 Okay, welcome everybody I'm going to get started right away there are a bunch of announcements I put them on canvas so I wouldn't have to say them today. And that will allow me to focus on the subject of the last two lectures of physics 1303. I like to add special topics and at the end, not every faculty member does it. We do have a set schedule of things we're supposed to get to, but I don't feel too bad about this one today because not only there's an interesting and engaging subject about which we all care in one way or another life. What does it mean, could it be elsewhere and things like that. It also ties into gravity, which was the subject of last week's lecture and I'll show you how. So, okay, that's going to keep happening today. So I'm going to pause that. That's called stealing focus. Okay. Let's begin with a disclaimer on the subject of alien life and the probability of life. And the subject is the disclaimer is fairly straightforward. I am a physicist. That's my disclaimer. Okay, so what does that mean. It means that I have an area of expertise, and I am definitely going outside of my comfort zone in this but I'm going to try to tie it back to physics as much as I can. So outside my area of expertise I am relying on expert sources as input material for this lecture and many of them are collected in the bibliography at the end of the slides, which I will make available later. So this is of course what biology which really if you think about it is kind of the thrust of these lectures although it ties neatly to chemistry and physics in its own way. This is what a biologist would view looking at a bird right they'd see, they know all the parts. They know that this is the crown the nostril the bill the lesser converts, etc. This is what a physicist sees when they look at a bird. It's got some bird over here. Oh there's definitely bird over here. Check out that bird. There's a lot of bird on this bird. Okay. So I just want to be clear that, you know, I'm I am doing this because I like a challenge at the end of a semester, I want you to have a topic that you feel engaged in I am going to drag you screaming back into physics during this lecture. But I do want you to be aware of the fact that this subject is too broad to cover in two short lectures, and it benefits from a wide variety of disciplines, looking through all of their lenses at the same question of what is life. Where can it exist and where might it exist. Okay, so with that in mind, the theme of today is life as we know it. Okay, and here's a snapshot from life as we know it as human beings. But of course when I talk about life I'm talking about all living organisms, anywhere for example on planet Earth will look beyond planet Earth in this part of the lecture. But this essentially is a snapshot of life as we understand it, you know we are bipedal life forms descended from a long ago from apes. Life forms appeared millions of years ago on the planet modern humans are only about 200,000 years old. And most of what we take for granted as part of our modern memory of civilization was really only laid down in the last 10,000 years. And in the cosmic expanse of time which is important to the formation of life in the universe. That's a drop in the bucket. So let's start off by pointing out that life as we know it is a very myopic view. And I'm going to come back to that theme for a lot of these lectures. So let's talk about how our view of life is constrained, our view of life is constrained by the fact that we human beings evolved on only one planet in a single solar system. The outer suburbs of the galaxy containing half a trillion stars, many of them like our own son, and our galaxy is one of hundreds of billions of galaxies that fill the visible universe. There are parts of the universe we can't see, we assume they also contain galaxies, it may go on forever. It hasn't been in existence for forever, but it may for all intents and purposes be infinite in extent today. And in many ways, I mean, we look up at the night sky and we wonder at the wandering stars in the sky, those are the other planets, Venus, Mars, Jupiter and Saturn are the easiest to see with the unaided eye. Here we are on the third planet from our son, we have a single moon that goes around us and so far as we know there's no nothing alive, as we would understand it on that moon. Everywhere we've looked, we have seen no evidence, no clear and compelling evidence of life as we understand it anywhere else, even in our own neighborhood. This is really our, you know, this is sort of our street, this is our cul-de-sac in the cosmos right here. Now let's talk about Earth since life did appear on Earth, and it did appear basically about as quickly as it could have in the ways we understand it now. It's important to understand what are the ingredients if you wanted to make another Earth. So Earth has some major pieces to it, of course, it's got the atmosphere, which we're breathing right now and we really take for granted. The atmosphere also plays an important role in trapping and maintaining heat energy that comes from outside and comes from underneath our feet. And we'll come back to that. We live here on what's called the crust. The crust is a very thin, stiff outer layer of our planet. Underneath the crust is the larger mantle, which is still rigid, but going deeper than that we reach a liquid part of our planet, the outer core. The inner core composed primarily of nickel and iron appears to be solid. Now we've never drilled down into the core. We've never gotten more than a few kilometers below the surface of the planet. We've basically only ever explored the crust of this planet. Everything we know comes from observations using seismic waves. If an earthquake happens somewhere over here, the waves will travel along the surface. They'll also travel through the bulk of the planet. And we can learn about the density of rock, the kind of material, liquid solid, things like that that are under our feet. On the surface, it's primarily water. That water can be in liquid solid or gas form. Ice appears naturally on the surface of our planet. Obviously liquid water, very common on the surface of our planet. And water vapor, water gas fills the atmosphere and is an important part of the atmosphere. It's a humid day today, you might have noticed. That means we have a high degree of water vapor saturating our air today, which helps to trap heat, which makes it hard for us to sweat, which is why it feels hotter than the temperature outside. Okay, the so-called heat index. Water is essential and annoying to all things that are living on the surface of the planet. The atmosphere itself, however, is primarily nitrogen and oxygen. Many things on the surface of the planet need both of those things to survive and I'll come back to that. The crust is mostly oxygen silicon with aluminum, iron, calcium, and then some of some about roughly equal amounts of sodium, magnesium and potassium. There's a little bit of hydrogen and then trace elements, all those rare earth elements that people are always freaking out about having for smartphones and so forth. They're buried somewhere in that half percent of stuff in the crust. Okay, so we need them, but they're rare and you'll see why they're rare. Hydrogen is also rare, which is strange, given what I'm going to tell you later, but there's a good reason for that. And if you're curious about it, we can come back to it. Helium is even rarer. Helium somewhere down in this trace elements thing. Now the mantle under the crust is similar abundance is not exactly the same, but pretty close enough for what I care about today. The core, as I said, is almost 90% iron, about 6% nickel, there's sulfur in there as well and then less than 1% of it is just trace elements, other things, heavy things. Why are the iron and nickel at the center? There was a time when the earth was molten and that means that atoms were free to move around. What forces were in play? Well, there was heat and convection, but there was also gravity and heavy things tend to collect toward the center of the gravitational potential, the center of mass of the body that's gravitating. So the iron and nickel settled down in the core and eventually hardened, but there's also a molten outer part of it there. This is all left over from the formation of the earth. The lighter stuff tended to float to the top. So if you think of the earth as once being a liquid ball of molten rock, that's a pretty good picture of what the earth looked like when it was born. It's settled down quite a bit in the first 500 million years of its existence. Now heat obviously plays an important role in all of this. We get a lot of heat from the sun. We're far enough away that it is not too cold and we're close enough that it's not too hot. That's how we're able to get all three phases of water on our surface. But we also have an atmosphere that helps to trap and regulate the temperature. If we had no atmosphere, it'd be really hot during the day and freaking cold during the night. We wouldn't survive. But the atmosphere regulates those swings from when you have daylight to when you don't have sunlight. So it helps to trap heat in the atmosphere and keep us kind of toasty down here. Now the earth also generates heat. There's heat left over from the formation of the planet. And there's also heat from radioactive decay of elements like uranium and thorium and potassium. In fact, how do we know this that about half the radiation radiating from the surface of our planet, which is 44 terawatts of power, 44 trillion watts of power radiates from the surface of the earth. How do we know half of that is coming from radioactive decay? The speaker today in the physics colloquium, Professor Bonnie Fleming studies the neutrino. Neutrinos are produced in radioactive decay. And if you have a sensitive enough detector that can trap neutrinos and figure out where they pointed back on their trajectory, you can make a photograph of the interior of the earth using these phantom particles. And that's what we've done. And based on just a handful of events, neutrinos interacting with detectors across the globe pointing back toward the deeper parts of the earth. And we know that the processes that make neutrinos, uranium, thorium decay, all that stuff. Those neutrinos are primarily coming from those things and the rate at which they come up from under the surface suggests to us that they're about half. That radioactive decay is about half due to radio isotopes. If we did not have unstable radioactive elements, the earth would not be as warm as it is today. That's a fascinating story. It actually bears on an early fight in physics between physics and biology about the age of the earth. Okay. But based on all of this today we know that the earth is about four and a half billion years old. It's been cooling for about four and a half billion years. Now, let's get to life, because life is the thing we care about. Now, I'm going to start off by saying there is absolutely not a universally agreed upon fundamental scientific definition that everybody walks away and quotes. There are some empirical rules of thumb about what it means to be a living thing. And obviously you need to define something if you want to go out and look for life. You need to know what you're looking for. You've got to have a functional definition. So for instance, in the 1990s emerged this thing which is now known as the NASA definition. NASA needed a definition of life if it was going to build missions that take 20 years to plan, 10 years to build, and 30 years to operate. If you're going to go looking for something, you kind of need to know what you're looking for, because you're probably not looking for the Star Trek concept of life. A bunch of semi humanoid looking bipedal aliens that also happen to speak English, it seems, okay, in every episode or at least have a universal translator that allows that to happen. Right. They all vocalize. They all use sound. They all have eyes. They all have seemingly, seemingly exactly the same senses as we do. That's a pretty narrow view of what it means to be an intelligent living organism. So the NASA definition is that life is merely a self sustained chemical system capable of undergoing Darwinian evolution. So what does that mean? It means the system can not only sustain, it can replicate, copy, manipulate the environment around it, but it has some mechanism by which it can pass along desirable traits that allow it to adapt to its environment and replicate in some way or another and sustain itself through memory effectively and replication. Okay. Now again, there are many definitions out there. So for instance, Patrick Fortair from the Institute Pasteur just says something like the following life and living processes are simply names for complex evolving forms of matter that are now present on our planet. That's pretty vague. That's pretty broad, right. So a big debate, not that long ago, maybe a century ago or so about whether or not crystals are alive, because crystals self replicate, organize, minimize their energy, adapt to their environment, right. They can fill whatever space you give them, they can grow within a space. But now it's sort of understood that crystals are not alive, are viruses alive. Okay, by the NASA definition, they're not because they can't self sustain. They don't have the internal machinery required to make copies of themselves. They have to inject their RNA into other hosts use their machinery to replicate, and then release more copies of themselves to then infest more hosts and make more copies. So but this is a lively area of debate. Alright, so I just want to tease this to you because I don't want you to walk around thinking that there's just a settled definition of what a living things. We're pretty myopic in our view. Okay. Now, the chemistry of DNA and the mechanism of heredity also mutation. These are important things. This is what allows evolution to occur. And it boils down on earth to a series of nuclear bases adenine cytosine guanine thymine and uracilin RNA. And these are the fundamental units of the base pairs that are the building blocks of genes and deep down inside make up RNA and DNA which are essential to replication, mutation, heredity and so forth. And this chemical subsystem on earth allows for traits to be passed along undesirable and desirable right. We all have the gene for sickle cell anemia. It's just a question of whether or not it's expressed or not. And you can actually show that if you expose a population to malaria over a century, that human population will naturally over express its sickle cell anemia gene, because there's a conferred survival advantage to being able to resist malaria and not die from that versus also having complications from sickle cell anemia. It is a complex and messy world out there. So in less than 100 years a human population can re evolve to re express its sickle cell anemia gene when it's needed, which I find just haunting and fascinating. Okay. Now nitrogen if you take a look at these molecular diagrams over here you see a whole lot of nitrogen hydrogen is important for the bonds that tie the base pairs together. Hydrogen bonds which fundamentally originate in dipoles which you'll learn about in second semester physics are essential to the zipping and unzipping of the DNA. But nitrogen is the backbone of this structure so you can already see nitrogen really essential to life as we understand it but also hydrogen oxygen and carbon. So thinking of carbon on earth we know from our own experience that carbon is essential to what we think of as life. It accounts for about half of all dry biomass on the planet. Okay, let that sink in for a second. Carbon compounds abound on the planet and they dominate in living systems. Okay, now why is that. And when you think about it it actually boils down to physics and chemistry. The physics is in energy and inertia. Carbon has the right number of free electrons, the right number of unoccupied places in its orbitals that it can pretty readily bond to most things and that makes it a good scaffold for molecules. That's an energy issue. Another energy issue is that carbon is in that heavy. Okay, it's only about 12 times the mass of the proton or the mass of the neutron. So it's one of the lighter elements, and that makes it easy to manipulate it has a little inertia compared to other things and I'll talk about silicon in the next lecture. So not only does it readily form chemical bonds. It has a lower mass and thus a lower inertia and enzymes after they come into existence, find it relatively easy to manipulate carbon based molecules. It's an energy in a force game. That's it. And that's why physics underpins a lot of this stuff the why of this why carbon. Why not silicon which has a very similar chemical structure, and the answer is carbons a lot lighter, it's a lot easier to move around and if you're in a cool environment like the one we find on the surface of this planet. Carbon is the lowest barrier for entry into building a scaffold for life, as we understand it. This all bears on the following question. Where did all this stuff come from. Where did carbon come from. Where did nitrogen come from. Where did iron come from all of this stuff is essential to our planet and life on this planet. And the story of where we come from as living organisms is tied directly to the story of the universe. We are inevitable in this universe. Let's begin at the beginning, the big bang 13.78 billion years ago this universe came into existence. Okay, we don't fully understand this moment. We have a pretty good idea what happened back to about a millionth of a billionth of a second after the big bang, but a lot happens in the first 10 to the negative 44 seconds of the universe. And that's a lot of stuff we don't understand yet. But here's what we do know. Once neutrons and protons and electrons came into existence electrons were basically there since the beginning protons and neutrons formed a little bit later. Once the universe cooled down so that protons and neutrons could bond through fusion. You basically seeded the formation of the atomic elements. So, nearly all the hydrogen and helium in our universe today, owes its existence to what happened in the first three minutes of the universe. And that baked in essentially all the hydrogen and helium in the universe today. That number has really not changed appreciably since the beginning of time. The hydrogen that is in your DNA and RN was forged, most likely at the beginning of time. The helium that is a byproduct of this process as well would go on to play important roles in the first stars and the deaths of the first stars. And in fact the deaths of those stars do in part to helium would see the universe in the remaining elements. So while it's true that hydrogen and helium were produced and in fact once you know basic particle and nuclear physics you can predict it a universe that formed the way ours did. What will be the ratio in this cosmos of hydrogen to helium. The helium should be one fourth of the universe by mass so you go out and you count, you do atomic spectroscopy, and you count how much hydrogens in that cloud, how much helium is in that cloud, how much hydrogens in that star, how much helium is in that star, how much hydrogen is in Jupiter, how much helium is in Jupiter, and you always get one fourth by mass helium to hydrogen. It's been baked in since the beginning of time. That is astounding. Lithium and beryllium, the first metals, they did come into existence near the beginning of time but fusion cut off very fast. There wasn't time to form anything heavier than beryllium. And so the universe would enter a neutral cold dark age, where molecular hydrogen and helium were sifting about through the universe slowly clumping under the force of gravity. 100 million years, 500 million years, something like that, they would collect into the first molecular clouds, the first cold clouds of gas that under gravity would compress and ignite into the first stars. And that's where the heavy elements came from. The first stars formed very early on, and without them, there would be no carbon, there would be no oxygen, there would be no silicon, there would be no iron in the universe. And these first stars formed from big molecular clouds. They collapsed and they were heavy. They were bright. They burned hot. They were much bigger than our sun, probably tens or hundreds of times bigger than our own son. And the problem with this is that stars operate by physics. And once you bake in the amount of hydrogen and helium and the mass of those things in a star, its fate is determined. And it will burn helium for, in the case of heavy stars, millions of years, a blink in cosmic existence. Remember, the first stars formed after about half a billion years. The universe is almost 14 billion years old. 10 million years is nothing to this universe. Okay. The first stars formed. They began to cluster into the first galaxies around that time as well. They live fast, and they died young. And when they died, they died spectacularly. They burned through their hydrogen, leaving helium in their core. The star begins to compress as the hydrogen runs out, which ignites helium fusion. And then they start forming things like beryllium again, carbon, oxygen, but there's not a lot of helium in these stars. So maybe in one tenth of the time it turned them took them to burn their hydrogen, they burn through their helium. And if they're not heavy enough at this point, that's where it stops. And now you have carbon and oxygen and nitrogen and all these other things. And it keeps going on. If they start burning carbon and eventually get up to iron, that's where it finally stops. That's burning iron is the desperate end of all stars. If you're forced to burn iron, it's like taking the remnants of your campfire and trying desperately to light it on fire to stay warm. You've run out of options at that point. It's not energy efficient to ignite ash on fire. Okay. And similarly, when you have iron ash built up in the core, and that's all you have left to burn, you have nothing left to burn. And so the star compresses and explodes, and it sprays all the heavy elements that it's made out into the cosmos. Those explosions can collapse nearby molecular clouds, reigniting star formation in those regions. Meanwhile, seeding the universe and heavy elements, iron, carbon, oxygen, nitrogen, these were forged in the first stars. That's the story of our cosmos. That's where the building blocks for life came from. They came from the life and death of stars in the very first billion years of the universe. And I love this quote from Carl Sagan, astrophysicist and science communicator who passed away about 30 years ago now. And in his famous series cosmos which was rebooted in the arts, maybe the teens of this century. He said the nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of star stuff. The thing that's original in our bodies to the big bang is the hydrogen. The rest of it had to wait for stars to die. Another way of putting it in a modern context is from the band modest mouse. We are between we are between. Yeah, we are somewhere between dust and the stars, the dust that was blown out in these explosions eventually re collapsed into new stars. And some of that was leftover informed planets and down the long chain of history here we are. Okay, we are we are made of old rocks and salt. And that's not wrong. That's my interpretation of the song. It's an indie anthem these days enjoy you can interpret it any way you want. Okay. So, let's take a look at a timeline of earth and life on earth. So, the time here is in giga years, billions of years. Okay, because that's the currency of the universe, a century for a human life barely shows up and I mean I won't show up in any of these decimal places. Okay. All right, so you need big time to think about the cosmos. So the big bang happened at zero time. Prior to now that would be 13.787 give or take a little bit billion years. Okay, that number is measured from various astronomical observations. That's the uncertainty on the age of the universe. It used to be a factor of two. When I was a kid it was a factor of two sciences come a long way in 30 years. Okay. The galaxies appeared about point two to point five 200 million to 500 million years after the big bang. That's about 13.3 to 13.6 billion years before now. So this column is always since the big bang, and this is before now. So if we think that now is our zero time we can go backward. Right. So the galaxy's core are sort of urban center in our galaxy that formed about 800 million years after the big bang or about 13 billion years ago. We're kind of where we live in the outer spiral arms that actually formed later. The Milky Way collided with another galaxy Gaia Enceladus something we only learned about recently and absorbed it. And formed the outer spiral arms of the Milky Way. That happened about 2.8 to 5.8 billion years after the big bang or about 7 to 11 billion years ago. The sun our star was born about 8.8 billion years after the beginning of time. Think about how many generations of heavy dying stars happened, right they all lived roughly 10 to 100 to maybe 1000 million years tops, and then seeded the space around us and the heavy elements which eventually then collapsed to form our sun and our planets. That all happened 8.8 billion years after the beginning of time or just about 5 billion years ago. The earth formed around the same time as the sun so the sun bursts into existence, spraying radiation out into this corner of space. Light elements hydrogen and helium too close to the star are blown away. The heavy elements iron, nickel, carbon, oxygen, they remain closer to the parent star. And so it's no accident that in at least in our solar system, we find rocky worlds close to the sun and gaseous worlds far from the sun. Those planets were formed from whatever was abundant and collected in the regions of the solar system where they now exist. It's also possible that planets have moved around in the in the history of the solar system. For the most part where we are is roughly where we've been since we formed. So, the earth formed about 500 million years after the sun so about four and a half billion years ago, and at that time it was molten and uninhabitable think of rock and dust slowly clumping together in space under gravity getting bigger and bigger. The gravity gets stronger and it compresses and as it compresses it heats and the rock becomes molten and liquid. This was not a hospitable place to live this was quite literally hell on earth. But something like 200 to 400 million years after our planet formed as a molten ball. It does appear that the first life as we understand it appeared which suggests that the crust settled down. It might still have been being bombarded by things. And there was a lot of carbon dioxide in the atmosphere that had to be drawn out and that was done by carbon chemistry and the rock. What liquid water could finally settle down on the surface of the planet when it wasn't molten anymore and being vaporized by by this hot magma. So what are collected and then multi cellular life. So life appeared multi cellular life appeared only about one and a half billion years ago. It's a many billions of years after the earliest simplest forms of life appeared on the planet. Modern humans appeared only about 200,000 years ago. But how remarkable is it, how young a species we are, how much we've learned. I find that fantastic. So, if the earth is habitable. Are there other worlds that are habitable. What makes a habitable world. And that's the part of the of the net that's the next part of this lecture, because clearly, things worked out on earth. It was a ball of magma, and then it wasn't. It was a carbon dioxide rich atmosphere. Chemistry took care of that water settled down liquid solid gas, and somewhere in all of that mix, what we now think of as life emerged and relatively quickly, after the conditions, sort of basically what we what we experienced now ish on this planet had settled into what we kind of see at this point. I mean I'm sure it wasn't pleasant back then in the early earth, but it wasn't as bad as when it was molten rock. Okay, so are there other places in the universe where we might find life as we understand it, forming under similar conditions. So let's go back to the idea of what's known as a habitable zone around a star or the Goldilocks zone for those of you unfamiliar with the fairy tale of Goldilocks. It's about a young girl that sneaks into a into a house owned by three bears. Find some porridge. Alright, so the father's porridge is too hot so she's going to taste it and spits it out. The mother's porridge is too cold she tastes and spits it out. The baby's porridge is just right. The father's bed is too hard, the mother's bed is too soft, the baby's bed is just right. It's the just right zone. Okay, so not too hot, not too cold, just right. That's the Goldilocks or habitable zone. What do we mean by just right? Just right so that water stays put in any of its various phases. That's kind of what we're talking about because water is the essential solvent in which life as we understand it formed. Water is a fantastic solvent. All kinds of chemicals can be dissolved in it. Reactions can take place in it. I'll talk about silicon based life tomorrow and the solvent that might be required for that to occur. It's not unreasonable and it does exist in our own solar system in large amounts. But water is a nice solvent. I mean any kind of still tell you it's pretty good for dissolving things, right? Not everything but many things. Okay, so of course different stars are going to have different habitable zones. A star that's bigger and hotter than ours, you're not going to want to be this close to it. You're going to need to be further away for water to be liquid solid and gas and stay put on the planet. For a planet that's for a star that's smaller than ours is so-called dwarf star, like a red dwarf, the smallest and most compact and oldest of all stars in the universe. They can live 100 billion years before they die. Our sun will only live 10 billion years. Red dwarf stars can live 100 billion years. In other words, the red dwarfs that formed near the beginning of time are still burning today. Their habitable zones are much more compact because they don't burn as hot. That's why they live longer. They're a little cooler to take it easy. They're coasting. The big stars, they're the ones that live fast and die young. Our star is kind of in the middle somewhere. It's going to live about 10 billion years. It's got 5 billion left. Okay. So these habitable zones depend on the star that you're in. How would we find planets in habitable zones around other stars? We've been cataloging stars for centuries with greater and greater accuracy and precision in the last 100 or so years. Although ancient astronomers were pretty damn good at what they did. They just didn't have the technology yet to do it at the speed, rate and precision we can do it. This is building on the shoulders of giants. How do we find planets going around those stars? Well, there are four ways. You can directly go on a telescope and look. Do I see a planet or not? Direct observation. You can take advantage of gravity and center of mass, something called the wobble method. You can look for the star to dim periodically as if something's going in front of it between us and the star. That's the transit method. I'm not going to talk about gravitational microlensing, but you can take advantage of this feature of space time that it can warp. The planets can warp space time and you can look for microscopic effects of the warping of space time from changes in the star. I'm not going to get into that. That is a way that planets have been discovered, but I won't talk about it. That's a little beyond the scope of our course. Direct observation is the easiest one. The problem is stars are so damn bright. If you look at a star with the telescope, it dominates. It dominates because it's really small. Even Alpha Centauri, which is four light years away, is just going to look like a bright, starry point of light in a telescope. Those sort of starry lines, that's just an optical effect. In truth, that star should appear as a perfectly bright point, but the limits of optics make it stretch out in your telescope. What you need to do is you need to find a way to subtract the star so you can literally mask over the star with something small, or you can image the star in multiple wavelengths of light and subtract them from one another, deduct the star from the picture, and then see what's left. That's what these researchers did. Around this star, HR8799, they actually found three planets. There it is, B, C, and D. We label the planets with letters. The star is always A. The planets are always B, C, D, E, F, G, H, I, J, etc. This is an A-type star. It's a lot hotter and a lot bigger than our star. So planets in its habitable zones would be really far away from the star. That's how we're able to see them. For a sun-like star, this technique really doesn't work. Their habitable zone is much closer to the star, and we haven't really found a reliable way to directly observe planets in those habitable zones. They're too close to the parent star to play games like this. This works with big stars where the habitable zones are multiples of the orbit of Jupiter away from the star itself, really far away, so that you can block the central star and then hope to see the worlds around it. That's exactly what was done right here. Great, that's my cue. Another way you can do this is you can use what's called the wobble method. When I kind of illustrated this a little bit in class, when I talked about center of mass and gravity, we orbit the sun, but the sun is wobbling as we go around it because we're actually co-orbiting a common center of mass. Now, you don't notice it. You don't see the sun getting closer and further away, but in fact it does by a little bit, and if you're really good at astronomy, you can actually measure it. We've done this with other stars, and the way we do it is as a planet goes around its parent sun and the sun goes around their common center of mass, it gets a little closer and a little further away from us during the planetary orbit. So our sun technically wobbles as a result of our orbit once a year, back and forth, one oscillatory cycle, one sinusoid, one cosine. Now, how do we see this? We can't actually see the planet. We're looking at the star. How do we know it's moving back and forth? And we use something called the Doppler effect. So just like when a siren on an emergency vehicle is approaching you and it gets higher and higher and higher in pitch, but then as it moves away from you, as it goes past you and moves away, it gets lower and lower and lower in pitch, light does that too. It gets bluer as it approaches you and it gets redder as it recedes from you. So as the sun, as that star wobbles toward us, we see bluer light from it, and as it moves away from us, we see redder light from the same star. So we just look for these red-blue wobbles in the star, and it's actually a fantastic way of seeing this. Many worlds have been discovered, not directly, but so-called indirectly, around stars using this method. The other method is the partial eclipse, the transit. Mercury and Venus transit across the sun every now and then. If you have a telescope with a solar filter, you can actually look at the sun. Don't do it with the unhated eye, burn your retinas. But there are solar filters that you can put on telescopes and you can look at the sun and you can watch Mercury pass in front of the sun, in between us and the sun. Venus too, much bigger, easier to see. When a planet passes in front of its star, it dims it, right, blocks some of the light. So all you have to do is look for the light of a star to dim and then wait. Sometimes stars dim all on their own. They don't need our help, okay? But the transit method, just you have to be patient. You have to wait for the planet to go around a few times, okay? And this is especially easy in small dwarf stars. The planets orbit really close. They go really fast around the parent star, and they transit frequently. So you get lots of periods of the oscillation that you can observe with your telescope. Okay, so you're just looking for the light from the star to be some amount, some average output. And then at some time when the planet goes between you and the star, you'll see it begin to dim, boom. Okay, it's not as sharp as this usually, and then it brightens again when the planet's done going in front of the star, and then you get that average light back again. That's the transit method. When you have multiple planets along the line of sight, you have to do some fancier statistical techniques to deconvolve these effects, but you can see many worlds going around planets using the transit method. In fact, there's a really famous one I'm going to show you at the end of the lecture today. So these are so-called exoplanets. These are planets not in our own solar system, but orbiting a different star. And one of the key missions that just exploded our knowledge of exoplanets in the universe was the Kepler mission, which is no longer operating. I think it stopped taking data in about 2008, 2009. There's another satellite now, TESS, the Transiting Exoplanet Survey Satellite. That one is used now to look for transiting planets around stars. And I just want to show you how many planets this thing has discovered. Every dot, every blue dot on here is a planet of a certain size with a certain orbital period observed by Kepler using the transit method. A total of 4,034 candidates, 2,335 have been verified as definitely planets, not some other effects. The yellow dots are just the ones that came out in the 2017 Planet Counting Catalog. They do periodic releases of catalogs as they observe new planets. The data is still there and you can still look for it. You can go planet hunting in this data if you want to. We've discovered a whole lot of Earth-like planets. Earth-like planets would be about here and there's tons of them. There are Neptune-sized planets and Jupiter-sized planets, but you can see there's a big collection of planets kind of around the size of Earth. These may be small gas worlds, but they're most likely rocky planets. There are actually ways of seeing whether or not they have atmospheres. And if you're curious about that, you can ask during the Q&A. Here's the Hall of Fame from the Kepler mission. These are small habitable zone planets about the size of Earth, roughly. There's maybe slightly super-Earths in many cases. They're all various distances, so 62E. We're 62E. 62E right there, pretty close in size to Earth, a little bit bigger. About 1200 years at the speed of light away. The closest one in this catalog would be, I guess, 186F. Yeah, it looks like 186F, which is right here, very close to us, very similar in size. It's in the constellation Cygnus. It's in that direction. And it's only about 580 years at the speed of light away. Now, the closest star with planets is Proxima Centauri, four light years away, four years at the speed of light. And it definitely has a super-Earth going around it, Proxima B. That star is complicated, like many other dwarf stars. Proxima is a dwarf star, and it's kind of a nasty environment around those stars. So it may or may not be habitable, we don't know. That's going to take more time to figure out. Habitable as we understand it. Okay, so I want to show you one of my favorites here, which is the trappist system. So the trappist system, the trappist star, okay, it's named after the telescope that observed it, the trappist instrument. And it is a red dwarf star. It's habitable zone is a whole, very close to the star itself. These planets orbit much faster than the Earth does around our own sun. So a whole bunch of planets, roughly in the habitable zone of this one star, were discovered about four or five years ago now. It's about 40 light years from Earth, okay. And the way it was discovered, again, as you look at the brightness, these are brightness measurements, and you look for dips, dips, dips, dips, and you wait and you see is there a periodicity to these dips and their intensities. The intensity tells you the size of the planet. The depth of the dip tells you the size of the planet, how much light it is from the parent's star. The frequency is just counting the gap in time between the dips and inverting it to get the frequency, okay. It's actually relatively straightforward to see this once you know the recipe, right. So the dip tells you the size, big dips, big planets, small dips, small planets, okay, they don't occlude as much of the star. And you just look at the average and then look for dips from that, right, and then count the frequency, and then what's the, look for regular periodicity between the same dips and the data. So a whole bunch of worlds were discovered this way. Now, here's where intro physics comes into play here, okay. A quick intro physics tie in. You can actually, you know, having gotten the period of the orbit from that data, you can actually estimate the radius of the orbit, very simply using the law of gravity, Newton's second law, and rotational motion. So F equals MA, I hope that's okay by now for all of you. So F is gravity, it's the mass of the star pulling on the mass of the planet little M at some separation R and this assumes a circular orbit but you'll see it's a pretty darn good approximation. The acceleration is centripetal, our old friend from the beginning of the class V squared over R, there are orbital radius. In rotational speed, we can rewrite that as omega squared, the angular speed squared times the radius, you can work that out and see that that works out but that's what you get. Now you just solve. So now you just go ahead and shove that into F equals MA, do a little manipulation. Oh, the mass of the planet cancels on both sides, that's cool. Now in real orbital mechanics, there are some corrections to this that where that doesn't quite work out, but this is good enough. And then, oh great, well, omega, omega is the angular speed, that's just too pie over the period, I measured the period using my telescope. No problem, so now I can relate the period and the orbit radius the only thing you need to know is what's the mass of the star. And astronomers have been able to figure out masses of stars from the color of their light, there's a relationship between those things, color begets temperature, temperature begets mass, those are all baked into a star, it's inevitable for stars, that the mass determines the temperature determines the color of the star. So we know that the trappist one star is an ultra cool red dwarf, it's color category is what's known as M, we're a G type star, M is a smaller than us, it's the smallest type, it has a surface temperature of 2600 Kelvin, and its mass is just about 9% out of our own sun, that's just from astronomy, that's an independent measurement. This equation right here is known as Kepler's third law of planetary motion, and it applies to circular orbits. So let's plug in some numbers, we can take trappist one F, it has a period that's observed from the data of 4.049959 days, that is precise to the last decimal place there, okay, that's how good these satellites are at measuring time and periods from these occlusions, these transits. Fine, plug it in, crunch the numbers and you find out that the orbital radius of that planet is about 3 billion meters. In terms of what's known as astronomical units, one AU is the distance between our earth and our sun, one AU, okay about 93 million miles, that's 0.0223 AU, the accepted measurement from astronomical methods for this is 0.02227 plus or minus 0.00019 AU, that ain't bad for estimating from intro physics, okay. You are more powerful than you think you are at this point. You are like little gods who with just a few numbers can actually calculate a whole bunch of things about the universe without ever touching a planet in the trappist system. The trappist star is about 8 billion years old. And it's 40 light years away. And the planets that orbit it, let me go back, well I'm not going to go back to it, the planets that, actually this is good here, the planets that orbit it are what are known as phase locked, like our moon. The same side of the moon always faces down on the surface of the earth, okay. There is no such thing as the dark side of the moon, the moon goes around the earth once a day. It's backside gets lit by the sun, okay. It's only dark to us because we don't see it. The same side of the moon is always facing down on us because we're tidally phase locked. It's actually an inevitable consequence of orbital mechanics. It's more common than you think it would be. The planets around the trappist system go in these resonances. You can look at the periods of their orbits and the ratio of the periods and they are phase locked to their parent star. The same side always faces the star. The same side always faces the star. So it's a roasty toasty on the sun side, freezing cold on the dark side because it never gets sunlight on that side. But in the transition zone between the two, that's where ice and gas and liquid water could exist. So in the transition zones between night and day, you go 20 miles this way. It's day. You go 40 miles back. It's night and right in that region and that strip of dark is the zone. Life as we know it could have emerged. So that's the question I'm going to leave you with. This is a very volatile star. In fact, this is what an artist's conception of what a view would look like from the trappist 1G, I think, planet. You'd see all your sister planets in the sky. These orbits are days. So things move around in the sky a lot. It's straight out of sci-fi. And because these transition zones would have the right temperature at this distance, it could have the right temperature at this distance. So these planets are in the Goldilocks. They're in the habitable zone of this star. If there's water there, it could look like this. And the sky would be this. We don't know what the chemistry of the atmosphere is there, but it could look reddish orange because of the light of this star. This is pretty much what it would look like unless there's interesting chemistry in the atmosphere of these planets. And we can actually work out the chemistry of the atmosphere, believe it or not. So I'm going to leave you with that. I'm going to leave it here for four minutes. Here are two minutes. Any questions? Don't pack up yet. Pause. Ask questions. Savor the moment. Okay. Any questions? Must have something. Milly's got a question. She's just hiding it. I can see it. And a yawn. It's okay. I'm right there with you. Oh, yeah. What's up? Yeah. Yeah. So it's sort of a conspiracy of two things. One, based on the thermodynamics of an early Rocky magma world, how long would it have taken to cool and for carbon dioxide by carbon chemistry to be pulled out of its young atmosphere? Because it would have been a carbon dioxide rich young atmosphere. So once that CO2 gets pulled down into the rock in carbon chemistry, you get, you know, all that sort of like the white clips of Dover kind of rock that's forming, right? That's where a lot of carbon is trapped is in rock formations like that. Then you've got a more hospitable atmospheric environment for the earliest, simplest kinds of single celled life to start emerging. And the evidence of their existence in the history of our planet goes back to about the same roughly few millions of years where the planet would have probably just cooled and drew its carbon dioxide down. At that moment, it could have been hospitable. And it seems like the records of life's existence in the geological record where we can date it back that old. There's evidence that there were living things at that point. They were simple living things. They weren't plants. Plants wouldn't evolve until much later, right? And then take advantage of carbon dioxide and then produce oxygen, right? And so forth. We rely on them heavily today. So that's what it means is sort of the thermodynamics of a cooling planet and carbon chemistry lines up roughly where in the geological record, we see evidence of the earliest forms of life to within our ability to actually measure those things accurately. And it emerged around 3.8 to 3.9 billion years ago. So certainly within the first few hundred million years of the planet. I mean, after the planet came into existence about four and a half billion years ago as a body, there was a bombardment at that stage. The evidence of that bombardment is not left because it was bombarding a liquid rock world and it absorbed those blows and then took that rock for itself. But that bombardment period ended around 3.8 billion years. Boy, this thing loves to steal the focus. And as a result of that, it's around that time that the evidence of life emerges on this planet. I mean, it's really quite remarkably fast. And when we get to Drake equation, the probability that intelligent life exists elsewhere in the universe, this is something we factor in. I mean, the probability of life is could be one on a planet like this. It could just be inevitable. If you've got carbon, oxygen, nitrogen, hydrogen, it's good to go. Like it's just waiting for the temperature to get to the right place. But that's not something that's going to be very, very difficult to do. But there's a lot of things that are kind of, you know, unsolvable and that are a lot of things that are not solvent on the surface for those molecules kind of do their thing. Right? I will say though, you know, the study of the origin of life is still fraught with mystery. You know, there's a famous experiment from like the 1950s or 60s where somebody sparked all the stuff that they thought was in the early atmosphere and they got some amino acids and that's cute. the origin of life because a lot of people have come in and mucked that up scientifically. Funding agencies are a little hesitant to hand money out for that because they're afraid of getting caught up in some kind of political nightmare. I just want you to be aware it's a legitimate area of scientific study but because it intersects in this boundary of the general public sort of interest and ire sometimes it's fraught with peril and I just want your eyes open on that. That's all. There's no reason not to go into it. It's just a reason to be brave and be bold. Okay. All right. You've been a very excellent audience and I'll see you tomorrow for the last day of class.