Worlds in Transit
The exoplanets discovered so far have been mostly around relatively nearby and bright stars. As a result, the host stars are mostly (i) in the Galactic disk, (ii) relatively massive, and (iii) relatively metal rich. After a review of the current status of exoplanets, I will describe the SWEEPS project which was aimed at extending our knowledge to stars which (i) are in a different part of the Galaxy, (ii) have low masses, and (iii) have a large range of metallicities. We used the Hubble Space Telescope to monitor 180,000 F, G, K, and M dwarfs in the Galactic bulge continuously for 7 days in order to look for transiting planets. We discovered 16 candidate transiting extrasolar planets with periods of 0.6 to 4.2 days, including a new class of ultra-short period planets (USPPs) with P <1 day. Radial velocity observations of two brightest candidates support the planetary nature. These results suggest that planets are equally abundant in the Galactic bulge and around low-mass stars, and the metallicity distribution holds even for the stars in the Galactic bulge. The USPPs occur only around low-mass stars which may suggest that close-in planets around higher-mass stars are irradiately evaporated.
