc is the fraction of civilization that develops technology that releases detectable signals in the space.
L is the length of time for which civilizations is detectable (Nadis 2010).
Fermi Thinking
It is not easy to solve and come to a single number if we look at the Drake equation as a whole. However, if we take a Fermi like thinking approach to break down the overall problem, then we can probably come to a solution (Christian 2012). For example, we do not have information about all 100 billion stars in our galaxy. In fact, we do not know for sure how many stars are there in our galaxy. Therefore, it is better to start with a small sample and then extrapolate the result for the whole Milky Way galaxy. We have more information regarding the 1,000 stars that are nearer to the earth. Then that result can be extended for 100 billion stars to arrive at the final number (N) for our galaxy (Nadis 2010).
If we use the Fermi like thinking of calculation, then from the findings of nearest 1,000 stars we get the following (Christian 2012):
There is a 68% probability that a star has a planet
7.6% of those stars are solar type (like sun)
50% of those stars are non-binary
40% of the stars have a planetary system
5% of the stars are earth sized
6% of those stars are in the habitable zone
Using these numbers, we deduce that 0.028 stars among the 1000 of our neighbor stars are expected to have planets in the habitable zone. This assumes the last four parameters of the Drake equation to be =1. By extrapolating for the whole Milky Way galaxy, we may deduce that 2.8 million stars have habitable planets.
Fermi Paradox
The main problem with the above number is that if we have so many stars that have a habitable planet, then given that fact that the universe is 100 billion year old, by this time, we should have been contacted by intelligent species of the other worlds, or by this time, the whole galaxy would have been colonized by millions of civilizations (Nadis 2010). As far as we know that this is not the case. This is Fermi Paradox. Either we are not being able to detect the presence of others, or the numbers we are using in the Drake equation is wrong (Christian 2012).
Rare Earth Theory
The rare earth theory says that the complex form of multicellular life on the earth requires almost an improbable combination of astrophysical as well as geological events. For example, in the Drake equation it is assumed that if any earth like planet is in the habitable distance from a sun like star, then at some point in time, life should start in that planet. However, rare earth theory suggests that there are many other factors. For example, not all the stars in a galaxy are suitable for habitation. Stars near the core of the galaxy are not suitable even if their size and temperature match the criteria (Matsos, 2003). Also, the stars far away from the core also do not have the right property to sustain habitable planetary system. Many astrophysicists also argue that the spiral galaxies like Andromeda have lower chances of habitable planet when the chances of collision between the heavenly bodies increase, which is not ideal for sustaining life (Matsos, 2003). There are other factors that have been covered by the rare earth theory, but are not captured in the drake equation, such as the right arrangement of planets in the planetary system, continuously stable orbit, geological factors like plate tectonics, large moon, and the right time in evolution. If we consider all the factors in the rare earth theory, then the result states that the earth is almost a unique object in the whole universe. Finding another earth is almost improbable.
Problem with Last four Parameters in Drake Equation
The main usefulness of the Drake equation is that it provides a simplistic model for a very well-discussed problem. However, arriving at some of the numbers of the Drake equation is not easy. For example, in the original Drake equation, the last three parameters (3 before L) were taken as =1.0. However, they can vary significantly from 1. For example, to estimate fl (the fraction of planets that actually develops life has habitable condition), we need some empirical evidence or strong biological support (Nadis 2010). The only example in our hand is the earth that already has a life system. This one sample prediction is full of bias, and the number can be anything as there may be other unknown biological factors. Another major factor not easy to estimate in the Drake equation is L (Walia 2014). L can be 100 years for some planet and can be 10 billion years for other planets. The average of L will not give the actual picture. For example, if we have 9 civilizations that existed 100 years and 1 civilization that went on for 1 billion years, then the value of L will be 100 million years, but when we are searching for life in the other planets, there is a high probability for us to find something in only one planet as others are too small in length (Walia 2014).
Analysis of the Calculation of the Drake Equation
In my estimation, I have used the Kepler mission data that are pretty accurate. However, the big question is that can we really extrapolate the data to all the stars of the Milky Way galaxy? As the rare earth theory suggested that stars too close to the core of the Milky Way or too far away from the core cannot create habitable planetary systems (Matsos, 2003). These considerations were not taken in my calculation, as there is no way to incorporate these factors in the Drake equation.
My conclusions support the claim that there are many planets that have the earth like habitable condition, and sooner or later, we will be able to establish contact with them. However, as we have seen in the rare earth equation as well as the assumptions made in the Drake equation, there may be a huge difference in the values we have used and the actual values because of uncertainty. Therefore, although the values show that there should be many life forms in the Milky Way galaxy, in reality, the situation may be different and the Drake equation may be incomplete and it needs more variable (Walia 2014).
Critique of the Rare Earth Equation
The Rare earth idea provides a good critique on the Drake equation, but the rare earth idea does not provide any clear guidance on how to calculate N. In fact, it suggests that the earth is so unique that finding another earth is almost impossible. However, with new discoveries, more and more planets with habitable conditions are found in the extrasolar star system, which increases the probability of finding another earth enormously. Also, the rare earth theory suggests that oxygen is required for the earth like life formation, but recent studies have suggested that multicellular life like anaerobic metazoa and spinoloricus nov sp can exist without oxygen (Matsos, 2003). This also suggests that the earth may not be so unique. Furthermore, the rare earth theory suggests that planets should have earth like undulating surface for sustaining an atmosphere and constant heat and life. However, there is no evidence or research to support that claim, and it is too broad based.
Conclusion
It is not easy to accept the fact that we are all alone in this vast universe. There are so many stars and galaxies that it is almost impossible statistically for the earth to be a unique planet. The Drake equation suggests that there are many planets like the earth, but the Fermi paradox questions that if there are so many plants like the earth, then they should have been found out by now. In fact, till date, we have not found any extra-terrestrial life. On the other hand, the rare earth theory with its many astronomical as well as geological parameters suggested that the existence of another earth like plant is almost impossible. However, many assumptions made in the rare earth theory were logical, but without evidence. Therefore, like the Drake equation, the rare earth theory parameters also have huge variability. There is no one equation to predict the number of intellectual life form, but I certainly believe that there are more earths in the universe. SETI should continue its mission to find out life forms beyond the earth.
Work Cited
Walia. , Arjun. “If You Think We Are Alone In The Universe – You Might Want To See This”. Collective Evolution. 20 June 2014. Web. 7 Dec. 2014 <http://www.collective-evolution.com/2014/06/20/5-reasons-why-everybody-knows-we-are-not-alone-in-the-universe/>
Christian, Hans and Baeyer, Von. “How Fermi Would Have Fixed It”. Arizona University. 2012. Web. 7 Dec. 2014 <http://zeus.as.arizona.edu/~dmccarthy/ASTR170B1/FermiCalculations.html>
Matsos, Helen. “The Drake Equation Revisited: Part I”. Astrobio. 29 Sept. 2003. Web. 7 Dec. 2014 <http://www.astrobio.net/topic/deep-space/alien-life/the-drake-equation-revisited-part-i/>
Appendix
First, consider a “nearby” sample of 1000 (i.e., 103) stars in our “solar neighborhood.”
1. Based on the astronomical evidence cited in Section C, how many of the 1000 stars in our “solar neighborhood” would be expected to host at least one planet? (HINT: Use evidence items #2 and 3 in section C.)
2658 / 156453 = 0.0170 (i.e., a 1.7% probability that a “Kepler star” has a transiting planet
0.0170 x 40 = 0.6796 (i.e., a 68% probability that a “Kepler star” has a planet in general)
0.6796 x 1000 = 679.6 of the 1000 stars in our solar neighborhood are expected to have a planet
2. Of those stars, how many are expected to be single main-sequence stars with properties like our Sun (i.e., G-type star which are not binary)? Use the data in the columns of Table #1 along with your answer to question #1.
90% of all stars are main-sequence stars: 0.90 x 679.6 = 611.6 stars.
7.6% of those stars are solar-type (i.e., spectral type “G”): 0.076 x 611.6 = 46.5 stars.
50% of those stars are binary: 0.50 x 46.5 = 23.2 stars in our solar neighborhood are Sun-like.
3. Of those remaining stars, how many are expected to host planetary “systems” containing more than one planet? (HINT: Use evidence item #4 in section C.)
40% of all the planets discovered by the Kepler Mission are Earth-sized.
0.40 x 23.2 = 9.3 Sun-like stars in our solar neighborhood are expected to have planetary systems.
4. Of the remaining stars, how many are expected to host planets with radii of Earth-size? (HINT: Use evidence item #5 in section C.)
5% of all the planets discovered so far are Earth-sized.
0.05 x 9.3 = 0.46 Sun-like stars in our solar neighborhood are expected to have Earth-sized planets in a planetary system.
5. How many of those planets would you expect to be in the habitable (“Goldilocks”) zone around their star? (HINT: Use evidence item #6 in section C.)
6% of Sun-like stars are expected to have Earth-size planets in the habitable zone.
0.06 x 0.46= 0.028 stars in our solar neighborhood are expected to host “habitable,” Earth-sized planets in planetary systems.
6. Assume this percentage from the solar neighborhood also applies to the ~100 billion stars in our Milky Way galaxy. Use the numerical skill of “scaling” (skill #33) to predict the number of Sun-like stars in our galaxy that have planetary systems containing an Earth-mass planet in the habitable zone.
The number of stars in our galaxy is 108 times larger (i.e., 1011 / 103) than the solar neighborhood, so we’d expect 108 times more planets.
0.028 x 108 = 2.8 x 106 or
2.8 million habitable, Earth-sized planets are expected to exist in planetary systems orbiting Sun-like stars in our galaxy.
