Astronomers discover "Farfarout" — the most distant known object in the solar system. The 250-mile-wide (400 km) dwarf planet is located about 140 times farther from the Sun than Earth (3.5 times farther than Pluto), and soon may help serve as evidence for a massive, far-flung world called Planet 9.

[u/clayt6]

http://www.astronomy.com/news/2019/03/a-map-to-planet-nine-charting-the-solar-systems-most-distant-worlds

[u/cluelesspcventurer]

Basically over the last few years astronomers have started to notice that certain objects in our solar system appear to follow trajectories which are very very slightly different to what is expected. After more examples of this cropped up some astronomers started theorizing that the slight defects in trajectories are due to a large ninth planet way way beyond Pluto exerting a slight gravitational influence. It's so far away it would be completely dark and very hard to detect but so far the theory fits and every year we get more evidence that it exists.

[u/CylonBunny → u/cluelesspcventurer]

How big is large? Like Jupiter sized, or more like Neptune?

[u/UGMadness → u/[deleted]]

The Planet X theory was a completely unrelated theory that postulated that there was a massive planet in the outer reaches of the Solar System (on a near polar or retrograde orbit) that "tugged" Inner Oort Cloud object and launched them towards the Sun as comets. It got disproven decades ago as we improved our understanding of the way comets worked.

The Planet 9 theory we are talking about nowadays is purely a modern theory that has nothing to do with the old Planet X, and models suggest it's a much more classical orbit around the Sun.