| Hundreds of planets have recently been discovered around other stars, revealing a startling diversity of planetary systems. However, these exoplanets lie almost exclusively within a few astronomical units (AU) of their host stars. A full picture of planet formation also requires a census and statistical analysis of planets at wide separations. This thesis uses the SEEDS direct imaging survey, together with archival data, to search for massive, >~5 M J companions tens of AU from their host stars. These objects are not sufficiently massive to fuse hydrogen, and simply cool and fade as they radiate away their heat of formation. As a result, SEEDS targets young, nearby stars using HiCIAO, a high-contrast infrared camera on the Subaru telescope. I first present an analysis of HiCIAO, deriving the distortion correction needed to reduce high-contrast data, and optimizing HiCIAO's entrance pupil to vastly improve its performance. I then describe ACORNS-ADI, software I have written to reduce HiCIAO data. This software includes several new algorithms that both improve its performance and efficiently compute each observation's sensitivity. I use ACORNS-ADI to uniformly reduce data from the SEEDS survey, including images of members of young moving groups and of debris disk hosts. The ages of these stars, together with substellar cooling models, are needed to convert our sensitivities from luminosities to masses. I therefore present a uniform Bayesian analysis of all targets, deriving a posterior age distribution for each using both proposed moving group membership and observed stellar activity. Finally, I combine the published SEEDS results with additional archival imaging to assemble a diverse sample of nearly 200 stars. A statistical analysis of five brown dwarfs and massive exoplanets discovered by HiCIAO provides a limit of ~50--200 AU, depending on the models used, beyond which the distribution of exoplanets at small separations cannot extend. By treating massive planets and brown dwarfs together, I further suggest that currently known long-period exoplanets are not an extension of the short-period planetary distribution. Instead, they may represent a low-mass extension of the brown dwarf distribution function, formed by gravitational fragmentation in a cloud or circumstellar disk. |
