Once, our solar system was home to nine planets, a fact that was etched in the minds of students and astronomers alike.

However, this classification was challenged by the groundbreaking work of astronomer Mike Brown, who earned the moniker "the man who killed Pluto." Brown, a planetary astronomy professor at Caltech, uncovered a distant world named Eris in the Kuiper Belt, a region teeming with icy relics from the solar system's formation, located beyond Neptune's orbit. This revelation in 2005 sparked a sequence of events that led to Pluto's controversial demotion from planetary status in 2006.

Now, in a twist of cosmic irony, the Kuiper Belt, the same area that seemingly took a planet away, might be on the verge of giving one back. The belt, which stretches 50 times farther from the sun than Earth, with an extension that reaches nearly 20 times that distance, is believed to contain remnants from the solar system's nascent stages.

Among the myriad icy bodies that reside there, Pluto, now reclassified as a dwarf planet, is but one of the largest, failing to dominate its orbit and clear it of other objects, thus not meeting the criteria set by the International Astronomical Union for full planetary status.

The search for an unobserved planet, inferred through the gravitational influence it exerts on surrounding objects, has been dubbed "Planet X" or "Planet Nine." Malena Rice, an assistant professor of astronomy at Yale University, emphasizes the significance of such a discovery: "It could completely reshape our understanding of the solar system and of other planetary systems, and how we fit into that context. It’s really exciting — there is a lot of potential to learn a tremendous amount about the universe."

This excitement, however, is not without its share of controversy. Competing theories about the nature of this potential planet exist, with some researchers even doubting its existence. Rice acknowledges the divide: "There are definitely full-blown skeptics about Planet Nine — it’s kind of a contentious topic. Some people feel very passionately that it exists. Some people feel very passionately that it doesn’t."

The quest for Planet Nine has been fueled by the work of Batygin and Brown, who have been actively searching for the elusive planet since 2014. Their inspiration came from a study by Scott Sheppard and Chadwick Trujillo, who first noted the peculiar clustering of orbits among known trans-Neptunian objects. They proposed that an unseen planet, several times larger than Earth and much farther from the sun, could be "shepherding" these smaller objects.

Batygin, in collaboration with Brown, has presented several lines of evidence supporting Planet Nine's existence, with the strongest being the perturbation of icy bodies into Neptune's orbit before their ejection from the solar system. He describes this as a "smoking gun" and a significant piece of evidence.

Planet Nine is hypothesized to be a "super-Earth" with a mass between five to seven times that of our planet, and an orbital period of 10,000 to 20,000 years. However, alternative theories propose a smaller planet, or even a primordial black hole as the source of the anomalies observed in the Kuiper Belt.

The debate is set to be resolved with the advent of the Vera C. Rubin Observatory, scheduled to begin operations in late 2025. Equipped with the largest digital camera ever built, this telescope will have the capability to survey the entire sky every few days, potentially uncovering more trans-Neptunian objects and providing definitive evidence for or against the existence of Planet Nine.

Rice highlights the importance of such a discovery, whether it be a super-Earth or a smaller planet, as it would provide invaluable insights into the common types of exoplanets discovered around other stars. She also notes the humbling fact that even the number of planets in our own solar system is not set in stone, reflecting the dynamic nature of scientific knowledge and the ever-evolving understanding of our universe.