There was a rumor going around that something terrible was going to happen on September 23, and whatever it was it was supposed to usher in the end times. It turns out the rumor was wrong, but the debunkers who argue that there is no threat because Planet X does not exist are not helping matters. They reason that if the planet were real and if it were headed toward the earth we could see it. The truth is somewhere in between. Planet X may exist although no one has seen it yet. And it is not going to hit the earth.
We’ve seen events that look very much like signs of the end of times—wars, earthquakes, hurricanes–so it’s natural for believers to wonder if the predictions about the end of the world found in the Book of Revelation are coming true. This is not just a quaint curiosity. If Revelation is not a prediction of the violent end of the world what is it talking about? If it has no meaning for our time what is it doing in the Bible? I plan to begin a discussion of Revelation in the next post, but to make a long story short it’s impossible to say whether Planet X is the cause of increased seismic and volcanic activity on Earth because the planet’s existence is more of a prediction than a sighting.
Planet X is not the first planet to be predicted before it could be seen. Neptune was found after Alexis Bouvard noted irregularities in the orbit of Uranus in 1820. Bouvard theorized that an unseen planet might be influencing Uranus’s orbit. His observations were repeated in the following decades, and led to a prediction of Neptune by French mathematician, Urbain Le Verrier. With the help of Le Verrier’s prediction, German astronomer Johann Gelle sighted Neptune on a single night of searching. Since that amazing demonstration of celestial mechanics astronomers have been intrigued by the possibility of a ninth planet. However the search has been a roller coaster of excitement and disappointment.
By 1910 conflicting predictions had been made for Planet Nine (Planet X). Edward C. Pickering and Percival Lowell led two unsuccessful searches for the planet, but predictions and searches continued until 1993. That’s the date when data from the Voyager spacecraft seemed to show that there were no positional anomalies in the solar system, implying that there is no Planet Nine. However this ‘proof’ was called into question almost immediately by ongoing discoveries in the Kuiper Belt. The following account is from Mike Brown, leader of the team who announced the discovery of Planet Nine in early 2016. 
At nearly the exact moment that Planet X was being put to rest, astronomers found the first new object beyond Neptune since the accidental discovery of Pluto (during a search for Planet X). As discoveries mounted, planetary scientists quickly realized that this population of objects in what we now call the Kuiper Belt is vast. To many of us who had begun to study this newest known collection of objects in the solar system, another thing became obvious: There was no chance that Pluto was going to be the only large object in the Kuiper Belt.
By 1998 Brown began a large survey of the sky from Palomar Observatory. The goal was to detect these large objects, but there was also the hope of finding something new beyond the Kuiper Belt. Astronomers Chad Trujillo and David Rabinowitz joined him five years later. They soon discovered a new object, now called Sedna, on an elongated 10,000-year orbit around the Sun. The closest approach of Sedna’s orbit to the Sun (its perihelia) was not in the Kuiper Belt, as it is for other Kuiper Belt objects with elongated orbits, and they concluded that something massive is, or was, tugging on Sedna’s orbit. Then it was found that the orbit of another Kuiper Belt object discovered three years earlier, 2000 CR105, was being pulled in the same direction.
Subsequently there have been many corroborating discoveries by astronomers in the U.S. and Brazil, and Brown says the odds that the similar alignment of these objects was due to coincidence are 0.007.
…we realized that everything we were seeing could be explained by a planet a little less massive than Neptune on an eccentric orbit that takes it from around 200 AU at its perihelion out to 1,200 AU at its aphelion—its further point from the Sun—over an approximately 20,000-year orbital period. Such a planet would capture Kuiper Belt objects with distant elongated orbits into stable orbits elongated in the opposite direction from the planet.
Moreover, it would pull the perihelia of these Kuiper Belt objects away from the Kuiper Belt…
With the many effects that Planet Nine is having on the outer solar system, we can infer many things about its properties. In practice, because the solar system is a complicated place, understanding these properties has involved massive amounts of computer simulation. We simulate a slightly larger planet, a slightly closer planet, a slightly more inclined planet, and each time we compare the results of our simulations with observations of the solar system that we know.
From these constraints we have determined that Planet Nine is about 10 times the mass of Earth, that its orbit is inclined by approximately 30 degrees to the plane of the planets, that it has an average distance of something like 600 AU from the Sun, and that when it is at its most distant point from the Sun, it lies toward the outstretched arm of the constellation Orion.
All of this relatively detailed knowledge might make it seem like we could, like Le Verrier, simply say to the world, “Go look; it will be THERE!” But we can’t. Le Verrier had the advantage of being able to analyze the full orbit of Uranus around the Sun to see its deviations. If we waited 10,000 years to fully track Sedna around its orbit, we, too, would be able to pinpoint Planet Nine.
Instead, though, we have only a snapshot of the orbits of a variety of different objects, and we must infer what should have happened in the past. In practical terms, that means that although we know the orbital path of Planet Nine through the sky, we don’t know where it is in its orbit. We no longer have to search the entire sky to find Planet Nine, but there’s still a lot of work to do.
The team is using the 8-meter Subaru Telescope on Mauna Kea and they expect other astronomers to join the search. As of June, 2016, Brown thought it was likely that someone in the world would spot the planet during the next five years.
 Mike Brown, How we discovered Planet Nine, Astronomy, June 2016)