Ground-based astrometric planet detection and confirmations

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We aim to confirm two radial velocity (RV) planet candidates in the binary stellar systems HD 19994 and γ Ceph by measuring the astrometric displacement (’wobble’) of planet host stars induced by the planet. For RV planets, only a lower mass limit m sin i of the planetary mass is known due to the unknown orbit inclination i. The astrometric wobble of the host star yields the planet orbital inclination and, hence, its mass. The star GJ 876 was the first case, where this wobble was detected with the Hubble Space Telescope (Benedict et al. 2002). As shown by others before (see Pravdo & Shaklan 1996) and demonstrated by us here, one can achieve an astrometric precision in the sub-milli-arc-second range from the ground with VLT and Keck AO. We are observing visual binary stars, where one of the stars has a RV planet candidate, so that the astrometric wobble is seen as change in binary separation. In the case of HD 19994, a ~ 2.3′′ binary star, one with a RV planet candidate, the wobble is 0.16 mas (or even more due to the unknown i in the m · sin i mass limit), and for γ Ceph, its ≥ 0.32 mas. The first successfull good weather run was done at ESO VLT with NACO in Dec 2004. The results presented here demonstrate that we can reach a relative astrometric precision of ~ 100 μas/√hr (hr is observing time in hours), the best precision ever achieved in single-aperture relative astrometry from the ground. These observations also serve as a feasibility program to investigate the astrometric capabilities of 8- to 10-m telescopes with AO, in particular regarding the detection and discovery of exo-planets. If successfull, we can embark on a larger survey for exo-planets around stars where the RV method does not work well. Numerous instruments and facilities are available for such programs, like NIRC-2/Keck, CIAO/Subaru or NIRI/Gemini-North, and in a few years Linc-Nirwana/LBT.