Core Viewpoint - Astronomers are reigniting the search for a potential unknown planet, referred to as "Planet Y," which may be influencing the orbits of distant celestial bodies in the Kuiper Belt, suggesting the presence of a mysterious force affecting these orbits [3][4]. Group 1: Discovery of Planet Y - A recent study by astrophysicist Amir Siraj from Princeton University proposes that the gravitational influence of "Planet Y" could explain the unusual orbital tilts of approximately 50 cold celestial bodies in the Kuiper Belt, which are tilted about 15 degrees [3][4]. - The study indicates that the observed orbital tilt is significantly beyond the gravitational effects of Neptune, with a confidence level of 96%-98% [4]. Group 2: Characteristics of Planet Y - "Planet Y" is hypothesized to have a mass between that of Earth and Mercury, located approximately 100-200 astronomical units from the Sun, with an orbital tilt of at least 10 degrees [5]. - This hypothetical planet is distinct from the previously theorized "Planet X" or "Ninth Planet," which is suggested to be much larger and farther from the Sun [7]. Group 3: Historical Context - The search for unknown planets in the solar system dates back to the 19th century, with the concept of "Planet X" introduced by Percival Lowell, which was later reignited by the discovery of Eris in 2005 [6]. - The "Ninth Planet" hypothesis, proposed by Mike Brown and colleagues in 2016, suggests a massive planet with a mass 5-10 times that of Earth, orbiting at a distance of about 550 times that of the Earth from the Sun [6]. Group 4: Future Observations - The upcoming Vera C. Rubin Observatory's ten-year "Legacy Survey of Space and Time" project is expected to provide critical data that could confirm the existence of "Planet Y" or the "Ninth Planet" [8]. - This observatory, equipped with the world's largest digital camera, will conduct systematic observations of the southern sky, potentially detecting "Planet Y" within the first few years of its operation [8].