  Gravitational microlensing events of high magnification provide exceptional
sensitivity to the presence of low-mass planets orbiting the lens star,
including planets with masses as low as that of Earth. The essential
requirement for the detection of such planets in these events is that the FWHM
of the light curve be monitored continuously, or as nearly continuously as
possible. The dependence of planet detectability on the magnification caused by
microlensing, on the planet mass and planet location, and on the size of the
source star, may be understood in terms of simple geometrical properties of
microlensing that have been known since 1964. Planetary signals of low-mass
planets are found to be approximately independent of the magnification caused
by microlensing. This implies that planets can be detected in events over a
wide range of magnifications, from moderately high values ~ 100 to very high
values ~ 1000. The former values are likely to yield more clear-cut separations
of the stellar and planetary features on the light curve, but they require
larger telescopes to obtain precision photometry. During 2007, twenty-four
events with magnification exceeding 50 were detected by the MOA collaboration,
of which about half were also detected by the OGLE collaboration. A quarter of
the events received essentially continuous coverage of their FWHMs by follow-up
collaborations, another quarter received partial coverage, and the remaining
half received little coverage. Final analysis of these events is still
underway, but casual inspection of the light curves reveals some possible
planetary detections amongst them. During 2008 it is hoped that fuller coverage
of events of high magnification will be obtained with the addition of further
telescopes to existing follow-up networks.
