 Greetings and welcome to the Introduction to Astronomy. In this week's Misconception in Astronomy, we are going to discuss Pluto and find out why Pluto is no longer classified as a planet by astronomers. So let's go ahead and take a look at this. And here is Pluto, and is Pluto a planet? Well, our quick answer is no. As of 2006, Pluto has been reclassified as a dwarf planet. And this is actually a new classification that never existed before for a new type of object of which Pluto is one of the first. Now, this is done because until 2006, we never really defined what a planet was. So what is a planet? Is it just an object that is orbiting around the sun? Well, we said no. We said it had to be able to orbit the sun, but it also had to do several other things. It had to have a spherical shape, meaning that it had enough mass to crush its gravitationally crush it into a sphere, which is the most stable shape under gravity. And it also had to be massive enough to clear its orbit of debris. Now, why did we make this change? Why couldn't we just leave things as they were with the nine planets that used to exist before 2006? Well, what we had found is that we started finding many other objects out there that were similar to Pluto. So, we see a few of those here, and in fact, a couple of these are also now classified as dwarf planets, Haumea, Makemake are two of the five dwarf planets that exist. Now, as you can see, these were discovered right around the time we started reclassifying Pluto in 2003 and 2005. Now, you notice that Haumea is actually not spherical, but flattened, and that's actually an ellipsoidal shape. So in saying spherical shape, I was kind of simplifying things, it's really an ellipsoidal shape that will be the most stable path of formation under gravity. So a sphere is one example of those and is quite common, but we do sometimes get those ellipsoids as well. So we started finding these other objects, and in addition, we thought previously that Pluto was much larger than we've now since learned that it is. And in fact, when Pluto was discovered in 1930, it was thought to be another planet out beyond Neptune that was to account for deviations in Neptune's orbit. However, we've since found that Neptune doesn't have any deviations and that Pluto in any case would be far too low mass to be able to affect the orbit in any way. So that's again one of those other reasons that it has been reclassified. It simply doesn't meet the criteria that are needed. And in all honesty, Pluto probably never would have been discovered in 1930 had it not been for an intensive search looking for that planet out there that was affecting Neptune's orbit. So a detailed search of the sky looking all over trying to find this object that could possibly be affecting Neptune. And again, we've since found that there are no problems with Neptune's orbit that Neptune orbits just fine and that another planet out there is not needed. Now, has something like this ever happened before? Well, yes it has. There have been other objects that have been classified as planets and then reclassified. One example of those is here. And this is Ceres. Ceres used to be the largest of the asteroids. However, when it was discovered in 1801, it was originally classified as a planet. It was thought to be a new planet that was discovered between Mars and Jupiter. And several years over the next few years, we found other objects out there and that led to it being reclassified as an asteroid or a minor planet. And then finally, Ceres was then reclassified again as a dwarf planet. So for part of the time that we've known of the existence of Ceres for the first 60-some years, it was classified as a planet. And that was not only a little bit less than Pluto was classified as a planet. But as we began to find these other objects, it then became reclassified and is now one of the dwarf planets and the only of the dwarf planets in the inner solar system. So let's go ahead and finish up with our summary. And what we've looked at is that Pluto is no longer classified as a planet as of 2006. It is now a dwarf planet which is the new classification that was created based on other objects that have been discovered in the outer solar system. There are now five confirmed dwarf planets which includes Pluto as well as other possibilities that may be dwarf planets requiring further study and confirmation. So that concludes this discussion on why Pluto is no longer classified as a planet. We'll be back again next week for another misconception in astronomy. So until then, have a great day everyone and I will see you in class!