Report - School of Physics

The first planet detection with the microlensing technique was published in May 2004
(Bond et al., 2004): The OGLE- and MOA-teams detected a clear caustic-crossing
microlensing signal in event OGLE-2003-BLG-235 or MOA-2003-BLG-53 which
could only be reproduced with a binary-lens model involving a mass ratio of q =
0.0039 +11
−07. The planetary deviation lasted about one week, with a measured maximum
magnification of more than a factor of 12. Assuming a low-mass main sequence
primary, this would correspond to a planet of about m P ∼ 1.5 M J at a projected
separation of about d ∼ 3 AU.
A summary of ongoing or planned microlensing experiments is given in Table 3.
(Stellar) microlensing events continue to be observed at large rates, particularly towards
the Galactic bulge (which is the prime search direction due to the high density
of background stars). Both MOA and OGLE regularly post their microlensing alerts
on their web sites. In the context of exo-planets, specialized networks have been established
(PLANET, MicroFUN) which perform follow-up observations of ‘alerted’
events at high time resolution and look for possible planetary perturbations in the
stellar microlensing light curves. In the 2004 observing season, OGLE alone had
detected and alerted on more than 600 stellar microlensing events. With the MOA
inauguration of a new 1.8-m dedicated telescope in December 2004, more than 1000
stellar microlensing events will be found per season from 2005 onwards.
A number of studies looked into the statistics of planets from microlensing searches.
They come in two kinds, either providing detection/exclusion probabilities for planets
in individual lightcurves or for ensembles of events:
In the case of MACHO 98–BLG–3, the estimated probability for explaining the data
without a planet is < 1%. The best planetary model has a planet of 0.4 − 1.5 M ⊕
at a projected radius of either 1.5 or 2.3 AU (Bond et al., 2002).
Very high magnification events are well-suited for showing signatures of planets,
because the relative track is very close to the central caustic which should be slightly
perturbed by the existence of any planet (at the same time, such planets are difficult
to characterise uniquely): In the case of MOA 2003–BLG–32 = OGLE 2003–BLG–
219 (Abe et al., 2004), with a peak magnification of more than 500, continuous
observations around the maximum did not show any planetary signature. This
enabled the authors to put very stringent limits on the probability of a companion:
planets of m P = 1.3 M ⊕ are excluded from more than 50% of the projected annular
region from ∼ 2.3−3.6 AU surrounding the lens star, Uranus-mass planets from 0.9–
8.7 AU, and planets 1.3 more massive than Saturn are excluded from 0.2–60 AU.
The best published statistical limits on the frequency of Jupiter-mass planets from
(lack of) microlensing signatures can be found in Gaudi et al. (2002), based on
the first five years (1995–99) of PLANET team data. They concluded that less
than 33% of the M-dwarfs in the Galactic bulge have Jupiter-mass companions
with a projected separation between 1.5–4 AU. Many more stellar events have been
monitored subsequently, and improved limits should become available soon.
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