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.
For a long time, scientists assumed that Earth's water was delivered by asteroids and comets billions of years ago. This coincided with the Late Heavy Bombardment (ca. 4.1 to 3.8 billion years ago), a period when planets and bodies in the Solar System experienced a much higher rate of impacts. According to this theory, the planets of the inner Solar System were unable to retain volatile elements such as water due to their proximity to the Sun. However, recent findings from the analysis of lunar rocks and regolith returned by the Apollo missions have cast doubt on this assumption.
