The PLANET microlensing follow-up network: Results and prospects for the detection of extra-solar planets

Among various techniques to search for extrasolar 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 giving 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 3 surveys groups that observe ∼ 10 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 to put constraints on the abundance of planets. For the event OGLE-Bulge-14, the PLANET data rules out heavy planets for actual projected separations in a broad region around the Einstein radius (∼4 AU) and in particular a planet with the mass of Jupiter at the orbital separation of Jupiter.