The big question I'm going to address in this segment is how did the solar system form, and how did we work that out. [MUSIC] The idea of how we actually form planetary systems has a long legacy. It goes back to the concepts of Immanuel Kant and Pierre-Simon Laplace back in the 1700s. So in Laplace's model for the formation of the solar system, essentially we start from a molecular cloud of gas and dust. It slowly condenses in under it's own self gravity. Forms a central bulge which eventually becomes the protostar. But as the cloud actually condenses in the small rotation the cloud has becomes accelerated. Becomes faster and faster and forms a central disc around that star region, and that disc is the material which eventually condenses out to form the planetary system we see today. So this is a picture of typical stellar nursery, this is probably a similar environment to where the sun and our planetary system first formed. And in the visible, the things the arrows are pointing to you barely see anything at all. These things are very, very faint invisible light, yet in the infrared, they're glowing like a Christmas tree. They're extraordinarily bright. These aren't typical stars. These aren't emitting a lot of light in the visible. They're emitting most of their energy down in the infrared as heat. This is typical of a dust disc around a newly forming star. It's obscuring a lot of the visible light, but it's quite hot so it's emitting a lot of energy down in the infrared. So what this Spitzer space telescope enabled us to see is the dust discs around newly forming planetary systems in a stellar nursery. We actually see these dust rings around other planetary bodies today. This particular dust ring is the Formalhaut dust ring. So, you can see the central glowing star and, also, a much more diffuse ring of dust around that glowing star. And, we think that's caused by the collisions of comets in the outer parts of the stellar system. This is another example of a dust ring. This is HR 8799 in Pegasus, and this is an infrared dust cloud all the way out to the stellar system's Kuiper belt, so for us that would be out past the orbit of Pluto. And this system has up to about three planets inside that dust cloud which are disturbing local comets and causing them to collide and generating this dust that we see in the infrared. When Laplace first developed his theory for solar system formation, he used the tools available at the time, which, being one of the greatest mathematicians of his age, was a mathematical analysis of the problem. These days, we can move beyond the simple pen and paper mathematics approach to actually simulating these processes in a computer. So we're building the physics, as we understand it, of these clouds into the models, and then we simulate them. We let them run forward in time and see what happens. And the structures we form from these models look remarkably like the structures we see. But there's still a few big unknowns. What actually caused the condensation of this molecular cloud to start with? Well probably somewhere in a stellar nursery was the reason that the sun formed when it did 4.5 billion years ago. The other big unknowns, how do you get from a diffused cloud of gas and dust, how do you bring that material together to start forming solid grains which then build up to form the planets? Or do you even need to? Do the planets form from initially solid grains accreting together into small rocks and asteroid-like bodies called planetesimals which then go on to form larger planets? Could you actually just take the gas from a disc and form a planet from it directly? These are still the big unknowns in modern Laplacian theory for the formation of the solar system. How do we know about what happened in the early solar system, and how might we actually understand what the environment was around the newly forming sun? One of the best lines of evidence we have actually come from meteorites. And when you actually look at meteorites there are number of types. There are iron meteorites, which are like big chunks of rusty iron. There are stoney iron meteorites, which are a mixture of silicate minerals and metal mixed together. There are normal stony meteorites, which might look like just a rock you find on the Earth today. Then there are these primitive meteorites called chondrites. And what they're made up of these little balls called chondrules. Which are these little spherical particles. They're a mixture of metal and silicates. And they formed extremely early in the solar system. And they're also associated with grains which may even be a little older called calcium aluminium rich inclusions which are small silicate inclusions we find embedded in the meteorites themselves. These guys are remarkably old. They seem to be the oldest solar thing that actually formed in the solar system as the gas and dust was acreating into individual grains, these seemed to be the first things that formed. And they give us a snapshot into the very earliest history of the solar system, and what processes were operating at the time. From these grains we can tell that a local super nova occurred shortly prior to the solar system forming, because we have a lot of short lived isotopes. Things like Aluminium-26 or Hafnium-182. These are extremely radioactive isotopes that decay very, very fast. And yet we see the products of the decay in the earliest grains. Which mean they must have been formed just prior to the solar system actually condensing. So that's cool in of itself, it tells us there was a supernova in our local environment just prior to the solar system molecular cloud condensing to form the solar system and planetary disc, and eventually the planetary system we see today. The other cool thing they'll tell us is the age of the solar system. We can use the radioactive decay of these short lived isotopes, to get a very tight constraint on how old those grains are, and by inference, how old the solar system is. Now we know the universe is about 13 billion years old, it turns out the solar system we can pinpoint extraordinarily accurately from these methods and it's age is 4.567 billion years old. While all of this is going on, we're getting massive perturbations in the protoplanetary disc itself. We see evidence of massive flash heating of the newly formed chondrules and calcium aluminum inclusions. These guys have just formed out of the dust from the protoplanetary disc, and we see evidence that they were heated up really, really quickly, flash heated up to about 1,000 degrees or so and then immediately cooled down to ambient temperatures. This is an incredible amount of heating to happen all at once. And the only two mechanisms people have suggested to account for this are either, giant lightning strikes in the gas discs as the planets are actually forming, or that this could be evidence for supersonic shock waves from the accreting material onto the disc. So as the solar system started to accrete solids, those tiny little grains like chondrules started to smash together and stick, and we started to form a protoplanetary disc with sizable solid components. These small grains accreted to larger rock size bodies, which accreted to bodies a couple of hundred kilometers across as asteroid size which we call planetesimals. The trick to this accretion though is things have to be bought together very slowly in order for planets to form. If you bring things together too fast, if these things are moving at very fast velocities, you end up just obliterating the objects and fragmenting everything. So planets have to be brought together gently. There is an argument at the moment over whether the planets actually formed from the accretion of this solid material, forming asteroids and planetesimals up to larger bodies, mostly from rocky cores or rocky metal core. Or, whether planets can actually form just from the self collapse of the dust and gas in the dust disc itself, something that's called a disc instability. One way to test this is to look at exoplanets, and in particular, from exoplanets we can look at their stars and we can tell the amount of heavier elements in the stars. What we call the metallicity, so the amount of heavy elements heavier than hydrogen or helium. And when we look at these planetary systems, the stars which have a higher metallicity, so have more heavy elements, tend to have more planets and they also tend to have bigger planets. And what this tells us is that we're probably need those heavy elements to form the rocky solid cores of a planet before they start bringing in the gas like hydrogen and helium and becoming giant, super, Jupiter-like planets. So, it seems that most the planetary systems we see, probably needed to form rocky cores first and it create the gas around it. Although it is certainly possible that a small proportion of them formed just from the collapse of the gas disc itself. As your dust is accreting into grains and growing into larger bodies up to planetesimal size, you're actually in a race in order to build your planets large enough while the gas and dust is still around, because at the same time you're growing planets, your star's getting brighter and brighter. And you start generating a lot of light and a lot of heat. And this essentially blows a lot of that gas out of the system. So you don't have very long to build your planets before a lot of the material that the planets are sourcing is gone. So this particular picture is from Herbig-Haro 46/47 and it's showing an embedded outflow from this planetary system. So you have a molecular cloud which is being blown out from the new star. And we see this in the infrared from the Spitzer space telescope. The gasses blown away from these newly forming planetary systems on a timescale of around six million years or so, so you need to form your planets fast, before you run out of gas and dust to form them from. Within the gas disc itself, things are quite heterogeneous. When you're closer into the Protosun. Things are quite warm. You're dumping a lot of energy from the newly accreting sun and that heats up the disc. As you get further and further out, things get cooler and cooler, but it's also more difficult to source material the further out you get. So this temperature distribution affects what we can crystallize out of the gas. Nearer in, you start crystallizing at very high temperature phases. Things like metals and silicates. Silicates are the minerals that form rocks on the Earth. Further out, eventually, things cool down enough that you can actually start crystallizing out, for instance, water ice. And the place where that happens, we call the frost line or the snow line. When you can crystallize that water ice you suddenly have a lot more material available to you to start forming your protoplanet. And, planets can get very very big, very very quickly. Even further out more exotic classes like ammonia ice start crystallizing and this dominates the composition of the very outer most portions of the solar system. So when you reach the frost line, you suddenly have a lot more mass available for you to form your planet. And this means you can form very big planets very fast. And we think, for instance, Jupiter and perhaps Saturn were formed just past the frost line in the early solar system, which enabled them to get so big so fast. And eventually, they grew that big that they could start holding onto the hydrogen and the helium from the protoplanetary gas disc and become massive giant gas bodies that we see today. So our solar system is fairly remarkable. In that it's been around for about four and a half billion years and the planets are in an incredibly stable orbit despite the constant gravitational tug of war between the big planets like Jupiter and Saturn. The effect they have on the smaller planets, they're still around. They're still orbiting in roughly the same position four and a half billion years later. So one of the incredible things is how you actually end up with a planetary system with this high degree of stability. One of the big revolutions in our understanding of the formation of planetary systems had been the realization that the planets are not in the place they started with. Planets migrate quite a lot. We've developed a theory of planetary migration primarily for looking at exoplanetary systems around a lot of other stars we see evidence of Jupiter-sized planets, or even bigger, orbiting extraordinarily close to the star in an environment where they could not have possibly formed. And we think they got there by the process of planetary migration. Planetary migration probably operated in the solar system during it's earliest period as well. Because Jupiter and Saturn got so big so fast, they start to interact viscously with the gas disc around them, and that causes them to start migrating, in fact probably start migrating inwards. There is a model at the moment that Jupiter actually rampaged into the inner solar system. We think it moved in as far as the asteroid belt wildly disturbing the population of the asteroid belt. The big question for planetary migration in our solar system is why did Jupiter stop? That it started migrating in. Why didn't it just rampage through the entire inner solar system, flinging all the small rocky planets out, and end up extremely close to the Sun like so many other planetary systems we see? The answer to that seems to be that we had a Saturn. Saturn got extremely big extremely quickly as well. Maybe not quite to the same degree as Jupiter, but it exerts a sizeable influence on Jupiter due to its gravity. And Saturn got that big it appeared to put the brakes on Jupiter's migration into the inner solar system. At some point during Jupiter's migration into the inner solar system, Jupiter and Saturn enter what we call a resonance. That's where Jupiter goes round the Sun every three times, Saturn goes round every two. And that is an extremely stable gravitational configuration, and the resonance between Jupiter and Saturn is what pulled Jupiter out of the inner solar system. They started exchanging angular momentum between them, and Jupiter's orbit moved back to its present position. The attraction of this model, with Jupiter moving in and then moving back out, is it explains a number of features of the solar system. One is why we have no planet where the asteroid belt is, is because Jupiter cascaded in, threw most of the material out, and sped up the rest of the material to the degree that there's really not the ability to form a planet with everything moving so fast. The other thing it explains is that by moving so close to Mars, it actually ejected a lot of the material that Mars could have used to form into a much bigger planet. And this explains why Mars is so small compared to planets like Earth or Venus which have a much larger size. The migration theory also suggests that Neptune and Uranus may have actually flipped positions, that Neptune may have formed closer in and actually moved past Uranus at some point in the early solar system. And this would've affected the Ault cloud and Kuiper belt and caused the massive bombardment of this material into the inner solar system at that time. Planetary simulation suggests we probably had hundreds of Mars-sized protoplanets in the early inner solar system. And of those hundreds of burgeoning protoplanets, only four survived to be the mature terrestrial planets we see today. So as these hundreds of Mars-sized bodies were being ejected from the inner solar system, some of them didn't quite make it out unscathed, and in fact we think the earth moon system is a product of a collision of something the size of Mars with the newly formed Earth. Earth's moon is peculiar, it seems to be made of exactly the same material as the Earths mantel, and it shares a lot of chemical similarities to the point that we couldn't actually explain the moon forming somewhere else and being brought into orbit around the earth. They really were made from the same material. There are a number of other peculiarities about the Earth Moon system. The Earth has a lot of water. The moon is essentially bone dry. The moon seems to have been entirely molten at some point early in its history. And the other thing is the Earth Moon system has an extraordinarily large amount of angular momentum. The moon is actually, is quite heavy and it's spinning around the Earth very fast. How do you explain these peculiarities? The only real, viable model to explain these features is that the Earth Moon system formed due to a giant glancing impact. In this model, a body the size of Mars, which we call Theia, came and impacted into the Earth and hit it kind of side on. This accelerated the Earth Moon's rotation. It caused a lot of melting of the Earth and the incipient lunar material. It formed a disc of molten gaseous rock vapor around the Earth which eventually coalesced to form the Moon, as we see it today. As we discover more about planetary systems outside our own, we're learning a lot more about the processes by which stars form, by which planetary systems form and evolve, and about even how moons form around those planets in those systems. So it's an exciting time for planetary science. We don't know what we're going to find next, but it's sure to tell us something very important and fundamental about our own place in the universe.
