Cosmology and astronomy
A framework for thinking about how many detectable civilizations are out there. Created by Sal Khan.
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- At9:02you mention the fraction of intelligent life as 1/10 of all life forms. Isn't it too optimistic?(69 votes)
- He is actually not saying that 1/10 of planets have life. He is saying that 1/10 of planets could sustain life, if it were present. Our solar system has eight planets, two of which are habitable. One is Earth, while the other is Mars. Its temperatures range from -80 to 30 degrees F. It has water ice (and likely liquid water under the surface), CO2, and traces of oxygen. So if anything, Sal is being pessimistic on that.(78 votes)
- is it true Jupiter's moon Europa could possibly have alien microbes under its icy surface(10 votes)
- Europa has an unusual amount of heat because of gravitational stress from Jupiter and Ganymede. Because of this, under the solid ice crust, there is a 50 mile deep ocean of water. At the bottom of this ocean, there are vents of hot gas rising from the core. Earth has similar vents but we know of specialized life that lives in the conditions under the ocean and live off of the vents. Because of this, we think that Europa might have life living off these vents under the ice.
The moons Enceladus and Titan (both of Saturn) have similar conditions and may also have life. The dwarf planets Ceres and Sedna may also have a subsurface ocean as a result of radioactive decay.(23 votes)
- woudn't it be possible for live to develop on a moon aswell?(3 votes)
- The moon, Europa in orbit around Jupiter has a vast ocean under its ice. Hydrothermic vents could support life similar to tube worms on Earth. Some say that Ganymede has similar conditions but I don't think so.
Saturn's moon, Enceladus also has a subsurface ocean. Titan also could possibly support life. We have found a microbe on Earth that not only could survive on Titan but it could reproduce there to.
There are more life supporting moons in the solar system then life supporting planets so yes indeed life could develop on moons.(8 votes)
- so is there anything like a telescope that can look at other planets that may have life?(3 votes)
- The only method we currently have for detecting planets in the "goldilocks zone" as "fiery blue thunder" mentioned, is a method that actually measures the doppler effect or wobble the planet has on its sun. Measuring the waves gives us a measurement of how far away from the sun the planet has and if it is a solid or gas planet. So far the Kepler telescope can only detect that, because all those stars are still too far away to see close up.(3 votes)
- If we are able to find another civilization, how are we going to communicate with them?(4 votes)
- thought basic undrstanding of the universe e.g. Gravity. in the way Lange can be pick up. things like picture and symbols will be used( see the sides of the voyagers). This will latter lead to an undrearstanding of culture and other matters. That what are teacher ( physics ) said any way...(3 votes)
- So can we detect different civilizations out there could we detect other life sources is the question we have been asking almost answered" are we alone is there alien life force out there"?(5 votes)
- We do have ways we think we could detect civilizations:
The first has been used for decades: radio waves. For years we have been searching for radio signals that were possibly sent out by alien civilizations, either intentionally or stray signals, like those we use for television and radio. We are also sending signals out, both the accidental signals, as mentioned, and intentional, for example prime numbers.
A second, very recent way of detecting civilization on other planets is watching their atmosphere. We hope to look for hight CO2, methane and other pollutants in the air, possibly signifying alien life producing energy, for example with combustion. However, the levels of these chemicals need to be much higher than even in our atmosphere, and we're choking in our own fumes as is.
I believe that there are other methods of looking for intelligent life, but these are the two I am most familiar with (at the moment). Unfortunately, though, the speed of light keeps us from watching what is happening on planets right now. I hope that there's life out there! I think it would be scary to be alone in the universe.(1 vote)
- How can we estimate the number of planets capable of sustaining life when other life forms could need totally different conditions than we do?(2 votes)
- Estimating is just from the current knowledge we have, you cant expect someone to know something they don't know.The estimate is considering the fact that liquid H2O is the most important or the most likely molecule for life to begin evolving (to our knowledge). Along with other necessary elements like Carbon. Planets which can hold these molecules are found at a particular distances from particular stars, where the temperatures and pressures are capable of holding onto liquid H2O.(3 votes)
- If all life on Earth started out as bacteria, can't possible bacteria on other planets evolve to look and act somewhat like us?(3 votes)
- aren't there any creatures in universe which can breathe nitrogen instead of oxygen ?(3 votes)
- Well following the evolutionary theory there used to be no oxygen on earth while we were still developing so yes it is possible that there are animals that can breathe nitrogen without the presence of oxygen.(1 vote)
- Guys read my answer below didn't have enough space to write it in the questions section (it's by Parth Agrawal, or allies4ever, whichever shows up.) Read it - I think its important(0 votes)
- There is no possible way that we can be 100% sure about anything. Your bed could have been made by intelligent zebras and placed in a furniture store. Was it? I don't know. I'm not be 100% sure that it wasn't. I agree that we should strive to find even more evidence to back up our theories but a theory can never be entierly proven. We can only look at our evidence and make a logical guess. We know other star systems have planets (not 100%, they could be brown dwarf stars). And we know that life is on Earth (Not 100%, we might be robots). So we guess that life can originate on other planets.(6 votes)
It's estimated that our galaxy, the Milky Way, has 100 to 400 billion stars, and when you hear a number like that the obvious question is: Are there civilizations on planets that are orbiting any of these stars and maybe even more interesting question is can we detect any of those civilizations? Have they gotten to the level of technological progress like us, that they are emitting electromagnetic waves into space and other civilization like ours can detect and say, "Hey, there is someone else out there watching television or using radio or whatever else they might be doing." So, what I want to do with this video is not answer that question. That's a big open question, we don't know the answer. We don't have anywhere near enough information to definitively answer that question, but I want to do is come up with a framework for at least thinking about that question, a way of actually estimating how many detectable civilizations there are in just our galaxy, and there is a formula, that you may or may not have heard of, called the Drake Equation. and what we are going to do is independently derive our own version of the Drake Equation. It's going to be slightly different, but it is the same thought process and in the future video, I'm going to maybe reconcile what we come up with, with the Drake Equation, and just so you know Drake equation is named for Frank Drake, who is a Professor at University of California Santa Cruz. He first, kind of put some structure around this problem, that's why the formula or the equation has his name, but the equation is not an equation you can apply on a daily basis and get results that you can use to build things, but what it's, it's the structures of our thinking around this question of How many detectable civilizations are there in our galaxy? And to answer this question, I am going to start little bit differently than Frank Drake did He starts with the number of new stars that're born each year, we will see that our definitions are actually pretty close to each other what I want to do, is start with the total number of stars, So, we are trying to come up with is, I will call it 'N' and this is number of detectable civilizations, number of detectable civilizations in the Milky Way, in our galaxy. and once again, there could be civilizations, looking back in the star field right over here this star right over here, maybe it has a planet that is in the right place that has liquid water and maybe there is intelligent life on that planet, but they might not be detectable, because, they aren't technologically advanced enough that they are using electromagnetic radiation or maybe they just figured out some other way to communicate or maybe they are beyond using electromagnetic radiation, you know, radio waves and all the rest to communicate so we will never be able to detect them. We are talking about civilizations like ours that are, to some degree, using technology not too different than our own. That's what we mean by 'detectable', so let's think about that little bit. I like to start with just the total number of stars in our solar system. So let's just start with, I will call it, N*, so this is the number of stars in our galaxy, and our best guess, I said is this's going to be 100-400 billion stars. We don't even know how many there are, some of them are undetectable, and the center of our galaxy is just a big blur to us we don't even know what's on the other side of that, e can't even see all the stars that are packed into the center, so this is our best guess, 100-400 billion stars. Now obviously, there is going to be subset of those stars that even have planets. So, let's multiply the times that subset; so lets multiply times the frequency of having a planet. If you are a star, this is the percent chance or the frequency or of the fraction of these stars that have planets (f sub p) so, I'll write this way "fractions that have planets", So, if this is a hundred billion, let's say I am making a guess here and we are learning more about this everyday, there are all these discoveries of 'exoplanets'- planets outside our solar systems, maybe this is one fourth. Then you could say, well that means there are 100 billion times one fourth, that means there are 25 billion stars that have planets around them. But that's still not enough to go to civilizations. We also need to think about planets, there could be a planet like Jupiter, and we don't know how life as we know it can survive on a planet like Jupiter or Neptune or Mercury. It has to have planets that are good for sustaining life. Preferably have a rocky core, liquid water on the outside, that's what we think are the ingredients we need for life, maybe we are just not being creative enough, that's what we know as life is being. So, let's multiply this times the average number of life sustaining or planets that could sustain life on them. So we don't necessarily know that they are going to have life, but they seem like they are just the right distance from the star, not too hot, not too cold. They have the right amount of gravity, water, all the other stuff, and we still don't know what this means, but this means average number, so given a number of solar systems with planets, what's the average number of planets capable of sustaining life, and once again, you don't know this answer. maybe it is 0.1, it's probably less than 1. Therefore any given solar system that has planets, the average number capable of sustaining life maybe its 0.1 maybe it's more than 1, I don't know. We don't know the exact answer here, but I will throw an answer in, I 'll throw out a guess, maybe it is 0.1. And here the fraction that has planets (f sub p), i don't know, i will throw that out as, and once again I am making up these numbers, we really don't know the right answers, this is one-fourth. But if we were to multiply this out we would have the average number of planets in our solar system that are capable of sustaining life that are around stars that have planets and these planets are capable of sustaining life. Now, n sub p will give us the total number of planets in our galaxy capable of sustaining life. Now, just because you have liquid water and right temperature and all the other rest of the ingredients doesn't mean that you will actually have life happening on your planet. So let's multiply that times the fraction that actually generate life (f sub l) So f sub p is the fraction that actually have life and this is actually a very , we dont know this answer, so this is the fraction that have life on them And this is a really big open question maybe if you have the ingredients, maybe every planet has life maybe it is a frequent thing that's happening in our galaxy and frankly our universe or maybe it's a very infrequent thing maybe it's just the right kind of freak set of circumstances that just have to happen i will throw out a number, just for the sake of having a number maybe it is one out of every ten planets that have all the right ingredients for life actually do generate life. My personal guess is probably higher than that given that life seems life seems such a robust and flexible thing we have seen in all sort of weird circumstances. Actually let me make it higher number than that so let me make it one half (0.5) assuming that we have all of the ingredients. So this should tell us essentially how many planets in our galaxy have had life on them at some point in those planets' lives. The life might have come and gone maybe destroyed itself through nuclear war. But this would tell us the number of planets in our galaxy that had life at at least one point in their history. Now we care about civilizations, about intelligent life.. So maybe if the asteroid never hit earth... the dinosaurs would have never evolved to the point of generating radios and tvs and telephones and all the rest.. and so it is kind of a freak circumstance that because they were destroyed these gaps in the ecosystem developed so we could emerge and and be intelligent and do all these crazy things like make YouTube videos and all the rest. So, let's multiply this times the fraction if you get all of this, the fraction that actually end up having intelligent life and maybe this fraction is, to a number of 1/10, then probably in the next video I'll calculate it all. Now this is very important to realise because once again, you could have life these are all examples of life right over here This is actually life on Earth, on our planet even though this looks quite alien this is a weeble, kind of looked at from very close-up, but there is all sorts of forms of life many of which we probably can't even begin to imagine but what we care is that if intelligent life starts to emerge on the planet because only intelligent life has a chance, we believe of being able to eventually communicate in ways that are detectable by us. Now I said intelligent life, but maybe not all intelligent life will eventually get to the technological sophistication, where they will be using radio waves, electro-magnetic radiation to communicate with each other maybe we might have stagnated at this stage if nothing of the right things didn't happen.. So what we need to do now, is multiply this so right here, we would have the number of planets in our galaxy that have had intelligent life on them at some point in their history maybe not at a time that coincides with ours, but what we wanna do is bring it down even more to the percentage that get to the point that they can develop technology that allows us to detect them. So, let me multiply times the fraction that are, I'll put a 'C' here, 'C' for, maybe they're using communication; 'C' for communications that allow us to detect them. So this is detectable, the fraction that are detectable. Now you might think that we're done, this will give you the Total Number of Civilizations, or life-forms in our Galaxies or the planets that have life forms that developed detectable technologies at some point in their history.. Now it would be nice if civilizations be not born and die but the reality is, they do die. They might destroy themselves or whatever, and they might exist for only a small period of time for the history of that planet or the history of that Solar System. So in order to make it the number of civilizations that are in existance now and I'll clarify what "now" means in the next video because really if we are detecting something from a star that's 10,000 lightyears away our "now" means we are just receiving their signals so, they released the signals 10,000 years ago. But what I want to do is get to know what is the fraction of these whose signals are reaching us right now. and here, I'll say what's the average lifespan of a civilization I'll put that "L", who knows what is, maybe 10,000 years so civilization lifespan as going to be that over the life of the star So, I'll put a 'T' here, for the average lifespan for the star, and I could say the average lifespan for the planet, or whatever, but we're assuming once our star supernovas, you're not going to have a chance for life on Earth to develop anymore. So, maybe this thing appears ten thousand years and this down here is maybe 10 billion years. and if you were to multilply this out, you should get the number of detectable civilizations in our galaxy right now. I'll leave you there, and next we'll discuss it a little bit more, and reconcile it with the more famous version of Drake's equation, and I'll also try and talk about this little piece of bit, because I think it might be a little confusing, and I'll try to diagram that out a little bit more..