Starting last year, it's been suggested by some astronomers that a large, undiscovered planet could be orbiting at the far end of our solar system, subtly throwing off other planets' orbits without otherwise being detected.
And before we get into a completely different conversation: no, this is not Pluto, which is no longer a planet. This hypothetical ninth planet is thought to be much, much bigger than any dwarf planet, and would be several times the size of Earth based on the calculations which point to its existence.
explaining how a ninth planet is currently the best explanation for several odd phenomena in the solar system. These phenomena range from odd behavior among far out objects, to several planets' orbital planes being non-level with the sun's equator (suggesting an object further out in space might be pulling at them).
The biggest question Planet Nine would sufficiently answer is how our solar system's plane of orbit is "wobbling," to borrow a phrase from NASA's press release. Batygin goes on to say that since many planetary orbits are tilted, it would make sense that a large planet which exerted a smaller gravitational pull over 4.5 billion years could create just such a tilted wobble in other orbits.
Beyond this, a planet of the same proportions would also explain how several objects in the Kuiper Belt (a collection of icy bodies like comets and Pluto which extend from Neptune toward interstellar space) have the same stretched, elliptical orbits that point in the same direction, with the same 30 degree tilt.
When astronomers first noticed this and used computer simulations to determine that a gravitational pull from a Planet Nine should also be causing specific changes in other Kuiper Belt objects, they were able to find matching bodies that fit the bill. Several objects in the Kuiper Belt are severely tilted to almost 90 degrees from the solar plane, and others were orbiting in the opposite direction from nearly everything else, both of which fit exactly with the computer simulation.
Shortly after that, the first papers on Planet Nine were published in January 2016. And the potential planet has been poked at several times since then, with some claiming Planet Nine could actually be an exoplanet, although NASA left out that hypothesis from their statement.