Report - School of Physics

Figure 4: Astrometric signature, α, induced on the parent star for the known planetary systems,
as a function of orbital period. Circles are shown with a radius proportional to M p sin i. Astrometry
at the milli-arcsec level has negligible power in detecting these systems, while the situation
changes dramatically for micro-arcsec measurements. Short-period systems to which radial velocity
measurements are sensitive are difficult to detect astrometrically, while the longest period systems
will be straightforward for micro-arcsec positional measurements. Effects of Earth, Jupiter, and
Saturn are shown at the distances indicated.
Galactic bulge main-sequence stars. In about one case out of a million, sources in
the bulge are lensed by foreground (bulge or disk) stars which are accompanied by
the planets being sought. Observing high surface-density sky regions improves lensing
probabilities to sufficient levels that successful detections can be expected over
reasonable observing times. Space observations are considered mandatory to permit
the high photometric accuracy required for detection even in very crowded regions
where seeing limits the achievable photometric accuracy and hence detectability
achievable from the ground.
Microlensing probes particular exo-planet domains: for example, low-mass planets
can be detected, albeit usually at very large distances of typically 5–8 kpc. The
sensitivity of such measurements is highest at (projected) orbital separations of 0.7–
10 AU, but it will also detect systems with larger separations, masses as low as that of
Mars, large moons of terrestrial planets, and some 50 000 giant planets via transits
with orbital separations of up to 20 AU (the prime sensitivity of a transit survey
extends inward from 1 AU, while the sensitivity of microlensing extends outwards).
There are theoretical reasons to believe that free-floating planets may be abundant
as a by-product of planetary formation, and MPF/GEST will also detect these.
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