Searching for Microlensing Events Observation from the Ground

Searching for planets by means of gravitational microlensing events (see Sect. 2.2) requires monitoring gravitational amplification events photometrically as accurately as possible, with sampling every half-hour. Given the time required to obtain accurate photometry of objects, the programme to monitor events requires the provision of photometric alerts so that efforts are concentrated only on events that are in progress. Initially, gravitational amplification events were used to investigate dark matter in the universe, which was believed to be in the form of cold objects, of low mass and dark. Several teams (EROS in Europe, DUO, MACHO and OGLE in the USA) regularly provided photometric alerts. Currently two teams are still providing details of microlensing events (the OGLE team, which is equally interested in studying planets through transits, and which has discovered several candidates, and MOA). Accurate photometric monitoring of these events reveals photometric artefacts caused by the presence of planets.

The PLANET Collaboration

Photometric monitoring of microlensing events requires the greatest observational continuity possible. In the PLANET (Probing Lensing Anomaly NETwork) collaboration, several telescopes around the world (Table 8.5), together with open-access photometry software are being used to detect 'classical' gravitational amplification anomalies (photometric curves with Gaussian distributions). The location of instruments around the world is such that, in principle, an event may be monitored practically continuously (subject to the weather). Given the duration of artefacts created by passing across (or near to) a planetary caustic (typically a few hours at most), this continuous monitoring is necessary.

So far, the PLANET collaboration (Beaulieu et al., 2006) has detected one object whose mass is about 5.5 Earth masses. This is one of the least massive exoplanets yet detected. Several other events are currently being analyzed (cf. the collaboration's web page:

Table 8.5 List of telescopes used by the PLANET collaboration to monitor microlensing events (listed by site)




Diameter of telescope

Perth (Australia)



0.6 m

Hobart (Tasmania)




Sutherland (S. Africa)




Bloemfontein (S. Africa)




La Silla (Chile)



1.54 and 2.2 m

The MICROlensing Follow-up Network (MICROFUN)

The MICROFUN collaboration is another network dedicated to the follow-up of microlensing events. Like PLANET, MICROFUN uses microlensing events announced by the OGLE and MOA consortia, and provides accurate and, if possible, continuous light-curves of on-going events. The instruments used by MICROFUN are listed in Table 8.6.

Table 8.6 List of telescopes and sites used by the MICROFUN collaboration to monitor microlensing events (from




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