Although PLANET observing campaigns are an excellent way of studying normal variable stars in our own Milky Way that change their brightness rhythmically, the primary goal of PLANET's multi-band, rapidly-sampled photometry is the study of microlensing anomalies. Microlensing anomalies are departures from the simple, achromatic point-source, point-lens microlensing light curve that is usually seen when a foreground lens (star) passes in front of a background star. Such departures can be due to a variety of reasons: blended light from other stars along the line-of-sight, double lenses, double background stars, or lenses with planetary systems, resolution of a large background star, or complicated relative motion within the lens system (like parallax effects due to the Earth's motion). The new science to be obtained through intensive monitoring lies primarily in the fine structure expected on hourly time scales. For this reason, PLANET's telescopes must keep watch 24 hours a day. Detailed measurements of the sharp caustic crossings in binary events, which last a few days, but peak in a few hours, can yield much more information about double lenses. The binarity of one such event was discovered by PLANET in its first 1995 campaign (MACHO bulge event 95-12). Double lenses were also discovered by PLANET in three other MACHO events (MB9728, MB9741 and MB9842) toward the center of the Milky Way. The caustic structure of the double-lens event MB9728 acted like a sharp magnifying glass passing over the background star residing near the center of the Milky Way. Continous PLANET observations of this effect resulted in a measurement of the limb-darkening, or dimming of the light coming from edges of the stellar surface, for the first time in a star so distant. If the binary rotates appreciably during the time of the observations, the caustic structure will be altered; this effect was measured by PLANET in event MB9741 and used to estimate the rotation period of the unseen binary lens. PLANET monitoring of another MACHO-discovered binary event 98-SMC-01 in the Small Magellanic Cloud (a nearby small galaxy) determined the relative speed of the lens, thereby indicating that it was most likely a normal star in the Small Magellanic Cloud and not part of the dark matter believed to reside in the halo of our own Galaxy. Occasionally, the PLANET team has used its network to identify the fading, faint optical emission from distant, powerful gamma-ray bursts such as GRB 990510 and GRB 990712.
Microlensing monitoring is also a powerful means of searching for extra-solar planets: just as with binary lenses, the caustic patterns arising from the complicated lens geometry of a planetary system can induce sharp peaks (few hours to few days) in the microlensing light curve. Depending on the actual numbers of planetary systems and their typical planetary masses and orbital radii, a significant fraction of events are expected to exhibit detectable caustic crossings. Even outside the caustic region, the magnification pattern is disturbed by planets, producing detectable anomalies. In principle, even earth-size planets can be detected in this way. An ultimate goal of PLANET is to place constraints on the number and distribution of planets around other stars. Analysis techniques designed to remove some of the effects of atmospheric turbulence have made PLANET photometry more sensitive to extra-solar planets, and massive planets (like Jupiters) at certain distances from their mother stars can already be ruled out in some events.
PLANET has completed its 1995 pilot campaign, and subsequent 1996, 1997, 1998 , 2000, 2001, 2002, 2003, and 2004 observing seasons. The 2005 season starts on 15-May. With semi-dedicated access to several southern telescopes until September, many on-going microlensing events per night are monitored in the direction of the Galactic Bulge. The large number of microlensing alerts keep PLANET telescopes busy whenever the bulge is visible. Our longitude coverage provides the possibility of nearly continuous monitoring and a hedge against bad weather. Light curves are typically sampled every one to two hours in the R or I band. Close to the peak of high magnification event, or during anomalies, we increase our sampling rate. We are able to track the progress of the events in near real time with an accuracy of about 1-2%, and adjust our observing strategy when appropriate. All sites perform their own reduction and communicate with one another almost daily. With the beginning of the 2005 season, PLANET is joining forces with the UK-operated robotic telescope network RoboNet1.0 performing microlensing observations at the Liverpool 2.0m (La Palma, Canary Islands, Spain) and the Faulkes North 2.0m (Maui, Hawaii, USA) as well as the Faulkes South 2.0m (Siding Spring, Australia) as soon as it becomes operational. Observations at these telescopes are carried out as PLANET/RoboNet campaign, where the selection of target follows an automatic algorithm designed to maximize the detection efficiency which is combined with suggestions from PLANET. The The Daily Planet is the electronic hotline for on-going PLANET observations.