(Proposal ID) S17B-044 (PI) Yoshida, Fumi (Proposal Title) Hunting Planet Nine and RR-Lyrae stars with HSC (Abstract) In this proposal, we request 5 nights to continue our HSC g-band cadence observations (2 visits per night per field, 8 visits in total per field) over ~200 sq. degrees to hunt down the theoretically predicted giant "Planet Nine". The combination of large aperture, wide field-of-view, and exquisite image quality of HSC/Subaru enables us to reach the required depth of 25 mag (AB) with 90 sec exposure for each visit, allowing us to cover the entire high- probability regions where Planet Nine should be located. If discovered, Planet Nine promises to revolutionize our knowledge of the planet formation theory, the physics of giant planets, and the connection to exoplanetary systems. In addition to this high-risk, high-return scientific goal, the entire data set will allow us to carry out the following ancillary science: (1) discovery of ~2 times 10^5 main-belt objects and ~10^3 Trans Neptunian objects in this previously unexplored region and determine the faint end of their size frequency distributions, and (2) the discovery of ~1500 RR-Lyrae (RRL) stars in the halo region of the Milky-Way (MW), which allows us to study the structure of the MW dark matter halo and to possibly detect its outer edge.