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