Report - School of Physics

0.35 arcsec of the centre, i.e. much closer than with a classic mask; (2) simultaneous
differential imaging over 5 × 5 arcsec 2 : four images are obtained simultaneously
through 3 narrow-band filters. Two are taken in the 1.625 µm methane feature, and
the two others at 1.575 and 1.600 µm, outside the spectral line. The data are registered
simultaneously in the four channels, and the point-spread function (including
all its residual aberrations, and the speckles, including super-speckles) are identical
in all four images. This mode was designed to search for methane-rich objects near
very bright stars, with a contrast of 50 000 accessible. With these characteristics,
a Jupiter-like planet in a Jupiter-like orbit around a very nearby star (within 5 pc)
should be just detectable at its largest elongation. Detection performances are limited
by uncorrected phase residuals (mainly low-order aberrations). NACO should
be considered as a prototype permitting the study of novel techniques that will be
used in dedicated instruments such as ESO’s Planet Finder.
Planet Finder: The next step beyond the VLT AO facility NAOS-CONICA
(NACO) would be a dedicated VLT instrument optimised for the detection of extrasolar
planets. Two independent design studies are currently underway for such a
Planet Finder instrument at the VLT. The prime goal is to gain at least an order
of magnitude with respect to NACO in the detection of faint objects very close
to a bright star, ideally reaching giant planets. Much higher Strehl ratios than
NAOS, around 0.9 in the K-band, are targetted. Planet Finder will combine highorder
adaptive optics with differential detection techniques; multi-waveband imaging,
integral-field spectroscopy, and imaging polarimetry are foreseen for the focal
plane instruments. Planet Finder could become operational around 2009. A review
board for the assessment of the Phase A studies met on 16–17 December 2004.
Planet Finder may discover giant planets in different phases of their evolution. During
the ongoing contraction and accretion phases, the internal luminosity of these
planets exceeds the reflected light contribution by several orders of magnitude, e.g.
a Jupiter mass planet will be 10 3 times brighter at 1 Myr than at 1 Gyr. This raises
the possibility of detecting young planets around the closest young stars, in spite
of their relatively large distances. Planet Finder will also search for old planets in
the Solar neighbourhood. The S/N for the detection of exo-planets drops rapidly
with distance, due to the combined effects of inverse-square brightness losses and
the reduction in stellar-planet angular separation. The most promising targets for
old systems are therefore within 5–10 pc, exploring the range in separation down to
≃ 3 − 5 AU. For this reason, possible targets for Planet Finder may be found among
stars known to have planetary systems from high-precision radial velocity surveys.
Even more promising is the synergy with planet searches using astrometric perturbations:
giant planets detected by Planet Finder should give a signal in the tens of
milli-arcsecond range, clearly measurable with PRIMA and/or future space missions
(SIM, Gaia). This will provide an independent estimate of planetary masses.
The detailed science cases will probably differ between both groups because of different
AO system performances and focal instruments. In the French-led proposal,
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