Planet Nine - also referred to as Planet X - is a massive, hypothetical object in an elliptical orbit far beyond Pluto, roughly at a distance that would take 10,000 to 20,000 Earth years for it to complete a single trip around the Sun. .

Though no direct observations have ever been made of an object fitting the description of Planet Nine, unexpected patterns in the orbits of other, smaller objects in the icy outer reaches of the Solar System could be explained by the gravitational pull of such a body.

In early 2016, California Institute of Technology astronomers Mike Brown (famed for his role in demoting Pluto from its rank as the ninth planet in 2005) and Konstantin Batygin announced their observation of dwarf planet 2012 VP113's orbit lining up uncannily well with the orbits of five other extreme trans-Neptunian objects.

To date, the best evidence of an undiscovered planet in the farthest reaches of our Solar System remains this clustering, along with a strange inclination of their orbits and the orbit of 90377 Sedna that can't be explained by the presence of Neptune.

However, subsequent analyses carried out by the Outer Solar System Origins Survey, as well as several other survey results, have found no sign of clustering among other relevant trans-Neptunian objects, making Planet Nine's existence extremely contentious for now.

Errors, biases, or coincidences aside, strangely linked orbits among trans-Neptunian objects could be the result of several phenomena other than a big, hidden planet.