(Proposal ID) S16B-049

(PI) Yoshida, Fumi

(Proposal Title)
Hunting Planet Nine and RR-Lyrae stars in the Milky Way halo with HSC

(Abstract)
We request 20 nights to conduct an HSC g-band cadence observation (2 visits 
per night and 8 visits of the same field in total) over ~800 sq. degrees in 
order to hunt down the theoretically predicted giant planet, "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 currently located.  Planet 
Nine, if discovered, promises to revolutionize our knowledge of the origin of 
the solar system, the physics of giant planets, and the connection to 
exoplanetary systems. In addition to this high-risk, high-return scientific 
goal, the same data will allow us to carry out the following guaranteed 
science: discovery of ~0.2M main-belt objects and ~10^4 Kuiper-belt objects 
in this previously unexplored region, in addition to the discovery of more 
than 1,500 RR-Lyrae (RRL) stars in the halo region of the Milky-Way (MW), 
about 100 of which will lie at distances in the range 100-700kpc (the virial 
radius of the MW is ~300kpc). The distribution of RRL stars will allow us to 
study the structure of the MW dark matter halo and to possibly detect its 
outer edge.