 Greetings and welcome to the Introduction to Astronomy. In this video we are going to talk about Pluto and also the other groups of dwarf planets that we now know about out in the solar system. So this is really something new that has come up relatively recently. As dwarf planets, most of these were not known about even just a couple of decades ago. And essentially what happened is that we finally actually defined what a planet is and gave a definition for what a planet is. Until 2006 there was never a formal definition of a planet. So what was a planet? It was what we say it is. It was not it did not have anything specific that was defined. So let's look at what this definition became and there were three things that had to be done for a planet. First of all it had to orbit the Sun. That meant that if a large moon could be larger than some of the planets for for example but would not be considered a planet because it does not orbit the Sun. It must be massive enough to reach an equilibrium with gravity and that means it pulls itself into a spherical or any ellipsoidal shape. And finally it must be massive enough to clear its orbit of debris. So things like the Earth do not have a lot of debris floating around them. There are of course some asteroids that come close to us but there's not a whole debris field there as there is with for example the asteroid belt and the Kuiper belt. And that is one of the things that led to this change was that we discovered a number of our objects very large in fact comparable in size to Pluto that were existing in the Kuiper belt and it was a decision are they all going to be planets or is this just another group of objects like the asteroids much further out and Pluto happened to be the first of these. So let's take a look at Pluto specifically in terms of this so according to the definition of a planet does Pluto meet it? Well it orbits the Sun yep that's good it is massive enough to pull itself into a spherical or ellipsoidal shape that's correct what it does not do is clear its orbit of debris it is out there in the Kuiper belt so there are lots of objects out there that are Pluto size or smaller and Pluto never became gravitationally dominant it did not collect up all that material making itself larger it was not gravitationally dominant to be able to kick that material out of the solar system all together. So in addition to defining a planet we also made a definition of a dwarf planet and there are now five dwarf planets that are known in the solar system. So let's take a look at what these are what are the dwarf planets well there are five of them and the first one actually discovered was Ceres. Ceres was discovered in 1801 Ceres imaged right here although we'll see some better images. Ceres is the only dwarf planet in the asteroid belt it is the only object in the asteroid belt that makes those that three criteria to the two criteria to classify it as a dwarf planet essentially a dwarf planet has to meet the first two planetary criteria they have to orbit the Sun and they have to be massive enough to pull themselves into that spherical or ellipsoidal shape. Ceres when it was discovered in 1801 was thought it might be a new planet maybe there was another planet there in the asteroid belt and we'll take a look at that in a little bit. Now Pluto was the next one discovered in 1930 after an exhaustive search Neptune's orbit was thought to not be exactly as it should be and it was considered that maybe there was another large object out there causing its orbit to deviate and after searching the sky Pluto was found in 1930 it was visited in 2015 by the New Horizons spacecraft which gave us a up-close look at this object for the first time. So Pluto is seen here with its satellites the other ones also all out in the Kuiper belt were Hamea which is this one here. Note that this one is actually not spherical this is one of those objects that is in more of an ellipsoidal shape still in a state of equilibrium with gravity but not exactly spherical as some of the others we also have Eris which is comparable in size to Pluto and Makemake which is a little bit smaller but around the early 2000s we did start to find a number of objects and many of these were comparable in size to Pluto as you can see in the scale drawing here that Pluto at this size Eris very comparable in size and Hamea and Makemake is very similar sizes so let's look at these in a little more detail let's look first at Ceres Ceres is of course the largest object in the asteroid belt and the only dwarf planet there it was studied by the dawn spacecraft that was orbiting or it orbited around it and it was originally discovered in 1801 looking for an object between Mars and Jupiter and it was thought that it might be a new planet that was discovered there it was eventually reclassified from a planet to an asteroid so now it is no longer an asteroid it is actually sorry now it is no longer a planet it was classified as an asteroid as more and more large objects began to be discovered in the asteroid belt over the coming years and now its classification has changed again it is now classified as one of the dwarf planets in the solar system and the only one in the asteroid belt and we do see it imaged here as seen by the dawn spacecraft now the best known of the dwarf planets is of course Pluto was discovered in 1930 by Clyde Tombaugh and what he was doing was searching for a planet that was disturbing Neptune's orbit and causing it to deviate from what it said what it was supposed to do and eventually it was found detected in the sky and it was found later that it just simply wasn't needed the orbit of Neptune just wasn't well known enough and it was not there was not a need for any other object to be disturbing its orbit so the need for a Pluto to have been there was later found to be incorrect now many of the early studies were difficult because Pluto is so small and so distant it cannot be imaged clearly from the earth so we cannot see it directly from the earth although when we look at it with the Hubble space telescope we can see that there are some light and dark areas so we can see that there are some darker areas here and some lighter areas scattered around so we could get some tantalizing views there were some variations in the terrain but we really didn't know anything in detail about it even with the most powerful telescopes here on earth now when we look at Pluto with new horizons we were able to get a lot more detail so what did we see well we found out that it is an icy surface which we probably expected and it is like the large moons of the giant planet so things like Triton it is very similar to that a very large moon orbiting around Neptune is quite similar to Pluto we do see a few craters on it and you can see some of those in this image there are some regions where there are a few craters and some cratered regions the darker areas being more cratered but there are also some very light areas which show little sign of cratering and it is composed of ices and as we've talked about with others that's methane nitrogen ices that make up the surface the dark areas are seen to be more cratered some of the light areas have very few craters and therefore have to be relatively recent and when we look at it close we see that there are really a large variety of terrains that we can see so we can look at some of the close-up images and see a lot of detail there mountains of ice and some ice flows that are present as well so very different types of terrain and in fact as we look at another image here very distinct types of terrain right next to each other so likely some kind of flooding over from the material here that covered a lot of this and then some of the higher areas were still sticking out so formed little areas here that stick up above the rest of the surface so we're still studying the images that were sent back by new horizons to really try to be able to understand more of this very distant object now we also know that Pluto has an atmosphere it has a very thin atmosphere of nitrogen gas not much else is going to be gaseous at this distance so it does have a thin atmosphere as we can see here when we look for new horizons from behind you can see some of that atmosphere illuminated by the sun Pluto does have several moons and the large moon Sharon actually looks in some ways very similar to Pluto we can see signs of cratering on some parts of it we can also see some kinds of stresses some kind of structural changes on the planet as well perhaps some kind of tectonic activity the moon is about half the size of Pluto and it when it was it was imaged by the new horizons craft as well and interestingly it is tightly locked to Pluto and Pluto is tightly locked to Sharon so our moon is tightly locked to us just as all the moons the large moons and the moons of the giant planets are meaning that they keep one side facing towards the planet so Sharon keeps one side facing towards Pluto but because of their similarity in size and mass Sharon has also locked Pluto in a title locking meaning that Pluto keeps one side facing towards Sharon so if you are on the side of Pluto that faces Sharon you would always see it hanging there in the sky if you were on the other side of Pluto you would never see Sharon now that would happen the same thing for our moon if you were on the moon if you're on the near side you always see the earth if you're on the far side you would never see the earth however from the earth you can see the moon at various times rising and setting now there are also other moons that have been discovered as well in addition to Sharon which was found in 1978 and they are pictured here and we're seen to various extents by the new horizons probe as it flew by it flew by very quickly so there was not a lot of time to get images and of course we could not rearrange the orbit to have it pass by all of these moons so Nixon Hydra we did get relatively decent pictures of for scale Sharon is shown now part of Sharon is shown down here at the bottom but sticks and Kerberos were imaged but not able to see anything of any great detail certainly we can see structures and craters on Nixon Hydra which we would expect for these small icy objects in the outer solar system now Pluto is not the only dwarf planet there are others and Eris is one of those that was discovered and it is comparable to Pluto in size they're very similar in size and you can see Eris here the bright object and it's moon dysnomia orbiting around it and so it does have a moon of course this is about the best image like with Pluto we have very little detailed images of these we can see that they are there but we don't have anything good to visit to see them with so there's no way to get any more detailed images now if we look at the others Haumea now this is an artist's conception image this is not for sure what it looks like but it is based on the observations of Pluto and the studies as to what we believe this is like and it is the one dwarf planet that is ellipsoidal so it is not a sphere it's kind of compressed into a squash sphere any ellipsoid it does have two small moons as well that orbit around it and finally Maki Maki another artist's conception image of what it might be like if you were just above the surface and thought again to be similar perhaps to Pluto is that's the only thing we have to go by to really be able to get any kind of understanding of these objects so Pluto we know a lot about the others we know about their orbits and we can make some estimates based on our observations but we really don't know a whole lot about any of these other objects so let's finish up here with our summary and our summary we have that in 2006 we actually made a definition of a planet for the first time and that meant that Pluto was remained from the classification of a planet and reclassified as a dwarf planet there are five dwarf planets known at this time two of these have been explored by spacecraft that would be Ceres in the asteroid belt and Pluto out in the Kuiper belt and our detailed studies of Pluto are going to help our understanding of other objects in the Kuiper belt we can use that as a base to get started to understand those other objects however unlike seeing them up close we are then making extrapolations based on what we know of Pluto so to really learn about them it'll take time until other missions can be planned decades from now to be able to study other objects in the Kuiper belt up close the new horizons craft will explore another object in the Kuiper belt so we will see some objects some pictures of that coming up in the near future so that concludes our 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