Is there a ninth planet? If you haven’t kept up on things you might respond, “Sure, Pluto.” But Pluto was demoted to a dwarf planet in 2006 because it doesn’t control its own gravitational environment (Neptune does). So is there a space object vying to take its place as the ninth planet? Yes, but I don’t know what to call it. Some people call it Planet X and some people call it Nibiru. I call it most likely non-existent.
Whatever you call it, even though no one has ever seen it, some scientists say that they know how big it is (five times bigger than the earth) and approximately where it is located because of its influence on the other twelve or so extreme outer objects we have seen (bodies in the Kuiper Belt and beyond) like Sedna. How can they be so sure? Because all these trans-Neptunian objects are aligned in such a way that they all make their closest approach to the sun around the time that they cross the plane of the solar system (“remarkably similar arguments of perihelion”). So astronomers are thinking that the ninth planet must shepherd all of these other objects and keep them clustered together.
So what is the flaw in their thinking? It’s true that you would expect this alignment to have been randomized over the life of the solar system if the solar system was 4.6 billion years old unless planet nine was there to keep them aligned. But what if the solar system is only six or seven thousand years old? Then no planet nine is required.
Since 2016 the astronomers have been training their telescopes on the region of space where they know planet nine has to be mathematically. They know that if they are the one to find it, they will be able to name it and they will probably get a Nobel prize and be famous. But what if their presupposition about the age of the solar system is wrong? It means a lot of time staring at nothing.