 How many planets are there? Most people would probably say eight, unless you're one of the holdouts hanging onto the dream of Pluto one day being reinstated. Sorry to burst your bubble, but this video isn't about Pluto. In ancient times, a planet was anything that moved across the sky in relation to the stars. With the advent of the scientific revolution, the use of the term planet was changed to a body that orbited the sun. With this definition, there were six planets, Mercury, Venus, Earth, Mars, Jupiter, and Saturn. From 1801 to 1807, four new bodies, Ceres, Pallas, Juno, and Vesta were discovered that orbited the sun and so were added to the official list of planets. For nearly half a decade, these 11 planets stood. Then, by the 1950s, having learned more about the new planets and with the discovery of Neptune in 1946, it became clear that the four additions were drastically different than the rest of the celestial bodies and so they were removed from the official list of planets. While no official definition was made at the time, it was generally understood that a planet was any large body that orbited the sun. You see how that word large isn't very specific? Well lo and behold, that caused a lot of confusion a few decades later with the discovery of Pluto in 1930. Initial observations led to the belief that Pluto was much larger than Earth and so it was easily adopted as the ninth planet. Then, over time, as more measurements came in, Pluto got smaller and smaller and now we understand that it's less than 1% the size of the Earth. But that wasn't an insurmountable problem. It was still larger than any other known asteroid and seemingly didn't exist with any near neighbors. But then, in 2005, Eris, a dwarf planet past Neptune, was discovered that was 27% more massive than Pluto. And by this time, it became clear that there were far more Pluto-sized objects at the edge of our solar system than previously thought. So there were two options. Either Pluto is a planet which would make Eris and the likely many more Pluto-sized bodies that would come to be discovered planets or kick Pluto from the exclusive list. So in 2006, the IUPAC, the International Union of Pure and Applied Chemistry, agreed on a definition of planet that set it as a body orbiting the sun with enough mass to make a nearly round shape and it's cleared all neighbors out of its orbit. Because of the existence of Pluto's three close neighbors, Quora, Sedna, and Eris, Pluto was demoted to a dwarf planet. So now, what's all this about a ninth planet? Both Pluto and Neptune were mathematically predicted before they were first discovered based on their invisible pull on their neighbors. For example, Pluto was discovered because Percival Lowell, among others, was convinced that Neptune was continually in the wrong place. Why? Some massive object more distant than Neptune was tugging on it, competing with the sun gravitationally for control over Neptune. If you want to learn more about Percival Lowell, check out this video here about the mystery of the Canale on Mars. Interestingly, Neptune was almost not discovered by mathematical predictions and instead almost discovered by the first person who could have possibly discovered a new planet, the first to turn a telescope to the heavens, Galileo. Galileo turned his telescope on the planets and was immediately fascinated by the system of Jupiter and its moons. While he was observing Jupiter on January 27th, 1613, he recorded two stars in his field of view. One was a genuine star, but the other was actually Neptune. Remarkably, Galileo observed the pair again the following night and even noted a strange occurrence that the two stars appear to be further apart, so close to discovering that one of the stars was the planet Neptune. And now, Caltech researchers have found mathematical evidence suggesting there may be a planet X deep in the solar system. This hypothetical planet orbits our sun in a highly elongated orbit far beyond Pluto. The object, which researchers have nicknamed Planet 9, could have amassed about 10 times that of Earth, making it similar in size to Uranus or Neptune, an orbit about 20 times further from the sun than Neptune. It would take this new planet between 10 and 20,000 years to make one full orbit around the sun, where Neptune completes an orbit roughly every 165 years. This announcement does not mean there's a new planet in our solar system. The existence of this distant world is only theoretical at this point, and no direct observation of the object nicknamed Planet 9 have been made. This mathematical prediction of a planet could explain the unique orbits of some smaller objects in the Kuiper Belt, a distant region of icy debris that extends far beyond the orbit of Neptune. Since the announcement, astronomers are now searching for the predicted planet, even if only to snag dibs on the naming rights. The researchers calculated that there's only a 1 in 500 chance that what they're seeing is just observational bias, indicating that their predictions are probably quite accurate, or I guess that their models really aren't. So who knows, maybe we'll be back to 9 planets before you know it. Well, that's it for today on Everything Science. If you like this content, be sure to like and subscribe so you don't miss out on any future uploads, and remember, there's always more to learn.