The Study of Planetary Transits Ground Based Studies

Searching for transits of giant exoplanets from the ground is, in principle, a relatively easy exercise, because it does not have very stringent instrumental requirements. The amplitude of a transit of a giant planet is at the per cent level, so it is necessary to be able to obtain photometric curves to an accuracy better than one per cent (see Sect. 2.2). To achieve this end, a telescope of a few tens of centimetres in diameter and a good CCD are all that are necessary. This is why some twenty-odd teams are conducting such programmes. The principal features of the projects that are current or in development are gathered together in Table 8.3. (Projects that have now ceased are not included in the table.)

It is interesting to note the significant differences between the methods employed to detect transits, and the few results obtain to date (5 planets identified by OGLE, 1 by STARE (TrES-1) and 3 planets detected by radial velocities, confirmed by transits). This is despite estimates that were clearly optimistic. The discrepancy between predictions and results may be explained in several ways:

• the number of light-curves that may be effectively used in any stellar field is clearly more limited than previously estimated;

• the limitations on continuous coverage (because of daylight, weather or clouds), enormously reduced the discovery effectiveness;

• uncorrected, remaining bias and systematic effects on the light-curves (correlated noise) which prevent the identification of transits.

This is why later transit-search programmes tend to favour the quality of the observing site and of the photometry, as well as the coverage over time, as being able to optimize the return from the observational programme. Let us mention the A-STEP project, for example, which aims to install a 40-cm telescope in Antarctica to benefit from the good photometric conditions and continuous coverage over a period amounting to several tens of days (during the southern polar night). Observations From Space

Space allows us to avoid atmospheric effects, especially, as far as photometry is concerned, the detrimental effect of scintillation. Access to space is therefore indispensable for particularly stable photometry.


The CoRoT space mission (Convection, Rotation, and Transits, Fig. 8.7) is a small mission under the leadership of the Centre national d'Etudes Spatiales (CNES: the French space agency), the major contribution being French, but achieved in cooperation with Austria, Belgium, Brazil, Germany, and Spain, as well as Europe, through the European Space Agency (ESA). The objectives of this mission cover two main themes:

• asteroseismology (the study of the interior of stars by observation of their pulsations, Baglin et al., 2006)

• the search for exoplanets from space by the transit method.

Table 8.3 Principal features of ground-based programmes searching for giant planets by the transit method





No. targets

Limiting magnitude



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