  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. The aim of the
SWEEPS project is to extend our knowledge to stars which (i) are in a different
part of the Galaxy, (ii) have lower masses, and (iii) have a large range of
metallicities. To achieve this goal, we used the Hubble Space Telescope to
search for transiting planets around F, G, K, and M dwarfs in the Galactic
bulge. We photometrically monitored 180,000 stars in a dense bulge field
continuously for 7 days. 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 < 1.2 days. Radial-velocity observations of the
two brightest candidates support their planetary nature. These results suggest
that planets are as abundant in the Galactic bulge as they are in the solar
neighborhood, and they are equally abundant around low-mass stars (within a
factor 2). The planet frequency increases with metallicity even for the stars
in the Galactic bulge. All the USPP hosts are low-mass stars, suggesting either
that close-in planets around higher-mass stars are irradiatively evaporated, or
that the planets can migrate to close-in orbits only around such old and
low-mass stars.
