X marks the spot: The hunt for Planet Nine

Since the beginning of time, humans have tried to make sense of the cosmic chaos above, ranging from our immediate solar system to the edges of the known universe. So far, astronomers have discovered and confirmed the existence of seven planets orbiting the Sun alongside Earth: three terrestrial planets (Mercury, Venus, Mars) and four gas giants (Jupiter, Saturn, Uranus, and Neptune). Now, they are on the hunt for a new addition: Planet X.

Pluto was once considered the ninth planet in the solar system until it was demoted to dwarf planet in 2006. Planet X has the potential to replace Pluto for the title of “Planet Nine,” but only if scientists can confirm its existence far out in the sky. The seven current planets in the solar system are well–understood; with extensive research on their orbit patterns, moons, climates, and locations. Planet X, however, remains a mystery. 

In a 2016 study, California Institute of Technology astronomers found evidence of the possibility of a large, undiscovered planet orbiting far beyond Pluto. Nicknamed Planet X (or Planet Nine), this hypothetical planet would be around the size of Neptune and have a mass five to 10 times larger than Earth’s. Planet X would orbit the Sun on a highly elongated path about 20 to 30 times farther than Neptune, taking between 10,000 and 20,000 years to complete one orbit around the Sun.

“Planet X has the potential to replace Pluto for the title of ‘Planet Nine,’ but only if scientists can confirm its existence far out in the sky.”

The home of this mysterious planet lies near the outskirts of the solar system in the Kuiper Belt, a doughnut–shaped region of icy objects. The Kuiper Belt orbits the Sun like the rest of the solar system and contains two different types of objects defined by their orbiting periods. Short–period objects include the majority of observed comets, which use Neptune’s strong gravitational pull to be flung inward towards our Sun. Long–period objects are less understood. With extremely long orbital times, they are believed to originate from the Oort cloud, a much more distant spherical shell of icy objects.

Long–period objects have interesting characteristics and strange gravitational behaviors that would be explained by the existence of Planet X. The long–period bodies are clustered in their orientations, meaning that the position of their respective angles or directions are close to one another and form a distinct grouping in terms of spatial orientation. On average, they are tilted by about 20 degrees with respect to the plane within which the planets orbit the Sun. Furthermore, they also often cross Neptune’s path and orbit in a retrograde manner where their orbits are in the opposite direction to their rotation, likely due to collisions with a large body like Planet X. 

Telescopes all over the world are searching the night sky for Planet X in hopes that its existence may explain the unique orbits in the Kuiper belt and the gravitational patterns observed in the solar system. However, with no concrete evidence to support its existence, Planet X and the behavior of its cosmic neighbors remain a mystery.