Neil deGrasse Tyson may get the blame, but it was Caltech astronomer Mike Brown who really killed Pluto, and now he and his colleague Konstantin Batygin want everyone to change their planet count again because they've claimed to discover a Neptune-sized object floating way, way, way out in the dark. "Planet IX," as many have dubbed it, has not been seen yet—which will be tough since it is thought to be very distant, very slow, and very cold. The evidence for its existence however, is quite strong, and is similar to the way Neptune was discovered in the first place.
https://www.youtube.com/watch?v=42GeoCVaZQgBeyond Neptune's orbit is the Kuiper Belt, full of icy objects (including Pluto) that have strange, elliptical orbits outside the plane of the Solar System. In other words, all the planets from Mercury to Neptune orbit the sun in a big flat disc, but Pluto, Eris, Haumea and the millions of Kuiper Belt comets and ice chunks fly around at all sorts of wacky angles. BUT, scientists noticed, a handful of the largest ice chunks seemed to be coordinating their movements so that the moment they came closest to the sun was also the moment they crossed the plane of the regular eight planets. Los Angeles-sized chunks of ice don't normally coordinate anything, you see, because they're chunks of ice.
https://www.youtube.com/watch?v=jy6JcViPkWg
Brown and Batygin calculated that there was only a 1 in 15,000 chance that these ice chunks would have clustered in such a way randomly. What made far more sense was that a large object was pulling these frozen mountains into similar orbits. When they crunched the numbers, they found that there should be a large planet way beyond the orbit of Neptune or Pluto—one moving so slowly that it takes 10,000-15,000 years to go around the Sun.
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Even more impressive, the computer model suggested that if this Planet IX existed, it would push other Kuiper Belt Objects into orbits that were perpendicular to those oddly clustered ice chunks. And when Brown and Batygin looked, they found objects in exactly such perpendicular orbits.
Related: New images of Pluto's surface reveal majestic ice mountains at sunset, which is pretty neat.
But how did Planet IX get there? The current theory is that it formed alongside Neptune and Uranus, and is composed of the same stuff: mostly ice and methane. Somehow in the gravitational billiards game of the early Solar System, it was ejected and flung off into space. Instead of forever wandering the Milky Way alone in the dark, however, the gas cloud surrounding the early Solar System slowed its escape until it started to fall back towards the Sun...very, very slowly. Now it's wandering the Kuiper Belt in the dark, surrounded by other dark objects.
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The Subaru Telescope in Hawaii (the "flagship telescope" of Japan's National Astronomic Observatory) will now monitor the night sky for Planet IX, and they expect to get visual confirmation within the next five years. So, if you refused to admit there were only eight planets, congratulations! Now you can pretend there are ten. As a reward for reading all this, here's some Looney Tunes:
https://vimeo.com/64344734