 Greetings and welcome to the Introduction to Astronomy. In this week's special topic in astronomy, we are going to talk about Planet X and what that means and what it has meant in the past. So let's go ahead and take a look at this and what we find out is that, yes, Planet X is something that has been considered in the past and is still considered today. What is Planet X? Well, it's a potential giant planet that is located beyond the orbit of Neptune. And Neptune, as you know, is the outermost of the eight planets in the solar system. Now, this was actually considered back in the mid to late 1800s as a way to explain what were thought to be deviations in the orbits of the outer planets. Uranus and Neptune weren't orbiting exactly as we expected them to, so there was a search for a further planet out beyond these two that might be affecting their orbits. Now, this is not unusual. This is how Neptune itself was found. But it turns out that this was not the case. However, that search for Planet X actually led to the discovery of Pluto seen here in 1930. Now, more recent studies have ruled out the possibility of any closer planet, and that means within the Kuiper Belt. That does not mean that a Planet X does not exist, but that any Planet X must be at a tremendous distance from the rest of the solar system. So, well beyond any of the other planets, there cannot be something else. The studies show that you simply cannot have one present that close to the rest of the planets within the solar system. But further out is still a possibility, and that's something that we do look at. And here we see orbits of some of the objects here. Now, if you note, there's our solar system with the orbits of the three outer planets shown right here. And those are Saturn, Uranus, and Neptune. Now, many of these other objects that have been discovered that have very large orbits that go well out, highly elliptical orbits, and these are objects that have been discovered over the last 20 years or so, are all on one side. And does that mean there could be some unknown Planet Nine on the other side of the solar system? Possibly, but nothing that has been found yet. So, it's something that is being searched for. And here is an example of part of that search. So, we see again all of these other objects. Let's look in at our solar system. There's our solar system out through Neptune, and then including the Kuiper Belt is all way down in this inner area. That's where we've ruled out anything new close to that. We've got objects out here, and the Subaru Telescope is searching this area of the sky with the potential of finding some other planet. So, there is the possibility that such a planet is considered, is possible there, and there is potential evidence but needs further study to really be able to find out and finally to be able possibly to detect if there is another planet out there. So, let's go ahead and finish up with our summary, and what we've looked at here is that Planet X is a predicted large object that would orbit out beyond Neptune. At one point Pluto was thought to be this Planet X, but it's now found to be far too small to work. And current studies suggest there is the possibility of a Planet X but well beyond the other eight planets known in our solar system. So that concludes this lecture on Planet X. We'll be back again next week for another special topic in astronomy. So until then, have a great day everyone, and I will see you in class.