Report - School of Physics

52 000 disk stars for 32 nights, reporting some 100 transit candidates with periods
ranging from 1–9 days (e.g. Udalski et al. 2002a; 2002b) based solely on the dimming
of stellar lightcurves. Most of them were quickly identified as stellar binary systems.
Radial velocity follow-up for many of the candidate stars was difficult because of
the faintness of the stars (down to I ∼ 16 mag). Nevertheless, four events were
confirmed as transiting planets by radial velocity follow-up measurements (Konacki
et al., 2003; Bouchy et al., 2004; Pont et al., 2004).
The TrES telescopes belong to the class of small telescopes dedicated to transit
searches. All three telescopes of this transit search programme are small aperture
(10 cm) wide-field (6 ◦ ) systems. They are located at Tenerife, Lowell Observatory,
and Palomar Mountain, and thus span a range of longitudes (Alonso et al., 2004).
Recently, the first planet found by this system has been announced. The discovery
is again based on the lightcurves and radial velocity confirmation, showing that
small-scale systems indeed have the potential to find transiting planets, providing
their observational coverage is sufficiently high.
These examples show the potential of the transit method to find a large number of
planets, including small planets, in an unbiased sample of stars. The full potential
will be exploited in future space missions, from which Earth-mass planets can be
detected (Section 2.2).
In addition to the geometric information derived rather directly from accurate photometric
measurements of planetary transits, high-cadence, high S/N spectroscopy
of transit events can reveal properties of the planetary atmosphere and exosphere.
Extensive work on the first transiting planet, HD 209458b, has shown the level of
current photometric and spectroscopic capabilities, principally using HST before the
failure of STIS. Ground-based photometry was able to reach a precision of 0.2%
(Henry et al., 2000; Charbonneau et al., 2002; Jha et al., 2000; Deeg et al., 2001)
while HST/STIS has achieved ∼ 0.01% (Brown et al., 2001). The high-cadence capability
of the HST Fine Guidance Sensor (FGS) is also being exploited for transit
timing (Schultz et al., 2004).
The use of time-resolved spectroscopy by Charbonneau et al. (2002) showed that
the HD 209458b transit was 2.3 × 10 −4 deeper when observed at the sodium D lines.
Again using STIS, Vidal-Madjar et al. (2003) detect a very large (15%) transit depth
at Ly-α, showing that the planet is losing mass. Subsequently the same group (Vidal-
Madjar et al., 2004) reported a detection of C and O in the exosphere. In addition
to these lines, the possibility exists, in the optical band, of looking for the effects of
water (longward of 500 nm) and of Rayleigh scattering in the blue. Moutou et al.
(2003) searched unsuccessfully for He I 1083 nm absorption using the VLT with
ISAAC. Their upper limit of 0.5% at 3σ for a 0.3 nm bandwidth was limited by
the detector fringing properties at this wavelength. An alternative approach is to
search for an infrared signature during secondary eclipse. This method, applied by
Richardson et al. (2003) which they call ‘occultation spectroscopy’, searches for the
disappearance and reappearance of weak spectral features due to the exo-planet as
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