 Greetings and welcome to the Introduction to Astronomy. In this lecture we are going to talk about Pluto as well as the other dwarf planets in the solar system. So this is a relatively new object, at least in terms of definitions, as to a type of planet that had not been classified previously. So let's look a little bit about the definition of a planet. What is a planet? Well really it was never formally defined until 2006. Now in 2006, the International Astronomical Union decided that a planet must do three things. First of all it must orbit the Sun. That removes moons from being planets even if they are larger than some of the planets, and we do have at least one moon that is larger than Mercury. But if it doesn't orbit the Sun, it's not a planet. It must be massive enough to pull itself into a spherical or an ellipsoidal shape under gravity. So its gravity must be dominant that it pulls it in to that compact shape. It cannot have an irregular shape and it must be able to clear its orbit of debris. So things like the asteroid belt and the Kuiper belt that we'll look at with objects there would not be able to be classified as planets because there is still a lot of debris, lots of material in those in those regions. Now the discover what really necessitated this is that we began discovering a large number of large objects in the Kuiper belt that really necessitated this change. The asteroid belt there's only one large object. So it really wasn't as big of a deal there. However, when we started finding more objects especially those similar in size to Pluto, this started to become a problem. So is Pluto a planet? Well, Pluto does orbit the Sun so it meets the first criteria. It is massive enough to pull itself in a spherical or ellipsoidal shape. That meets the second criteria. However, it does not meet the criteria to be able to clear its orbit of debris. It orbits in the Kuiper belt which has lots of other objects, many small but some that are comparable in size to Pluto. So what was done was to make a new classification of object and that was a dwarf planet. And there are now five dwarf planets in the solar system that have been confirmed and we will look at those in turn. So let's look at overview here. What are the dwarf planets? Well, the first dwarf planet was actually discovered in 1801 and that was Ceres. This was the largest object in the asteroid belt and is the only dwarf planet in the asteroid belt. When Ceres was discovered in 1801 it was thought it might be a new planet. So it was originally classified as a planet existing between Mars and Jupiter. However, we noted shortly thereafter discovery of other objects in that area and therefore it was reclassified to an asteroid and then reclassified again to a dwarf planet. So Ceres has had many classifications over the little over 200 years we have known of it. Pluto was discovered in 1930 and was visited by the New Horizons craft in 2015. Only Ceres and Pluto have been visited by spacecraft. And then we've discovered Haumea in 2004, Eris in 2005, Makemake in 2005. Eris of these is the one that is comparable in size to Pluto. Maybe a little bit bigger, maybe a little bit smaller, about the same size though, and it's difficult because we don't have direct measurements of it. We have seen it from Earth but we have not visited Eris. So let's look at each of these in a little bit more detail. First of all, Ceres. Ceres is the largest object in the asteroid belt and is only about a thousand kilometers across. It was discovered in 1801 and as I've mentioned it was originally thought to be a new planet. Then it became an asteroid. Now it is a dwarf planet so it's gone through a number of different changes in its classification over the years. Now we'll look at this in a little bit more detail actually when we talk about the asteroids but it is one of those craft that has been studied, one of those objects that has been studied close up by spacecraft and we can see it is heavily cratered. Although we do see some very bright areas here, not ice as we saw on the moons of the outer planets but maybe some kind of salty deposit that is dug up in these impacts. So very heavily cratered, very old surface that's been around for a long time. No sign of geological activity. Now of course the best known of the dwarf planets is Pluto and Pluto was discovered in 1930 by Clyde Tomboff and he was searching for a planet that was disturbing the disturbing the orbit of Neptune. Now what that is is that there was, Neptune didn't seem to be orbiting exactly the way we thought. However what was known was we had used this method to discover Neptune previously because of irregularities in the orbit of Uranus and that led to the discovery of Neptune so it was thought to do the same thing. However it took an exhaustive study of many regions of the sky to actually find Pluto. When we looked at this from earlier studies from Earth we get images like this so here is looking for some companions stars, companion moons and we can see several moons there in the image and of course some of the best images would be from Hubble space telescope and we can see some lighter and darker terrain. So until about 2015 this was our best image of Pluto. We knew there was some lighter and darker areas but we really didn't have any idea of the detail. Now in 2015 it was visited by the New Horizon spacecraft and we found that it had an icy surface like the large moons that was pretty much expected. It had ices of methane nitrogen so those were things that would be expected in that part of the solar system. What was not expected was that there were some regions with very few craters so it's an active object because some certain areas have had regions that have been wiped out of craters and the darker areas which are more heavily cratered so in a way like our moon except in reverse. On the moon the darker areas are the lightly cratered ones. On Pluto the dark areas are more heavily cratered and we saw a variety of different types of terrain on the surface so we can look at some of those images. Here we see some of those regions and various different type fault lines very plain planes very low lying areas heavily cratered areas so it was very interesting surface that we saw and not quite what was expected. We saw signs of some of the things some of the close-up areas very very flat so we can see how nice and smooth some of these regions are and that they have been flooded and in fact some of this jumbled material could be left over material from craters that did not get filled in when the material flowed so this could have been the higher lying areas that were left behind when some icy material flowed through. Now we also saw that there was a very big variety just in a very short range here we have some of this region very very flat and smooth with some pieces sticking through where lava had flooded here a much higher region kind of on the edge of this that was left behind and then we go into this more jumbled region and then we have a more heavily cratered region here so just in a relatively small region we do see a number of different types of terrain and astronomers even nearly a decade later are still studying the images New Horizons sent back from Pluto to learn more about this distant dwarf planet. Now one thing we do know is that Pluto does have a very thin atmosphere of nitrogen gas this can be seen in an image like this one from behind actually when New Horizons was leaving Pluto it looked back towards the sun and could see Pluto's atmosphere in relief it's primarily nitrogen gas because nitrogen gas freezes out at a very very low temperature and at this point Pluto was still warm enough to have a thin atmosphere. Pluto is currently in an elliptical orbit and moving further away from the sun so that atmosphere will likely freeze out onto the surface for hundreds of years until Pluto comes in closer again. Now Pluto does have a number of moons including the large moon Charon which was discovered in 1978 and looks in many ways a lot like Pluto. We see definitely signs of activity on the on the surface some heavily cratered regions but some cracking on the surface as well and maybe some flatter areas as well with fewer craters so it's a very interesting image and it is tidally locked to Pluto and Pluto is tidally locked to Charon those two are locked together and means they always keep the same side facing each other so one side of of Charon always faces Pluto just like our moon does with Earth but also one side of Pluto points toward Charon so that means if you're on one part of Pluto you can see Charon in the sky and it will just sit there however if you're on the other side you never see the moon it will never rise or set it is locked in what we'd call a geo for Earth a geosynchronous orbit it is locked and will stay in the same point in the sky much as we do with geosynchronous satellites. There are also other moons that have been discovered for smaller moons we can take a quick look at here. Nix and Hydra were known prior to New Horizons leaving Earth. Sticks and Kerberos were actually discovered en route not by the New Horizons craft but by imaging from Earth and we're studied we're studied not imaged very well because the orbit had already been pretty much set so we couldn't get close to all of these and you note that some of them have not less detailed images Hydra has a much more blurry image than Nix it just happens to be where they were relative to the spacecraft when it flew by Pluto. Now these two are the best studied dwarf planets Ceres and Pluto however there are other dwarf planets as well and that includes Eris which is comparable to Pluto in size it does have a moon Dysnomia which helps us to determine its mass from its orbit and we can see both of those here here as seen from Earth but again as with Pluto we can't see much from Earth this is even further away so it's very difficult to get any kind of imaging of it in fact essentially impossible to get any imaging of the surface. We also have Homea and we can picture that here now this is not an image an image of it this is actually an artist's conception based on our understanding of Homea from observations from Earth we see that it has a couple of moons that we do know know it is one of the objects that is not spherical but is ellipsoidal so it's in a flattened shape this is also a stable configuration under gravity for a massive object and if you recall that was one of the requirements to be a planet or a dwarf planet was to have enough mass to pull it into one of these shapes and then there is Makemake again an artist's conception image of what it might look like from around that that dwarf planet looking back toward this relatively bright star in the sky a very bright star in the sky but that is our Sun so with this depth in the solar system the Sun is just an extremely bright star but it does not stand out the way it does here on Earth so as of right now these other three objects have not been studied and are not current plans to be able to go visit them to see but of course future missions may change that and may get us a chance to learn more about these dwarf planets so let's go ahead and finish up with our summary and what we've looked at is that in 2006 a planet was defined and that meant Pluto was removed from the planet classification and reclassified as a dwarf planet we now know five of these dwarf planets two of which have been explored by spacecraft and the studies of Pluto will help us understanding these other objects in the Kuiper Belt so that concludes this lecture on Pluto and the dwarf planets 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