Report - School of Physics

The planetary lensing events have a typical duration of 2–20 hr (compared to the
typical 2–20 weeks duration for lensing events due to stars), and must be sampled by
photometry of ∼ 1% accuracy several times per hour over a period of several days,
and with high angular resolution because of the high density of bright main-sequence
stars in the central bulge. The proposed polar orbit is oriented to keep the Galactic
bulge in the continuous viewing zone. Most of the multiple-planet detections in
the simulations of Bennett & Rhie (2002) are systems in which both ‘Jupiter’ and
‘Saturn’ planets are detected. Since multiple orbits are generally stable only if they
are close to circular, a microlensing survey will be able to provide information on the
abundance of giant planets with nearly circular orbits by measuring the frequency
of double-planet detections and the ratios of their separations.
Just over 100 Earths would be detected if each lens star has one in a 1 AU orbit. The
peak sensitivity is at an orbital distance of 2.5 AU, with 230 expected detections if
each lens star had a planet in such an orbit. Although the prime quantity obtained
from a microlensing detection is the mass ratio between planet and star, additional
information or hypotheses can be combined to estimate the mass of the host star,
the planetary mass, the distance to the host star, and the planet-star separation in
the plane of the sky.
One of the disadvantages of lensing experiments is that a planet event, once observed,
can never (in practice) be seen again – follow-up observations for further
characterisations are not feasible (unlike the case for any of the other principal
detection methods). Nevertheless, a mission like MPF/GEST will provide important
observational and statistical data on the occurrence of low-mass planets (Earth
to Jupiter masses), low-mass planets at larger orbital radii, multiple systems and,
significantly, free-floating planets formed as a by-product of the system formation.
Detection by microlensing could in principle also be included in the Eddington mission,
but the Eddington team has made no detailed evaluation of feasibility, and its
inclusion would be likely to drive instrumental requirements in a non-trivial manner.
2.2.4 Other Space Missions: JWST, Spitzer, SOFIA
JWST: JWST (http://www.jwst.nasa.gov/) is a collaboration between NASA,
ESA and the CSA. It will be a passively-cooled (40–50 K) observatory spacecraft
with an 18-segment primary mirror having an effective aperture of about 6.5 m
and diffraction-limited performance at 2 µm, equipped with four principal science
instruments and a fine guidance sensor. It is scheduled for launch in 2011 into an
L2 Lissajous orbit. The observatory is optimised for the 1–5 µm band, but will
be equipped to cover 0.6–28 µm with a combination of imaging (through fixed and
tunable filters) and low- to moderate-resolution (100 < R < 3000) spectroscopy.
The ‘Origin and Evolution of Planetary Systems’ is one of JWST’s five science
themes, and the science requirements are drafted accordingly.
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