Report - School of Physics

larger number of hot Jupiters would also be found in the short (asteroseismology)
observations. Predictions for multiple systems depend sensitively on the assumed
distribution of relative orbital inclinations.
The following missions, not dedicated to transits, can also be noted:
HST: although suitably placed above the atmosphere such that low-mass planets
could be detected by HST using this technique in principle, HST is not a dedicated
transit discovery instrument, and its discovery efficiency is constrained by its
limited field of view, and available observing time. Any searches using HST will
therefore likely be restricted to observations of especially high-surface density regions.
Observations of 47 Tuc (34 000 main-sequence stars monitored for 8.3 days)
failed to detect planets (Gilliland et al., 2000), whereas 17 would have been expected
based on radial velocity surveys. This non-result is currently attributed to effects of
metallicity (Gonzalez, 1998), ultraviolet evaporation (Armitage, 2000), or collisional
disruption (Bonnell et al., 2001) of the protoplanetary disks in this crowded stellar
environment. An advance in transit statistics should come from observations of the
Galaxy bulge with HST by Sahu et al., monitoring 100 000 stars to V = 23 over
7 days in February 2004, with results expected in spring 2005.
Nevertheless HST, and its successor JWST, have considerable potential for followup
observations of transiting systems discovered by other methods, for example by
Kepler. This issue is developed further in Section 4.
MOST: the Canadian satellite MOST is a 15 cm telescope launched on 30 June
2003. Dedicated to the long-term photometric monitoring of a small number of
stars primarily for asteroseismology studies, it has a photometric performance just
a factor of 2 better than from ground (Matthews et al., 2004). Although with limited
transit discovery potential, it will nevertheless aim to detect reflected light of a few
known hot Jupiters.
2.2.2 Space Astrometry Missions: Gaia and SIM
As noted previously, astrometric planet detection involves detecting the system’s
photocentric motion on the plane of the sky, in the same way that radial velocity
detection involves detecting the system’s photocentric motion along the line of sight.
The amplitude of the displacement, and therefore the system’s detectability, can be
characterised by the system’s ‘astrometric signature’, α = (M Planet /M star ) · (a/d).
This signature is measured in arcsec when the orbital radius a is measured in AU and
the distance d is measured in pc. Astrometric measurements can provide the planetary
mass directly rather than M sin i (as provided by radial velocity techniques) if
d is determined and if M star is estimated from stellar evolutionary theory.
Figure 4 shows the astrometric signature versus orbital period for the known exoplanets,
where the size of the circles indicates the planetary mass. The horizontal
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