[Music] one of the most wondrous questions of all concerns our place in the universe after 13. 8 billion years of cosmic Evolution 4. 5 billion years since the formation of Earth and at least around 4 billion years since life first arose on our planet human beings have achieved remarkable Feats for the first time at least that we have any evidence of planet Earth now houses an intelligent sentient technologically advanced civilization we can receive signals from across the distant Universe identify their Origins and properties and have even begun exploring outer space beyond the confines of our own planetary home although we've been looking for other signs of intelligent life out there in the universe for more than half a century searching for extraterrestrial intelligence we have yet to obtain robust evidence that it exists creating life involves many steps but the necessary ingredients are abundant everywhere even if you're searching for Life similar to ours the universe offers countless possibilities if the ingredients for life are common and we evolve naturally then where is everyone else to form rocky planets organic molecules and the building blocks of life enough heavy elements must be generated initially the universe lacks these elements after the big bang the universe consists mainly of hydrogen and helium elements like carbon oxygen nitrogen and others vital for life were absent these heavier elements are crucial for the formation of rocky planets like Earth and play a significant role in enabling complex chemistry including the formation of organic compounds this mix of elements provides the necessary building blocks for the diverse and intricate processes that contribute to the complexity of our Cosmic environment multiple generations of stars are needed to produce and distribute these elements stars must live burn fuel explode in Supernova and recycle heavy elements neutron star mergers are also crucial for creating the heaviest elements essential for life processes this process involves complex astrophysical phenomena even though Earth formed more than 9 billion years after the big bang the universe didn't have to wait as long scientists classify Stars into three main groups population one 2 and three population one stars are similar to the sun with a small percentage of their composition being heavier elements this results in solar systems containing a mix of gas giants and rocky planets suitable for Life population two stars are typically older and contain fewer heavy elements compared to the Sun their planets are often diffuse and gassy potentially too primitive and lacking in heavy elements to support life population three stars are the earliest in the universe completely devoid of heavy elements while not yet discovered they are believed to be the first stars ever formed when observing the earliest galaxies they predominantly contain population 2 Stars however nearby galaxies exhibit a mix of young and old Stars some rich in heavy elements and others lacking in them one of the most important findings from the Kepler Mission involves the system Kepler 444 despite being a population onear with planets orbiting around it the planet is significantly older than Earth while our planet is around 4.
5 billion years old the planet is about 11. 2 billion years old this suggests that earthlike worlds could have formed much earlier in the universe's history potentially up to 7 billion years before Earth did considering this possibility along with the rapid enrichment of heavy elements in regions like the center of our galaxy it's feasible that there are location within the universe and perhaps even within the Milky Way that are more conducive to the emergence of intelligent life than our sunar system in discussions about finding aliens two main ideas pop up the Drke equation and the fmy Paradox the Drke equation guesses how many alien civilizations we might find signals from the fairm Paradox suggests that if extraterrestrial civilizations exist we should have detected them by now as they would likely have spread throughout the Galaxy however since we haven't some argue that searching for their signals is feudal to understand how the universe creates intelligent life we need to consider two things first we must understand the steps that led to our own existence second we need to explore how intelligence could arise in different circumstances from ours and we should aim for accuracy in our analysis we must also avoid making unfounded assumptions or falling for logical fallacies such as mistaking absence of evidence for evidence of absence or assuming causation from Mere correlation avoiding two common mistakes when estimating what's out there is crucial firstly some people make Point estimates without considering uncertainty simply saying we estimate this parameter to be X isn't very meaningful to make a meaningful estimate you need to include an uncertainty range error bars or some other measure of likelihood Ood for example estimating odds as 1 in 100 with a 10% uncertainty is vastly different from having a 1,000% or two-tailed uncertainty where the odds could be as high as 1 and 10 with no lower limit however the other mistake that people make is the most common mistake of all when attempting to answer the fairy Paradox using the Drke equation now the Drke equation has many interesting lessons for us and when it was first put forth it was a Monumental achievement from a scientific perspective for the first time it broke up a seemingly unknowable question of how many intelligent space fairing civilizations there are within our galaxy today right at this very minute into a series of smaller questions that we could conceive of solving one at a time we could for example measure or estimate things like the rate of star formation in the Milky Way the fraction of stars that have planets the average number of potentially life- supporting planets per star with planets the fraction of planets that could have life that wind up with life the fraction of planets with life that go on to develop intelligent life the fraction of intelligently inhabited planets that emit detectable signals that announce their existence and the length of time that such civilizations continue to emit those signals multiply all of those things together and you'll get an estimate of the number of active civilizations that we could potentially detect today however we encounter significant issues right away firstly we have a good understanding of the rate at which stars form in the Milky Way unfortunately if we multiply this rate by the age of the universe since the hot big bang we'd end up with a very low number of stars according to this calculation the Milky Way should have formed only around 10 billion stars throughout Cosmic history while 10 billion may seem like a large number it's actually only about 2 to three % of the estimated total number of stars in our galaxy which is closer to 400 billion fortunately it's easy to see why this method of calculating the total number of stars in the galaxy is flawed the main reason is straightforward the rate at which stars form has not remained constant throughout Cosmic history initially the universe lacked stars and star formation began roughly 200 million years after the big bang it then increased for about the first 3 billion years before reaching a peak and gradually decreasing additionally locally within our galaxy there have likely been bursts of star formation when smaller satellite galaxies merged with ours we may even be experiencing another burst as the melanic clouds and Andromeda exert their gravitational influence on the gas dust and other matter within our galaxy but that's only The Superficial reason why the Drke equation is problematic today the deeper reason is that the Drke equation when put forth assumed the universe that we know today is untrue it assumed that the Universe was eternal and static in time as we learned only a few years after Frank Drke first proposed the famous equation that bears his name the universe doesn't exist in a steady state where it's unchanging in time instead it has evolved from a hot dense energetic rapidly expanding State a hot big bang which occurred for a finite duration in our Cosmic past instead it's much more productive to calculate the quantities we can speak about with some level of certainty today and then move on to the great Cosmic unknowns in as responsible a fashion as we can unlike over 60 years ago when the Drke equation was introduced today we have a much better understanding of our universe both within the Milky Way and Beyond we know about the different types of stars and populations that exist as well as the processes involved in forming heavy elements rocky planets and enabling complex chemistry for instance we understand how energy storing molecules can form from basic building blocks and starlight allowing for the possibility of important reactions moreover scientists have made remarkable progress in understanding the variety and prevalence of planets orbiting Stars other than our sun known as exoplanets just three decades ago we were only beginning to discover our first exoplanets now we've confirmed more more than 5,000 exoplanets while our data is biased as we tend to detect the easiest to find planets we have methods to adjust for and measure these biases accurately instead of relying on speculation we now have access to excellent data about our modern Milky Way we know the total number of stars in our galaxy how these stars are categorized into different populations the average number of planets orbiting each star the composition of elements on these planets is crucial for complex chemistry among other key parameters is the number of planets with conditions suitable for hosting life with this information it's relatively straightforward to estimate the number of potentially habitable planets in our galaxy we can perform the calculation in several ways there are approximately 400 billion stars in the Milky Way about 80% are red dwarfs 18% are sunlike and only 2% are two massive and short-lived for life to develop each Stellar system typically has about 5 to 10 planets of these approximately 1 to two are considered to be in the habitable zone around each star we estimate that around 20% of the planets orbiting sunlike stars are earthlike in size this percentage is even higher for terrestrial planets orbiting red dwarf stars which are more common if we assume that red dwarf systems are not habitable but sunlike systems are we can calculate the number of potential inhabited planets in the Milky Way as follows total number of stars 400 billion proportion of sunlike stars 18% expected potentially habitable zone planets per relevant star 1. 5 fraction of those planets that are earthlike in size 20% by multiplying these values we estimate the number of potentially inhabited planets in the Milky Way which is 21.
6 billion using fewer significant figures makes sense 20 billion is sufficient additionally it's important to acknowledge that all these numbers come with uncertainties for instance there might be as few as 200 billion stars which is half our estimate some Stars might have low levels of heavy elements making them unsuitable for life-bearing planets but this is likely less than 10% of stars similarly less than 20% of stars might lack planets moreover the habitable zones size could vary with roughly a 33% uncertainty added to our estimate furthermore we still need to thoroughly explore the lower end of the mass and radius spectrum of exoplanets our estimate of 20% earthlike planets could vary with about a 25% uncertainty taking all uncertainties into account there might be as few as 5 billion or as many as 50 billion potentially habitable planets in the Milky Way if Red Dwarf systems are also considered potentially habitable this number could increase by a factor of 10 interestingly many factors we previously considered such as whether a planet has a major Moon a jupiter-like world in its system its distance from the galactic center or its status as part of a single or multi-star system may not significantly impact its habitability however there are still significant unknowns where our Cosmic ignorance remains staggering while we know that the ingredients for life are abundant everywhere we look in asteroids in gas in the galactic center in outflows around massive newly forming stars and even in the atmospheres and on the surface of other planets and moons in our solar system we still don't know the fraction of potentially inhabited planets where life actually originated from non-life in Carl Sean's original Cosmos series he suggested a range of 0. 1% to 10% which he considered conservative that assumption may not hold true the emergence of life could be a challenging process just because it happened relatively early in Earth's history doesn't necessarily mean that a significant portion of planets Harbor or ever harbored life the likelihood of Life arising from non-life could vary widely ranging from nearly 100% to extremely rare perhaps as Slim as a one in a million chance or even less if we were to rewind the clock and replay Earth's history the probability of Life arising and thriving here could differ significantly our level of ignorance on this matter is truly astounding likewise once life emerges how often does it face Extinction compared to how often it persists for billions of years how frequently does it remain in a relatively simple State unable to evolve complexity differentiation multicellularity or sexual reproduction even after billions of years how often does life resemble the simple forms seen at the start of the Cambrian explosion on earth once more we lack any concrete understanding of these processes assuming a frequency of 10% for any of these scenarios might seem reasonable but so could 90% or even 0.
