  Among various techniques to search for extra-solar planets, microlensing has
some unique characteristics. Contrary to all other methods which favour nearby
objects, microlensing is sensitive to planets around stars at distances of
several kpc. These stars act as gravitational lenses leading to a brightening
of observed luminous source stars. The lens stars that are tested for the
presence of planets are not generally seen themselves. The largest sensitivity
is obtained for planets at orbital separations of 1-10 AU offering the view on
an extremely interesting range with regard to our own solar system and in
particular to the position of Jupiter. The microlensing signal of a
jupiter-mass planet lasts typically a few days. This means that a planet
reveals its existence by producing a short signal at its quasi-instantaneous
position, so that planets can be detected without the need to observe a
significant fraction of the orbital period. Relying on the microlensing alerts
issued by several survey groups that observe ~10^7 stars in the Galactic bulge,
PLANET (Probing Lensing Anomalies NETwork) performs precise and frequent
measurements on ongoing microlensing events in order to detect deviations from
a light curve produced by a single point-like object. These measurements allow
constraints to be put on the abundance of planets. From 42 well-sampled events
between 1995 and 1999, we infer that less than 1/3 of M-dwarfs in the Galactic
bulge have jupiter-mass companions at separations between 1 and 4 AU from their
parent star, and that less than 45% have 3-jupiter-mass companions between 1
and 7 AU.
