In Search of Planet 9: Why There Might Still Be Something Massive at the Edge of the Solar System

The Sun's gravitational pull extends out to more than 160 times farther than Neptune's. (Image credit: Vadim Petrakov via Shutterstock)

Is there a huge, undiscovered planet at the edge of the solar system? The concept has been around since Pluto was discovered in the 1930s. Renowned astronomers, dubbing it Planet X, proposed it as an explanation for Uranus’s anomalous orbit, which does not match the expected path of motion according to the laws of physics. The gravitational pull of an unknown planet several times larger than Earth was considered as a potential cause for this discrepancy.

The mystery was eventually resolved by a revision of Neptune's mass in the 1990s, but then in 2016, astronomers Konstantin Batygin and Mike Brown of the California Institute of Technology proposed a new hypothesis about the possible existence of Planet Nine.

Their theory revolves around the Kuiper Belt, a vast disk of dwarf planets, asteroids, and other objects beyond Neptune (including Pluto). Many Kuiper Belt objects, also known as trans-Neptunian objects, have been discovered orbiting the Sun, but like Uranus, they don’t all move in the same direction. Batygin and Brown argued that their orbits must be influenced by something with a strong gravitational pull, and they proposed Planet Nine as a possible explanation.