Today, we're going to talk about the nebular hypothesis and sketch out our basis picture of planet formation. And the basic idea in planet and star formation rests on conservation of angular momentum. As we discussed in the previous section, if we have rotating gas and dust, as it collapses down, it's going to form a disk. And this picture here shows what an early solar system looks like. A central protostar, accreting gas and dust from the disk, and a disk filled with gas, mostly hydrogen, and dust composed of carbon, silicon and some of the heavier elements that settle out and form these tiny grains. So, material flows along here. The star radiates and the star light, particularly the ultraviolet and far violet light from the star goes and heats up the disk and evaporates the edges of the disk. As the disk evolves, the dust grains start to collide and condense, and form larger grains and planetesimals. As these heavier objects start to form, they settle towards the disk. Eventually, the star light will evaporate the inner portion of the disk. And eventually, the whole disk is a hole, leaving behind a debris disk. This whole stage here last about 10 million years. Remember that our solar system is four and a half billion years old. So, this formation timescale is much shorter than the age of the solar system. Here again are the basic steps in the process. The dust settling to the mid-plane. The dusk range merging to form planetesimals. The planetesimals merging to form planets and the planets themselves accumulating mass around them. Now, as you look at this picture, you can already see that one of the aspects of this is there's going to be many collisions and mergers taking place, because planets are built up of smaller units. We say this when we talked Earth and Mars. We think that the Earth formed when, and the Moon formed when a Mars-sized rock came in, hit the Earth and then sent off the Moon. We saw, when we looked at Venus, that Venus's orientation was tilted, probably due to a late-time collision. With other, sub-units building up the form of the inner solar system. So, the basic picture is building things up from very small units. We start with dust grain that are micron size. These dust grains stick together and coagulate. Building up bigger and bigger systems. Once we start to get structures that are on the kilometer scale, they become massive enough that they attract other material by their own gravity. And finally, if the system gets massive enough, say ten times the mass of the Earth, it's able to not only attract lots of dust and smaller planetesimals, but also attract gas from the region around it. And a planet like Jupiter accumulates a great deal of gas around it's hard, rocky core. So, this gas accretion phase is the final stage of growth. In this picture of planet information, and during this gas accretion phase is how, when you build up planets like Jupiter, Saturn, Neptune and Uranus, that have a lot of gas around their rocky cores. We know this stage must happen relatively quickly. Because the gas disks are short lived. Gas disks live only about 10 million years before they evaporate. Now, how do we know this? How do we know that gas disks are short lived? What we can do is observe lots of other forming systems. And we could basically count how much time a star spans at a given stage by counting up the number of systems like this. I think of this as, you're an alien landing on the Earth, and you want to know what is the evolutionary phases of a human? And you simply count the number of people you find of different types. You'll notice that there are relatively few infants on the planet, that for every adult you see, you find, can say 30 adults for every infant. From that you conclude that people spend about 30 times as long as an adult, as they do in the infant phase. And if you count up the number of toddlers, say, we'll call a toddler someone between ages one and two and an infant someone between ages zero and one, you'll notice that you find roughly equal numbers of infants and toddlers. You can do this by just observing the planet for one minute. Just a very quick survey. If you're an alien with a very effective observing system, counting up all the, the people you see. You notice equal numbers of infants and toddlers. That tells you you spend equal amounts of time in the infant phase and the toddler phase. But if you notice that, let's classify children as ages two to ten, that there are eight times as many children as infants, even without knowing how an individual child evolves, just by assuming that the population of humans is not changing a great deal over time. Approximation actually for humans isn't great, because the human population is growing. But just based on the ratio of children to toddlers to infants, you could conclude how much time we spend in each stage. Well, that's what we do as astronomers. We count the number of systems we see that have disks like this photo-evaporating disks, debris disks, ordinary stars. By measuring the number in each stage, we can figure out how much time a planet spends, are, a planetary system, spends in each stage in its evolution. And we know that these first stage is here, are relatively brief. They last only about 10 million years. And since Jupiter's and Saturn's and Neptune's must accumulate all of their gas during that early stage. We know that this planet formation process must be relatively rapid. And we also know that during this formation process not only do we need to form Jupiter, but we need to form moons like Europa and Titan. From disks that form around each of the forming Jovian planets. Another important concept that affects our understanding of planet formation is the idea of a snow line. As the planetary system forms, the inner parts are hot, the outer parts are cold. The inner parts are going to be close to the star while the outer parts are further away. Let's consider water. In the inner parts of the Solar System, when it was forming and it was very hot, the water is going to be in a gas or liquid phase. In the outer portions, it's going to be in a solid phase. If it's in a solid phase, it could condense on the dust grains. And when we think about things like comets, we're looking at objects that are made mostly of ice. These are objects that formed in the outer portions of the disk where it was cold. So, to go back to this picture here. Here's our forming disk, near the sun we're going to have rocky particles because it's too hot for the water to form ice. Out beyond the snow line, we will have icy particles because out here, it's cold enough that the water condenses and freezes on the dust grains and enshrouds them with ice. As the system evolves, the rocky particles form rocky planetesimals, then rocky protoplanets, then rocky planets. And then, in the case of our solar system, we think these icy particles formed icy planetesimals, icy protoplanets. And these icy protoplanets, in the case of Jupiter and Saturn, then accumulate a great deal of hydrogen and helium gas from the forming disk. And for Uranus and Neptune, these icy protoplanets merged to make the ice giants. So that this led to our current form of the solar system. With planets in the outer solar system containing a lot of water, because they're beyond the snow line. But planets in the inner solar system, being relatively dry. That's because the water in this inner solar system was in the gas phase, and when the solar, the sun photo evaporated the disk, the gas in this inner region was lost. And when the gas was lost, the water was lost. As a result, the inner planets are very dry. Now, you might say, wait. The Earth is covered with water. But that's only a very thin layer. You think about what the Earth looks like. Most of its core is fairly dry. And it's covered with a relatively thin skin of water. I think of it being like an apple where you have a thin apple skin that's covered with water and the core is solid and made of carbon and silicon and iron. And in fact, we think that a lot of the water on Earth might have even come from the outer solar system. That some of the water on Earth and certainly the ice we've seen on the poles of Mercury come when comets come in and strike the inner solar system. This basic theoretical model, where we have water vapor forming in the outer regions and condensing to form ice, has been confirmed by our observations of young forming stellar regions. And here's some observations from the Subaru telescope of a nearby protostar with the musical name HD142527. These are observations with the coronagraph. And you can see the coronagraph has blocked the central star so that we could observe the protoplanetary disk around it. So, we're blocking the star. And observing the forming protoplanetary disk. And what is seen here is the surface brightness of this disk, what the astronomers did was they took a spectrum of this portion of the disk corresponding to a region out here. When they took the spectrum, they saw an absorption line due to ice. And they were able to detect the presence of ice in the outer portions of the disk and these same observations show that the inner portions of the disk were ice free. This confirms, in this young-forming system, our basic picture of a snow line, where water only condenses in the outer portions of the system. Now, our picture of the early solar system is confirmed somewhat by looking at fossils of the early solar system. Things like asteroids and comets are remnants of our early solar system's history. When we look at asteroids in the asteroid belt between Mars and Jupiter. They tend to look like rocky systems, like this one here, and when we look at comets, we can actually see in some comets, as the comet becomes, comes closer and closer to the sun, it gets hot enough that it starts to evaporate the ice, forming cometary tails. And by taking the spectrum of these cometary trails we can measure their composition. And that confirms our basic picture that comets are snowballs that are remnants of this material forming out here, out beyond the snow line. So, what happens next? We have this disk forming. Planets start to form in the disk. The planets forming in the disk start to interact with the disk. The planet of, own gravity starts to affect the disk. So imagine, you have a planet moving around in our disk and let's go back and look at our edge on disk here and imagine what happens when either a rocky protoplanet or an Icy protoplanet forms. These protoplanets are going to interact with the material around them. Some of the material will fall directly on to the protoplanet while other material be affected gravitationally by the protoplanet. It will feel the gravitational field of this forming protoplanet. When we look at the effects of that, what it does is it sweeps up a gap. So, in here is a planet. And that planet is sweeping up a gap. This is an illustration showing what's seen in computer simulations of disk formation. The basic ideas in these computer simulations have been confirmed by observations. These are observations with the Hubble space telescope on the left of a young protoplanetary disk called TW Hydrae and this is observed in the infra red, here's the star. Here's the disk. And you can see the planet has swept up a gap in the disk. What we think is going on in the system, though we have not directly detected the planet, is we're seeing the planet's gravitational effects as the planet sweeps up a gap. This is something we've seen in our own solar system when we looked at Saturn. When we look carefully at Saturn's rings, we can see gravitational interactions between tiny moonlets, these are tiny moonlets in Saturn's rings, and the rings itself. And we can see the moonlets sweep up gaps in the ring. And we can see the moonlets actually trap tiny ringlets between them, as these are particles that are effected by the gravitational interactions of both of these, what are called, Shepherd Moons. That shepherd these thin structures in Saturn's rings, in fact, Saturn's rings show a number of gaps associated with tiny moons and we could see, basically, an analog system to our young protoplanetary disk. By observing Saturn's rings. Now, interesting things happen when you have these gap interactions. You have a process called planet migration, and these planets interact with the disk, create a gap. That gap serves as a source of drag, and the planet starts to migrate in. So, as shown in this figure here, you could have a giant planet form in the outer portions of a disk, maybe a giant gaseous planet or a giant ice planet. And that planet could migrate. In the gaseous disk, migrate all the way in some computer simulations what happens, the giant planet is it migrates all the way in and actually collides with the proto sun and in some systems the star, like a Gods in ancient Greek myths, swallow up their children. And all of the protoplanets spiral in to the disk and get swallowed. A lot of this depends on how long the disk lives. If you have a long-lived disk, you may swallow up all of your protoplanets. If your disk doesn't live for very long, there's not much time for dynamical interactions. And your protoplanets survive. In the intermediate case, you have enough time for the planets to spiral in, but perhaps not enough time to completely drive them inwards. And particularly, if this inner region of the disk is depleted of gas because of a hot star. That might stop planetary migration. Now, this whole process of migration and how much planets migrate, and how they interact through gaps, is an area of active study. Which means, we don't fully understand this. We think the complex architecture of planetary systems that we're finding with the Kepler telescope. The fact that we're finding many systems that are in resonance with one planet orbiting around once and its the next planet out coming around with double, or perhaps triple its period. These kind of resonant effects are telling us things about planetary migration. But we haven't put together the whole story yet. What we're trying to do right now, is develop computer simulations and analytical theory, tie them to observations, and try to get an understanding of how these processes of planet assembly, from planetesimals to planets, planet migration through gaps and dynamical interactions with their disks determine the structure of the systems. I'm going to show you just a quick movie, next, which will convey one of the things that we're doing in these simulations. This simulation shows the interaction between the planet, you see that bright white spot's the planet orbiting in a disk. You can see what the planet's doing. It's exciting a spiral density mode in the disk, and as it does that, it's driving material outwards, and it's spiraling inwards towards the center of the protoplanetary disk. And with each revolution, it heads further and further inwards. As it's losing angular momentum and energy to the gas disk around it. The process of planet migration probably plays an important role in shaping the structure of a planetary system. What happens if we have a Jupiter like planet migrating inwards? And, a smaller planet inside Jupiter's orbit. Well, that planet will probably be driven inwards with Jupiter. We think processes like these, help explain some of the diversity we're seeing in these multi-planet systems. And here are images of some of some of the multi-planet systems that Kepler studied. We have learned from the Kepler observations that while planets are common, most stars do not have large planets, at least in their inner solar system. Kepler does not find planets most of the time. While it's found 2000 planets, it looked at a lot more stars. So, somehow planet formation doesn't, or star formation doesn't always produce planets. Or, at least, it doesn't always produce planets that survive. On the other hand, some stars have multiple planets and they show a wide range of properties. And we find some systems that have a large number of planets. Some of the systems we see will have five or six planets within the orbit of Venus, so some systems seem to be really good at forming planets. One of the things we're trying to understand right now is this tremendous diversity in planetary systems. When, before we had observations of extrasolar planets, we assumed that all solar systems, if they are out there, would be like our own. With massive planets in the outer solar system and rocky planets in the inner solar system. Yet, as we begin to explore the universe around us, we're finding a much richer set of possibilities. And before we go on to the next section, I'd like you to think about this question. Why do some systems have hot Jupiters? How do we form gaseous planets or how do we get gaseous planets so close in to their host star. Some planets have cold Jupiters. Planet systems like our own have massive planets far outside. Yet, other systems have no Jupiters at all. So, I'd like you to think about that and a possible answer. And then we'll come back and talk more about the diversity of solar systems.
