Report - School of Physics

line at the top of the figure indicates the Hipparcos astrometric accuracy, and shows
immediately why Hipparcos was unable to detect new planetary systems. Neverthess,
Hipparcos data was useful for placing some constraints on the masses of planet
candidates (Zucker & Mazeh, 2001).
Gaia (ESA) and SIM (NASA) are two very different approaches to space astrometry,
both approved and under development:
Gaia: Gaia is a scanning, survey-type instrument, with a launch around 2011 (Perryman
et al., 2001). Its detectability domains are shown in Figure 4: periods below
about 0.2 yr will not be detectable because of the relatively long times between successive
observations dictated by the scanning law, while periods longer than about
12 yr will result in photocentric motions indistinguishable from rectilinear motion
over the mission’s measurement duration (about 5 years). As seen in the figure,
Gaia will therefore contribute substantially to the large-scale systematic detection
of Jupiter-mass planets (or above) in Jupiter-period orbits (or smaller); some 10–
20 000 detections out to 150–200 pc are expected (Lattanzi et al., 2000; Sozzetti et al.,
2001), including confirmation of most of the (longer-period) radial velocity detections
known to date. Planetary masses, M, rather than M sin i, will be obtained.
Full orbital parameters will be obtained for some 5000 (higher S/N) systems. Relative
inclinations can in principle be obtained for multiple systems with favourable
orbits (Sozzetti et al., 2001), important for studies of formation scenarios and orbital
stability of multiple systems. Some 4–5000 transit systems, of the hot-Jupiter
type, might also be detected (Robichon, 2002). Gaia might also detect a handful of
protoplanetary collisions photometrically (Zhang & Sigurdsson, 2003). Gaia cannot
observe systems at epochs other than those determined by its fully deterministic
scanning law, and will not detect planets with masses much below 10–20 M ⊕ unless
such systems exist within 10–20 pc.
SIM: SIM is a pointed interferometer with a launch around 2010 (Danner & Unwin,
1999): accuracies of a few micro-arcsec down to 20 mag are projected. Such faint
observations will be expensive in terms of observing time, and brighter target stars
are likely to be the rule. Of 15 key projects and mission scientist programmes currently
studied, three focus on planetary systems: (1) A Search for Young Planetary
Systems (Beichman): this will survey 200 stars with ages from 1–100 Myr (mostly in
star-forming regions at 125–140 pc, but including TW Hya at 50 pc) which expects
to find anywhere between 10–200 planets, depending on whether the occurrence rate
is the canonical 5–7 % from current radial velocity surveys, or 100 % of all young
stars. The survey will be sensitive to M J planets at orbital distances of 1–5 AU.
(2) Discovery of Planetary Systems (Marcy): this will focus on searches for 1–3 M ⊕
planets within 8 pc, and for 3–20 M ⊕ planets within 8–30 pc. The survey is also
considered as a reconnaissance for TPF. Target stars will be selected from ongoing
surveys of the nearest 900 GKM main-sequence stars in the northern hemisphere
with the Lick 3-m and the Keck 10-m telescopes, the nearest 200 GK stars in the
south with the AAO 3.9-m, and the planned 6.5-m Magellan survey extending to a
further 600 GKM stars in the south. (3) Extra-Solar Planet Interferometric Surveys
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