Now that well over 100 planets have been discovered, we no
longer need to ask the question "Are there planets out there?"
Instead we ask, "How common are planets?" and "What are their
properties?" In short, we have moved from the initial discovery
phase to the characterization phase. The objective of XO is to
find additional hot-Jupiters (like HD 209458b) that transit
stars bright enough for detailed follow-up studies to be done
with existing telescopes such as HST, Keck, and the VLT. Given
that hot-Jupiters are in transit roughly 3% of the time, and
given statistics of stars and hot-Jupiters, at any given time we
could observe one bright (mv < 11) star being transited, if only
we knew where to look. Transit observations by themselves
provide no information about the mass of the planet and radial
velocity studies can yield only minimum masses. However, the
combination of the doppler obit plus the photometric detection
of transits will yield a full solution for the physical
characteristics of the candidate. To