Welcome, so today we are going to talk about what makes a habitable planet? What are the conditions, the star, the stellar properties, the planetary properties that makes a planet a place that life can evolve and grow and thrive? We going to look at this problem in a couple of different ways. So, we're going to begin by talking about the Faint Sun problem, talk about how the evolution of the Sun through time affects the Earth, then come back to the theme we talked about earlier, when we discussed Earth, Venus, and Mars. Is the Earth just right? Or is there a wide range of environments in which life might be able to thrive? And then, you, once we've discussed these to find the habitable zone, to find the range of distances from a star where we think it's possible for life to evolve and thrive. So let me begin by reviewing some of the things we talked about earlier in the course. What sets the average temperature of a planet? The planet's temperature will depend upon the temperature of the star, the star's size, the distance the planet is from the star, of course, the further away the planet is from the star, the colder the planet is. We'll look at, as we start talking about the habitable zone, the trade off between the stellar temperature, size, and the distance, so, if you have a more massive star, it's going to be bigger and hotter. So, the habitable zone, the zone in which you can have liquid water, will be further out when you have a more massive star. On, the other hand, as we'll see, when you have a smaller star, a low mass star like an M dwarf, the habitable zone will be quite far in. The star, the planets average temperature, will also be determined by the planetary properties. What is the albido of the planet, if it's covered by a high layer of clouds that are very reflective and bounce back much of the light from the star. Then that will lower this temperature at the surface, because most of the insulation from the star will not get to the surface. So, we are affected by the planetary albedo and we're also affected by the optical depth of the planet to re-emit the amplitude of the greenhouse effect. The more carbon dioxide, water vapor, methane, ammonia, the more gasses we add to the atmosphere, that absorb infrared radiation, that will serve effectively as a blanket for the planet, as we've discussed, and keep the surface warmer. So, we need to know both the internal properties of the planet, and properties of the solar system, to determine the planter's average temperature. And of course this is just the average temperature, as we know from our own planet, it's a lot harder at the equator, a lot colder in the Pole. And on any planet there'll be a range of temperatures and perhaps, some environment in which life can thrive and others which are either too hot or too cold. So now let's turn to Sun, talk a bit about stellar evolution and the history of our Sun and how its properties have evolved in time. Back when early life formed, we think that the Sun was only about 70% of its current luminosity. And as this plot shows, this is the Suns luminosity relative to today, so this is it's luminosity today. Here we are today, as we look at what we expect versus time, we expect that the Sun has become hotter, and more luminous with time. So in the past, the Sun was much less luminous, that means that the Earth got much less energy from the Sun. If the basic properties of the Earth stayed the same, four billion years ago, we would have expected the Earth's temperature to be 20 degrees colder, 20 degrees Centigrade colder. That's a big difference, if it was that much colder, most of the water would freeze, we would enter snowball phase, and if we keep the Earth's properties the same, the fact that the Sun was so faint in the past would imply that through most of the early history, of our planet the Earth was a frozen snowball. And while we know that the Earth has undergone several brief snowball phases, in it's history, the geological record implies that the early Earth was actually quite warm. How do we reconcile the two? This is what we call the faint Sun problem, this is an old problem. Back in 1958 Martin Schwarzschild, one of my mentors here at Princeton, ask the question of paper when he first worked out the evolutionary properties of a star like our Sun. Back in the 1950's and 60's astrophysicist developed much of our current understanding of stellar evolution. And when Schwarzschild realized, that the Sun would evolve like this, he asked the question back then in a paper. Can this change in the brightness of the Sun some geophysical or geological consequences that might be detectable? And for a long time, astrophysicists ignored this problem, because they thought perhaps we didn't understand enough about the Sun's properties. To predict its long term evolution. And one of the causes of their doubts was something called the solar neutrino problem. The Sun, when it burns hydrogen to helium, produces neutrinos, and experimentalists did not see the predicted number of neutrinos from the Sun. It was only later work on the neutrino properties, confirmed by further physics experiments, that offered an explanation for what was going on in the Sun's interior, and once the solar neutrino problem was solved, we had some confidence in our stellar models. And I remember being at a talk here in Princeton, where John MCall one of the great pioneers in understanding the solar nutria problem, Identifying it and pursing it over his career. Got up and explained how we now had a solution to this problem. We now understood the fault line not, in the Sun, but in the neutrino, and the old solar models were right, and Martin Schwarzschild, then in his late 70's, stood up and said, we'd better start paying attention to the faint Sun problem. And in many ways, we should have started, some people recognize that we needed to do this earlier, that Martin was right back in 58. And in fact if we look at stellar evolution, this evolution of the Sun is just an inevitable consequence, of converting hydrogen to helium. As the Sun's core converts hydrogen to helium, the mean density of the Sun goes up, because the mean density goes up. The average temperature of the Sun, or its central temperature and surface temperature, also must rise in order to have pressure balance gravity. So just hydrostatic equilibrium, and the process of converting hydrogen to helium inevitably leads to this rise in temperature versus time, and this rise, by the way, is going to continue. If we extrapolate to the future, in another say, two billion years, the Sun's temperature will be roughly 15% higher, and the Earth will be hotter. So why wasn't the early Earth frozen? As I mentioned already, if the conditions stayed the same, the Earth would be a lot colder, yet the geological record doesn't show a frozen Earth. The solution that a number of scientists have offered going back to early work by Sagan, is the idea that there were substantially more greenhouse gases. So let's go back to our temperature equation here, the, if the stellar temperature was higher and lower in the past, Stellar radius was smaller in the past, the way we could balance these terms is to make this term bigger. So if these terms gets smaller, if we can make this term also smaller the two terms can balance. So if we could and if the Earth in the past, had more greenhouse gasses, then greenhouse gasses would make the planet warmer, and the balance would be such that the temperature would stay the same. And, perhaps we've been very fortunate, and that, as the Earth evolved, as the Sun's temperature rose with time, the level of greenhouse gases dropped with time. And the two trends worked in directions that nearly cancelled each other, just keeping our planet's temperature, close to a constant livable value, across the last four billion years. Well, what can those greenhouse gases be? The first proposal put forward, back in the 70's was ammonia back in the 70's, many geologists thought that Earth's early atmosphere was reducing. They thought that the Earth's core formed slowly, and then back 3 to 4 billion years ago, the Earth's the iron in the Earth had not, sunk to the cooling core, but the iron was interacting with the atmosphere. And the Earth has a lot of iron, and as you probably all know, iron interacts with oxygen to make things like rust, and that iron would remove much of the oxygen from the early Earth's atmosphere. We would thus have a reducing atmosphere that was dominated by hydrogen, and ammonia would be quite common. Now, based on the geochemical evidence, the fossil record looking at the composition of the Earth's atmosphere three, four billion years ago, suggests that the core formed rapidly, and does not show any evidence for a reducing atmosphere. And in fact when we think about the interactions of ammonia, which the early Earth's ocean, most of the ammonia in the early Earth's atmosphere would have been destroyed. And we've now pretty much eliminated ammonia as a plausible greenhouse gas. Another candidate is methane, a methane's a very effective greenhouse gas, in fact, methane's a more effective greenhouse gas per unit mass, than carbon dioxide. And methane is fairly hard to destroy, but if we had a thick methane atmosphere, that methane would form a high haze and the methane would interact with ultra violet radiation from the Sun. And the early Sun had more ultra violet radiation even though it was cooler because it was a more active star probably was rotating faster. That methane in the upper atmosphere would form a kind of smog, a kind of haze and that smog would be highly reflective. So, while methane would increase the optical depth, making it warmer, that high haze On top of our atmosphere would increase our albedo and make the Earth cooler. So methane alone doesn't seem to be a solution to this faint Sun problem. The next candidate's carbon dioxide, and you know, carbon dioxide is a pretty effective greenhouse gas. But, in order to produce a 20-degree Centigrade temperature rise, we need a lot of carbon dioxide. It would have to contribute about 30% of the current atmospheric pressure. That's a thousand times the pre-industrial value. So, if this is in fact the solution, then the greenhouse level, gas level, or carbon dioxide gas level in the Earths distance past was far higher than today. Though back then of course we wanted it more because the Sun was fainter. One of the attractive features of carbon dioxide is there's a very natural cycle by which the carbon dioxide could have been high in the past and then through silicate weathering through the carbon cycle, something we discussed earlier the carbon dioxide would be drawn out of the atmosphere enter into the ocean redeposited into carbonaceous rocks, and carbonates in rocks and the carbon dioxide would decrease in time. There are some objections though to his hypothesis, the current best geological record seems to suggest we look at rocks and the fact that we see rocks like siderite FeCO3, that suggests an abundance of carbon dioxide. In the early atmosphere, less than 0.03 bars, or 3% of the current atmospheric pressure. Not enough to be an effective greenhouse gas at the level needed to keep the early Earth warm, when the Sun was producing only 30%, 30% less than its current energy levels. Perhaps at this point what it, might be the most attractive solution, is a combination of these, that the early Earth's atmosphere had both more, excess carbon dioxide, and excess methane. The combination would not produce this high pace, you'd have significant contributions to Greenhouse warming by both gases and it seems that the combination of the two might have been just right to keep the Earth's temperature nearly constant. So, let me ask you know to consider and go back to another possible solution. To the Faint Sun problem. When we've talked about the Faint Sun problem, we've assumed that the Sun's properties had been constant with time, that the Sun, today, has the same mass as it did in the past. I'd like you to go back, either look back at your notes on stellar evolution, or look at some of the notes provided on the Coursera page, and you'll see the information you need to answer this question, about the evolution of the Sun. So why don't you look at that, and then we'll come back and talk some more.
