hello and welcome to general astronomy lecture 10 the formation of the planets to understand how the planets asteroids and comets formed we start by considering a cold and low pressure solar nebula before it was warmed by an emerging protozoan at the low pressures that prevailed a substance does not form into a liquid state but must exist as either a solid or a gas as an example you can refer to this figure which shows a dust grain of the sort that would have been present throughout the solar nebula other substances in the early solar nebula would have been in the form of small ice crystals like snow although at high temperatures these ices would evaporate to form a gas together these solids referred to as ices dust grains and ice coated dust grains were mixed with gaseous hydrogen and helium but things began to heat up so this highly magnified image shows one of those microscopic dust grains they came from interplanetary space and entered Earth's upper atmosphere and was collected by a high-flying aircraft dust grains of the sorts are abundant in star forming regions like that shown in a previous lecture so these tiny grains were also abundant in the solar nebula and served as the building blocks for the planets themselves the planets began to form after the solar nebula collapsed and flattened into a disc of perhaps diameter of about 200 astronomical units the churning and mixing of the gas in that solar nebula should have ensured that the nebula had the same composition throughout about 98% hydrogen and helium and then two percent heavier elements how then did the terrestrial planets end up so different from those of the jovian planets well the key clue comes from their locations terrestrial planets formed in the warm inner regions of the swirling disc while jovian planets form in the colder and outer regions of our forming solar system so we first need to understand the condensation temperature which determines when a substance forms a solid or gas if the temperature of a substance in the solar nebula is above the condensation temperature this substance is a gas on the other hand if the temperature is below the condensation temperature the substance would solidify or condense into tiny specks of dust or ice Frost you can often see similar behavior on a cold morning so the morning air temperature can be above the Condon says above the condensation temperature of water while the cold windows of a parked car may have temperatures below that condensation temperature thus water molecules in the warmer air remains a gas that being water vapor but it may it may form solid ice particles or Frost on your cold air car windshields so that gives you an idea of how this might work so you'll have a gaseous state near the warm protosun and you'll have more solid ices further away the temperature in the nebula varied significantly rising about rising above 2,000 degrees Kelvin closer to the hot protosun and dropping below 50 degrees Kelvin in the outermost regions with heat from the emerging protosun the solar nebula was radically changed into two distinct regions these regions inner and outer had very different properties and eventually formed different planets so for the inner region which was the planets that were made out of rock and metal in the hot inner region of the nebula only substances with high condensation temperatures could have remained solid these rocky and metallic materials in the form of solid dust grains eventually formed much of the rock and metal of the terrestrial planets hydrogen compounds such as water methane and ammonia remained as a gas so they could not condense into solids in the warmer inner regions of the solar system so basically it was so warm that only rocks and metals could solidify everything else was gaseous and not help build the planets that we know to be the terrestrial planets today as for the outer region well in the area between present-day orbits of Mars and Jupiter the temperature had dropped below about 170 degrees Kelvin in the low pressures of the solar nebula this is the condensation temperature at which water vapor forms ice one aspect of this what we call snow line is that which is the distance from the Sun at which water vapor solidifies into an ice or frost is that beyond this line the solid ice particles can join with the rocks and metals to form a more massive planet right so in the inner parts of the solar system we only had rocks and metals but now beyond this snow or ice line we have icy materials to help build the planets so this can give us an idea as to why the outer planets are bigger and we'll get to that more at throughout this lecture and so even further out there are an even cooler temperatures methane and ammonia ice lines as well so basically the further we go the more material you have to build some planets so here's a great graphical representation of everything we're talking about so the graph on the left shows how temperatures probably varied across the solar nebula as the planets were forming and the present-day position of the planets note that the note of the general decline in temperature was increasing distance from the center right so you see here's basically where the Sun would be or the protosun and the further away you move the colder it gets so between present-day Mars and Jupiter which is right here where you see this blue dashed line the snow line marked where temperatures were low enough for water to condense and form ice beyond about 16 astronomical units and then you can see also where that's expands from Ambika units is where methane can condense and form more ices excuse me so the idea is here so right where the snow line is the snow line happens to separate our terrestrial planets from our jovian planets so this plays a huge role in the formation of our planets now on the right it just shows a nice figure so terrestrial planets formed inside of the snow line where the low abundance of solid dust grains kept the planets small the jovian planets formed beyond the snow line where solid ices of water methane and ammonia added to the masses to build larger cores and attract surrounding gas so now we have kind of the stage being set up for how this all comes together so in our last lecture we talked about how we go from just a random cloud out in space into a disc forming a solar system and now we know and now we know to an extent about where the planets are coming from so let's dig into this a bit more the frost line or the snow line marked the key transition between the warm inner regions of the solar system where terrestrial planets formed and the cool outer regions where jovian planets formed inside the frost line only metal and rock could condense into solid seeds which is why the terrestrial planets ended up being made of metal and rock beyond the frost or snow line where it was cold enough for hydrogen compounds to condense in ices the seeds of the planets were built of ice along with that metal and rock moreover because hydrogen compounds were nearly three times as abundant in the nebula as metal and rock combined the total amount of solid material was far greater beyond the frost line than within it the stage was set for the birth of two planet types planets born from seeds of metal and rock in the inner solar system and planets born from seeds of ice as well as metal and rock in the outer solar system from this point the story of the inner solar system seems fairly clear the solid seeds of metal and rock in the inner solar system ultimately grew into the terrestrial planets that we see today but the planets ended up being relatively small in size because rock and metal made up such a small amount of the material in the solar nebula the process by which smaller seeds grew into planets is called accretion so a chrétien began with the microscopic solid particles that condensed from the gas in the solar nebula so just tiny little particles these particles orbited the forming Sun was the same ordered orderly circular paths as the gas from which they were condensed individual particles therefore moved at nearly the same speed as neighboring particles so collisions were more like gentle touches although the particles were far too small to attract each other gravitationally at this point they were able to stick together through electrostatic forces the same kind of quotes mean static electricity that makes hair stick to a comb small particles therefore began to combine into larger ones as the particles grew in mass gravity began to aid the process of their sticking together accelerating their growth into boulders large enough to count as planetesimals which means pieces of planets so here we see this general process visualized the planetesimals grew rapidly at first as they grew larger they had both more surface area to make contact with other planetesimals and more gravity to attract them some planetesimals probably grew to hundreds of kilometers in size in only a few million years a long time in human terms but only about one ten thousandth of the present age of the solar system however once the planetesimals reached these relatively large sizes further growth became more difficult gravitational encounters between planetesimals tended to alter their orbits particularly those of the smaller planet testicles with different orbits crossing each other collisions between planetesimals tend to occur at higher speeds and hence the came more destructive such collisions tended to shatter planetesimals rather than help them grow only the largest planetesimals avoided being shattered and can grow into terrestrial planets so this just kind of shows you what it would look like visually at first the material that coalesced to form proto planets in the inner solar nebula remained largely in solid form despite high temperatures close to the protosun but as the proto planets grew they were heated by violent violent impacts that collided with other planetesimals as well as by the energy released from the decay of radioactive elements and all this heats all this heat caused melting thus the terrestrial planets began their existence as spheres of at least partially molten rocky material material was free to move within these molten spheres so the denser iron-rich materials sank to the centres of the planets while the less dense silicon rich rocky material floated up toward their surfaces this process is called chemical differentiation in this way the terrestrial planets developed their dense iron cores so if you recall I think it was in our previous lecture I had a question for you that asked about the average density of a planet so we have solid cores of heavy material because all that heavy material when the planets were molten in forming sank to the center so our planet doesn't have the same density from surface to core right it gets more dense as you go further in as a result of this differentiation so here I have a great video or it's a simulation so this isn't a real forming system but it's just 45 seconds and it shows you how you can go from a disc like how we started out in at the beginning of today's lecture to forming some planetesimals so let's just take a look and see how some of this material can de cretes and coalesce so you can see now that a lot of the material has formed into these lumps so these might be the planetesimals that we're talking about and over time you might see one of them fall into the protosun or some of them collide and it's a pretty violent process and this is occurring over many many years but you get the general idea so there at the bottom we saw two planets come close together those two well it's hard to narrate there we go so those two on the right just collided into form one so you can kind of see that's just a simulation so somebody made that video and decided that you know with these certain properties this might be how it looks but it's a great visualization of what we're discussing and that's along the lines of what we're looking at in this lecture so I hope that helped a little bit the reference is down at the bottom since this one I just decided to keep in the lecture so that's the terrestrial planets but what about the jovian planets well a chrétien should have occurred similarly in the outer solar system but condensation of ices meant that meant both that there was more solid material and that it contained ice in addition to metal and rock the solid objects that reside in the outer solar system today such as comets and the moons of the jovian planets still show this ice Ridge composition however the growth of icy planetesimals cannot be the whole story of Jovian planet formation because we know that the jovian planets contain large amounts of hydrogen and helium gas the leading model which is called the core accretion model for Jovian planet formation holds that these planets formed as gravity drue gas around ice ridge planetesimals much more massive than Earth because of their large masses these planetesimals had gravity strong enough to capture some of the hydrogen and helium gas that made up the vast majority of the surrounding solar nebula this added gas made their gravity stronger allowing them to capture more guests ultimately the jovian planets accreted so much gas that they bore little resemblance to the icy seeds from which they grew due to the lower temperatures in the outer solar system Jupiter's large seed mass could capture and retain hydrogen kiwi and helium gas so this picture where a Jovian protoplanetary core captures gas and grows by accretion again is called that core accretion model as Jupiter grew its gravitational pull increased allowing it to capture more gases and grow even larger until most of the available gas in its region had been captured because hydrogen and helium were so abundant again about 98% of the solar system's material Jupiter quickly grew to be more than 300 Earth masses farther out in the solar nebula Saturn would have gone through a similar process about one-third of the mass of Jupiter Saturn's 95 earth masses would have taken longer to accumulate forming a new forming a few million years after Jupiter Uranus and Neptune formed well beyond the snow line where temperatures were cold enough for additional ices of carbon dioxide methane and ammonia to form the bulk of these planets this model also explains most of the large moons of the jovian planets the same process of heating spinning and flattening that made the discs of the solar nebula should have also affected the gas drawn by gravity to the young jovian planets each Jovian planet came to be surrounded by its own disk of gas spinning in the same direction as the planet rotated moons that accreted from ice-rich planetesimals within these discs ended up with nearly circular orbits going in the same direction as their planets rotation and lying close to their planets equatorial plane however the jovian planets have also share oh excuse me the jovian planets also have smaller bodies called irregular satellites that orbits in the opposite direction and these were probably captured after the planet was formed so there are some small objects moving in the opposite direction around these larger Jovian worlds and we believe that they were captured into orbit not formed the planet the vast majority of the hydrogen and helium gas in the solar nebula never became part of any planet so what happened to it apparently it was cleared away by a combination of intense radiation from the young Sun and the solar wind a stream of charged particles continually blown outward in all directions from the Sun although the solar wind is fairly weak today observations show that stars tend to have much stronger winds when they are young the young Sun therefore also should have had a strong solar wind strong enough to have swept away huge quantities of gas out of the solar system the clearing of the gas sealed the compositional fates of the planets we now look at migration which refers to changes in orbital distances of the planets so at this point we basically have an understanding now of how all of our planets formed in the inner parts of the solar system we only had rocks and metals the planets formed by accretion we're basically just chunks of rock slowly built up in size collided with one another to form the inner planets in the outer parts of the solar system we have the core accretion model which is where you still have these like solid cores or what we call seeds but they became large enough to capture a lot of the hydrogen and helium gas that was out there beyond the snow line and so that's how we get those gaseous worlds so now we're going to figure out how the solar system formed its general shape why we have certain planets in there certain positions so migration of the planets results from interactions that is the gravitational pulls and tugs between planets and different parts of the forming solar system within the first few hundred thousand years long before the terrestrial planets formed Jupiter's interaction with the gaseous disk of hydrogen and helium led to an inward migration of Jupiter followed by an outward migration so it had an inward migration I don't know if we talk about this too much but because it's colliding with all this other material out there it kind of slows it down and it falls in word so we'll get into this a bit more but at its closest distance to the protosun Jupiter actually migrated inward to about 1.5 astronomical units which happens to be near the current orbit of Mars with its large mass Jupiter gravitationally deflected many of the planetesimals near the current Martian orbit as a result when Mars eventually formed in this region it ended up with a low mass of about one tenth of Earth's mass in fact it was the miss it was the mysteriously low mass of Mars that prompted this analysis of Schubert's inward migration even more inward than outward migration of all the jovian planets which we call the Grand Tek model also solves a long-standing mystery involving the asteroid belt so basically what I'm getting at is Jupiter was kind of the bully of the solar system it moved inward while it was forming and it basically took all the material where Mars will have formed and kind of kicked it all the way so that's why Mars is so much smaller than Earth or Venus it's because if you patern migrated inward kicked all the material out before moving back and then Mars started to form but it didn't have as much material there as it would have so that's the general idea but this does again also explain the mysteries involving the asteroid belt the asteroid belt contains rocky objects typical of the inner solar system but surprisingly it does also contain icy objects expected to have formed beyond the asteroid belt in the grand tech model as both Jupiter and the other jovian planets migrate outward they deflect planetesimals inward to form the asteroid belt so this kind of has to do with some of like newton's laws where you have an equal and opposite reaction so we have the bigger Jovian worlds moving outward in their migration so they're kicking things inward some of these planetesimals come from the inner solar system but some also come from much farther out beyond the snowline providing a very natural explanation for the icy objects that are found within the asteroid belt this early migration along with the Jovian planet formation was over within the solar system's first few million years or so another type of migration occurred over the next few hundred million years and reshaped the outer solar system astronomers have long suspected that some planetary migration must have taken place in the outer solar system to see why consider Neptune Neptune's current large orbit a large orbital distance the timescale necessary to build a planets of Neptune's large size by core accretion model is much longer than the time that the protoplanetary disk was around so in other words with how long our solar system was there to form there's no way the Neptune could have formed where it was because it would have taken too long to collect as much material as it has however planets grow much faster closer to the protosun where the protoplanetary disk is denser therefore astronomers suspect that Neptune formed closer in and then migrated outward this leading theory or the leading theory described here for late migration of the jovian planets is based on the nice model in the nice model the jovian planets would have all originally formed within about two 1ps gives me 20 au of the protosun even though the outermost planets today that being Neptune the outermost planet lies at about 30 astronomical units so it must have migrated about 10 astronomical units outward there was also a disk of planetesimals beyond the outermost Jovian planet at 20 astronomical units and through gravitational encounters these planetesimals on average were scattered in word when a big planet knocks of small planetesimals in word the planet itself is kicked slightly outward over a few hundred million years as numerous planetesimals were knocked inward by Saturn Neptune and Uranus these jovian planets slowly migrated outward to their current locations most of the inward moving planetesimals even made it close to Jupiter with its much greater mass Jupiter did not migrate inward very much but it did gravitationally flying most of the planetesimals clear out of the solar system however a small fraction of these icy objects would not have made it all the way out of the solar system and are therefore currently orbiting at above 50,000 au from the Sun these planetesimals would be very loosely bound to our Sun and gravitational deflections from passing stars would spread many of their orbits into a spherical halo we call this hypothesized distribution of icy planetesimals the Oort cloud which formed beyond the objects of our next topic the Kuiper belt gravitational interactions with the planetesimals eventually destabilized planetary orbits leading to the elongated or eason eccentric orbits that we see today with elongated orbit the planets gravitationally interact with each other more strongly and many computer simulations this nearly doubles the orbital distance of Neptune sending Neptune out to its current distance of 30 astronomical units as not to move outward it's gravity flings nearby planetesimals to greater distances as well creating an orbit orbiting collection of icy objects called the Kuiper belt the nice model seems to solve another mystery of the solar system astronomers think that there was a cataclysmic period when the planets and moons of the inner solar system were subjected to a short but intense periods but an intense period of large impacts this hypothesis is referred to as late heavy bombardment so what could cause a big jump in impacts well in the nice model the destabilized and elongated planetary orbits also led to deflections of planetesimals throughout the inner solar system so this leaves us now at a point where we have a general idea of how the solar system formed how the planets formed from that disk that the solar system came about in and now how each planet got to where it was so in summary we end with these three slides to showing you again a visual overview of everything that happened from the beginning of lecture 9 into this one so you begin out begin with a large clump of gas it has a slight rotation and it so that big ball of gas might get hit with a shockwave from a nearby explosion or something and triggers its collapse once it starts collapsing gravity will assist that collapse and cause it to occur more quickly but as it gets smaller just like a figure skater pulling her arms in that rotation speed increases and as a result it starts to flatten out into a disc so you have a lot of dense material upton to be get up in the center and less dense material outward so it starts heating up as things start colliding with one another and that inner parts of their solar system formation and you begin to heat up enough to form a proto Sun and eventually that material starts to clump up and you get lots of just dusty debris around and it's warm enough now to where we can define a snow line a region where condensation occurs for water so over time after a few million years we have a nice solid protosun formed the snow line is starting to mark what's going on in solar system the first planet form as we mentioned was Jupiter so Jupiter starts growing in the outer parts of the solar system it's sweeping out the material and then over time Saturn Neptune and Uranus form closer than they are today though right but they start to move outward as they fling materials inward at this point the inner planets are still forming they're slowly building up from planetesimals well then we finally get to the point where we have a present-day solar system so the jovian planets migrate outward as I mentioned and Jupiter flings objects out of the solar system to form the Oort cloud that you see here in the spherical halo around it so if there's a lot going on here but this is again just a very quick in general overview of the formation of our solar system and it's planets so from here we're going to take a dive into each planet a little bit more so we're going to look at their terrestrial worlds and their geology of them and we'll talk a little bit more about the outer world as well so as always thanks for watching and have a great day