 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to present an overview of our solar system. So we're going to look at all of the different types of objects that exist in our solar system and talk a lot about them a little bit. Now if we will go over them in great more detail in other lectures we'll go into each of these in much more detail. So let's start off by looking at what we have in our solar system. So if we want to look out and take a quick inventory of our solar system so what do we see in the solar system? Well we can look at this two different ways. What do we know now? Here's what we know now that our solar system contains. Here's what we knew in ancient times. So it was a little bit different in ancient times. We looked at our solar system as the universe. So we saw that it had the sun but it also had the sphere of the stars. There were five planets known. Uranus and Neptune had not yet been discovered and the Earth would not have been considered a planet. There was only one moon known that was ours and we knew of meteors what we see as shooting stars and comets. Now over the years and now we see a lot more and we have a better understanding of it. Now we know that our solar system contains one and only one star and that the rest of the stars are actually could be parts of other solar systems. We are now up to eight planets, five dwarf planets and more than 150 moons known throughout the solar system. Asteroids have been discovered which were not known in ancient times. Interplanetary gas and dust. We see some of that as meteors but now we see it as also other parts of the solar system. Kuiper Belt objects again completely unknown in ancient times and of course we still know of comets but we have a much better understanding of them. So let's look at a little bit in each of these in a little more detail. So let's look at the masses first of all. How massive are these objects? Well the sun is essentially all of the mass of the solar system. 99.8% of the mass of the solar system and here we see in a chart the yellow part of course being the mass of the sun and the only planets that are actually showing up there are Jupiter and Saturn. They're the only ones that are massive enough to actually show up. The rest of the solar system is just the rounding errors within the mass and in fact Jupiter and Saturn would contain 90% of the mass that remains after you take out the sun. So essentially there's only a very small fraction 0.02% and if we split this down a little bit more let's look at just the planets here. So Jupiter is most of the mass here in the orange and then the gray for Saturn here takes up 90% of the planetary mass of the solar system. So really in terms of the whole solar system mass that would take out the sun and a couple of the very most massive planets there's only a tiny fraction for everything else left in the solar system. So let's look at these planets and what do we mean first of all by a planet? Well a planet is now about back in 2006, 2005, 2006 was actually defined and here we see an image showing each of our planets. These are the inner or terrestrial worlds, the earth-like worlds and the outer or Jovian worlds, the large gaseous planets. But what we have done is to give a specific definition for a planet and a planet has to do three things. First of all it has to orbit the sun. So no matter how big the object is, if it does not orbit the sun it's not considered a planet. So that will eliminate the larger moons that we'll see in the solar system. It has to be massive enough to have pulled itself into a spherical or ellipsoidal shape. That is essentially that it has reached equilibrium under gravity. So gravity has pulled it down together and a sphere or an ellipsoid will be the most stable shape that can come out of that. And finally it has to have cleared its orbit of debris. So it will not be orbiting in things like the asteroid belt or the Kuiper belt. If it is there it has not become gravitationally dominant and will not be a part of the planetary systems. Now there are a lot of other objects in the solar system so let's look at some of those smaller objects we see in the solar system as well. Here we see, for example, the moons which orbit a planet or another larger object. So we see a number of the moons here in the solar system. There's our Earth for scale and there's the Earth's moon. You can see that some of these are incredibly small so things like Phobos and Deimos are almost completely invisible at this scale. Some of the larger moons are actually larger than our own so things like Ganymede, Callisto and Titan are actually larger than our own moon here around Earth. So there are a number of moons around these various planets and in fact we see them around all of the giant planets, Jupiter through Neptune as well as the round in the inner solar system around Earth and Mars. Mercury and Venus have no moons. Now there are also a new class of objects that came up when we defined what a planet was. We also made the definition of a dwarf planet are objects that meet the first two components of the definition of a planet that they orbit the Sun and that they are spherical or ellipsoidal. So right now at this point there are five dwarf planets that have been defined and those include Pluto seen here in the middle, Eris, the next object that was discovered and Hamea and Makemake which are two other objects out in the depths of the solar system in Pluto what we actually call the Kuiper Belt and we see Ceres which is known as the largest object in the asteroid belt and is also now classified as a dwarf planet. So a number of different objects there that we see that are all now classified as dwarf planets, they're like planets but you see how small they are compared in scale to our Earth here. So much, much smaller than our Earth and much smaller than any other planet that we see in fact smaller even than the larger moons that we see in the solar system. And finally there are also some other objects that have been discovered things like the asteroids and here we have some images of some various asteroids that have been studied Ceres and Vesta have both had spacecraft orbit around them to be able to study them so we've actually looked at those in more detail some of the other ones we've actually also visited so we've now visited some asteroids and we've been able to study these they are very small you could take all of the asteroids in the asteroid belt and not even make one moon, make up one moon so we can see them here they actually look like the lunar surface but now we're getting smaller objects they do orbit the Sun but that's the only thing that they have in common with the other objects they are not spherical some are maybe Ceres which was classified as a dwarf planet would be that Eida, Eros and Gasper are actually not spherical and they just look like old cratered objects like little pieces of our own moon. Now the other objects that we see here are comets now comets also orbit the Sun they are by no means spherical we have an image of the comet here that was observed by the Rosetta spacecraft that actually orbited that comet and this comet then has a big bulge on one side and another in a much thinner area in the middle so we've now actually been able to see comets and study them up close in ancient times what we saw was something like this this would have been the comet that we would have seen from the Earth's surface the actual nucleus the part that we see in large tier that we saw in great detail would have been a tiny speck in the middle of this comet that we would see in the sky so we've actually not only seen the comet and their tails and all of that but we've actually been able to study them in much more detail and in fact have orbited comets and in fact actually collected pieces of debris from comets. So next thing we want to look at is kind of an idea of what the solar system is to scale and we look at this that if we just imagine taking something that is about 25 centimeters to represent the sun that would be a ball about 10 inches in diameter and then we looked at what everything else would be so if we were to scale everything down scale the sun down to a ball of that size we would find that mercury would be less than a millimeter in size and would be 10 meters away so you'd have this ball 20 meters away from it you would have the first planet in between those two is nothing the earth would be a little bit over a millimeter in size and would be 27 meters away so 27 meters would be if you want to put that into feet if you're more familiar with that multiply it by about 3 and you'd get close to 80 between 80 and 90 feet away so you'd have to have quite a different distance a distance for this to be able to be seen so there's really a lot of empty space there now Jupiter would be 2.5 centimeters in size and would now be 140 meters away or more than a football field away Pluto would be much tinier smaller even than mercury about half a millimeter and would be a kilometer away from the sun so in order to make a scale solar system we would have to need a kilometer's worth of space to be able to to make this and most of that would be very empty there would only be those scattered little objects reaching as much as 2.5 centimeters over that kilometer if you wanted to look at where the nearest star would be alpha centauri is about the same size as our sun and would be about 7500 kilometers away so what we want to look at here is how empty everything is space is very empty the solar system is very empty so if we try to put everything to scale some of the pictures that we looked at where you see the sun and the planets and everything looks close together are really not realistic representations of what the solar system is actually like the one that we want to look at here is talking about how we name some solar system objects it all depends on what the type of objects are there are various things that are set to be able to name the types of objects so if we look at for example planets and moons they are named after the gods and heroes from Greek and Roman mythology with one exception Uranus and its moons what are named of characters from English literature so Uranus has a little bit different it's well it is named after from Greek mythology its moons are actually named for things from English literature when we do the names of features on some of these there are different things that are taken up for the features on the different objects some are named after scientists such as features on the moon others are named after various artists and other people now when we look at other objects comets are named differently comets are named after the discoverer so whoever discovers a comet gets their name attached to it or their names if multiple people happen to discover it at the same time differently are not named after the discoverer but are named by the discoverer so if you discover a new asteroid you get to pick the name for it subject of course to probably to certain restrictions you can't use anything that has already been taken of course and you can't use anything that would be an inappropriate name but for other objects the naming conventions are set by the international astronomical union and that would define exactly how you name them and leaves for some consistency as to how things are named so let's finish up here with our summary which says that we're looking at the solar system it has a wide variety of types of objects and we've looked at a number of those but the most of the material is our sun so the sun has almost all of the material in the solar system the smaller solar system objects have a very varied structure and features so we'll look at those in other lectures if you want to look at them in more detail you can look at some of the other lectures that will cover those in more detail and finally a scale model solar system is essentially impossible to construct because of the immense distances you can either get things to scale in terms of distances or in terms of sizes but trying to do both at the same time means that you need a lot of space and a lot of it has to be empty and again that's the key point here the solar system and space itself are very empty so that concludes our lecture on the introduction to the solar system and we'll be back again next time for another topic in astronomy so until then have a great day everyone and I will see you in class.