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Days since 31 July 2005 UT

Fig. 2.13 The trace of the gravitational-amplification event OB-05-390, where the artefact indicates the presence around the deflecting star of an object of 5.8 (After Beaulieu et al., 2006)

This method is, in principle, extremely sensitive, and enables giant planets to be detected - for example, the candidate associated with the MACHO-97_BLG-41 event, with a mass estimated as about three Jupiter masses (Bennett et al., 1999). It also allows lower-mass planets (terrestrial planets) to be detected. It was using this method that, in 2005, Beaulieu et al. detected the object 0B-05-390 b, which is currently the lightest planet (5.8 Me to within a factor of 2) ever detected orbiting a star other than a pulsar (Beaulieu et al., 2005). The difficulty in this case occurs in monitoring the gravitational-amplification event. The teams that are carrying out the search for planets by this method are dependent on other teams who provide details of the gravitational-amplification targets as soon as an event begins. To search for planets, as continuous a curve as possible is required so that it may be examined for artefacts. To obtain adequate sampling (about one point every 30 min) and continuous coverage of the amplification events, a collaborative effort (PLANET) has been devised, using several telescopes around the world to obtain 24-h coverage of any event.

The gravitational microlensing method is most sensitive to objects at a moderate distance (a few AU) from their parent star. It is not suitable for close objects (but there are the transit and radial-velocity methods for that). It allows the preferential detection of objects orbiting cool stars, because the stellar population of lenses is dominated by low-mass (and low-mass) objects. The object OB-05-390 b is orbiting a cold star (of spectral type M) at a distance of 2.6 AU. Given the distance of the objects, the surface temperature of this planet cannot be higher than 50 K, which means that it compares with that of a Pluto-type object in our own Solar System.

At present only the OGLE and MOA teams are continuing to issue the gravitational-amplification alerts that are likely to lead to the discovery of other planets by suitable follow-up observations. The permanence of the method thus depends, in a crucial manner, on these programmes of systematic observations.

Period (years)

Period (years)

Fig. 2.14 A diagram summarizing the domains over which the various methods of indirect detection are most sensitive

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