Planet X is a term used to describe a hypothetical major planet in the outer solar system, beyond Neptune, whose existence was initially inferred from unexplained gravitational perturbations of Uranus and Neptune. Although the search was largely satisfied with the discovery of Pluto, the concept of a distant, undiscovered world has seen a resurgence following new astronomical observations.

Historical Context and Early Search

The initial search for Planet X began in the late 19th century due to discrepancies in the orbital paths of the outer gas giants.

The discovery of Pluto temporarily ended the search, but its small mass soon led scientists to realize it was not massive enough to account for the observed orbital anomalies.

Modern Revival: Planet Nine Hypothesis

In recent years, the idea of a distant planet has gained new traction, driven by observations of the Kuiper Belt, a region of icy bodies beyond Neptune.

Evidence from Clustered Orbits

The primary evidence supporting a modern "Planet X" (often referred to as Planet Nine) comes from the peculiar orbital patterns of several extreme Trans-Neptunian Objects (eTNOs).

• Clustered Perihelia: The closest points to the Sun (perihelia) of a group of distant objects are all clustered in the same quadrant of the sky.
• Similar Inclinations: The orbital planes of these objects are tilted in a similar, unexpected way.

Astronomers suggest that the gravitational influence of an as-yet-undiscovered massive planet is required to herd these small, distant objects into their observed configuration.

Estimated Characteristics of Planet Nine

While the planet has not been directly observed, models predict the following characteristics: