What's Planet 9 and where could it be hiding in our solar system?

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Artists illustration of Planet Nine with the Sun and orbit of Neptune (ring) in the distance.— ESO
Artist's illustration of Planet Nine with the Sun and orbit of Neptune (ring) in the distance.— ESO

A recently submitted study to The Astronomical Journal persists in the search for the elusive Planet Nine, a hypothetical celestial body thought to orbit in the outer fringes of the solar system, well beyond Pluto's orbit, Universe Today reported. 

Conducted by Dr Mike Brown, a Richard and Barbara Rosenberg Professor of Astronomy at Caltech and the study's lead author, the research aimed to pinpoint potential locations for Planet Nine. 

By utilising data from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the study covered the largest region to date.

Dr Brown explained the motivation, stating, "We are continuing to try to systematically cover all of the regions of the sky where we predict Planet Nine to be." 

The study successfully eliminated about 78% of previously calculated possible locations and provided new estimates for Planet Nine's approximate semimajor axis and Earth-mass size. 

While the study didn't discover Planet Nine, it significantly narrowed the search area, covering approximately 80% of the predicted regions.

Looking ahead, Dr Brown identified the Legacy Survey of Space and Time (LSST) as a promising avenue for discovery. LSST, set to begin in a year or two at the Vera C Rubin Observatory in Chile, is a 10-year program designed to explore the southern sky. 

Dr Brown expressed optimism, stating, "If Planet Nine is there, [LSST] will find it." The potential discovery of Planet Nine holds significance as it would be the fifth-largest planet in our solar system, offering a unique opportunity for study. 

Dr Brown emphasised that Planet Nine is crucial for explaining various anomalies in the outer solar system, and its existence could unlock new insights into the solar system's formation and dynamics. The quest for Planet Nine continues, shaping the future of astronomical exploration.