 Have you heard? Another planet may be lurking in the distant shadows of our solar system. It's not out of the realm of possibility. In fact, a couple of scientists think it's basically a certainty at this point. Say hello to Planet 9, or Planet X. Well, you can't really say hello to it because we haven't seen it yet, but two researchers at Caltech, Mike Brown and Konstantin Batygin, are convinced it's out there based on the way objects are moving around at the edge of the solar system. And if it does exist, it's a pretty sizable planet. The two estimate it's about 10 to 12 times the mass of Earth, so kind of like a smaller Neptune. And it orbits way out there. At its farthest from the Sun, the planet is somewhere between 500 and 1200 AUs. For reference, Pluto's farthest distance is less than 50 AUs from the Sun. So it's possible that this planet takes up to 20,000 Earth years to complete just one orbit. Right now, the stranger is mostly going by the name of Planet 9, but not everyone is keen on that title. I don't like them calling it Planet 9. The astronomer that discovered Pluto, his name Clyde Tombaugh, he did it in 1930. He was 24 years old, and people had looked for a generation, and nobody could find the planet they were looking for. So the reason I don't like the term Planet 9, when by my count the next one would be about Planet 29, is that it diminishes Clyde Tombaugh's legacy. What he did was absolutely an amazing feat for 20th century astronomy. And I think it's insensitive and a little bit rude to try to diminish that. But Planet 9 is the name that stuck, so I guess we'll go with it for now. Anyway, the search really gained steam in 2014 when Mike and Constantine decided to look at the outer solar system a little bit more closely. They were inspired by the discovery of a new object of the Kuiper Belt, that's the cloud of icy bodies that orbits past Neptune. The announcement of this new object called their attention to the most distant objects in the solar system. These are the ones that take more than 4,000 years to orbit the sun. They noticed a strange pattern with six of these far out objects, including the one that had just been found. They all had a common argument of perihelion. Don't worry, the Kuiper Belt objects aren't in a fight or anything. The argument of perihelion is this really obscure orbital feature, and it basically means whenever these objects make their closest approach to the sun, they're all at the same angle to a plane of reference. That was immediately strange, because this parameter should basically be random for each object. It was the first indication that their orbits had been manipulated, possibly by another planet. Then a closer look showed even more interesting similarities. All of their orbits lie in the same plane, and they're all kind of tilted away from the plane of the solar system by about 20 degrees. Plus, they're all highly elliptical and swing out from the sun in the same direction. Normally, these objects would trace out randomly in a flower-like pattern. It's as if some thing had arranged these objects so that they cluster in space together, and that something had significant gravitational influence. So, the two researchers set out to figure out what was behind this strange pattern. At first, they rejected the idea that a planet was the culprit. You can't just be pulling planets out of thin air. I mean, the vacuum of space. But after ruling out all the other mechanisms that could be causing these objects to cluster, they decided there was no other explanation except for a planet. It's not a totally crazy idea either. Neptune was predicted purely through math in 1846. So, inspired by that process, Constantine came up with a model of the solar system as it evolved over 4 billion years. He threw Planet 9 into this model, and sure enough, it evolved beautifully to the solar system we see today. And since this first discovery, even more objects have been found in the outer Kuiper Belt that fit this pattern. But there are extra clues that Planet 9 exists too. For one, it may be responsible for why the sun is tilted. For many years now, we've known that the sun's axis is tilted, or oblique, by about 6 degrees in relation to the rest of the solar system. Basically, all of the planets orbit in the same flat plane around the sun. But the sun looks like it's spinning at a slight angle in relation to everything else. It's been a long-standing mystery for a while, but the Caltech researchers say Planet 9 could be behind this. The planet's gravity has torqued the solar system out of alignment with the sun, so basically all of us are the ones that are tilted. Then there's another clue. A bunch of objects right next to Neptune are orbiting the wrong way in the solar system. One recently discovered object, NICU, is basically perpendicular to the rest of the solar system, and it's going in the opposite direction of most other objects. Neptune says Planet 9 is responsible for this too. It's doing something called the Kozai effect. It's actually tilting the orbits of objects on their sides, and with the help of Neptune, they end up orbiting the sun in the wrong direction. Alright, so we have all this evidence for Planet 9, but we still don't have this smoking gun. Seeing the damn thing. The Caltech researchers have been observing the night sky with Subaru, a huge telescope located in Hawaii. The goal is to completely map out the orbit of the planet so they can eventually catch it. It's a process that may take a while, probably a couple years, and it may be hampered if Planet 9 is orbiting against the backdrop of the Milky Way. Since this planet is so far away from the sun, it's super faint, so it'd be much harder to see against a bunch of really bright stars. But it's not so ridiculous to think that this planet might actually exist. One of the biggest findings from NASA's Kepler mission is that smaller Neptunes like Planet 9 are very common throughout the galaxy, so having one in our neighborhood actually makes this pretty normal. Constantine says he's confident we're going to find it eventually, but we may just have to wait a while. So, fun fact, Mike Brown is actually referenced in another video that we did about what makes a planet a planet, because he's often credited with helping demote Pluto to a dwarf planet. So the irony here is that he got rid of one ninth planet and then replaced it with his own ninth planet.