Fig. 2.14 A diagram summarizing the domains over which the various methods of indirect detection are most sensitive
In the previous sections we have considered the different methods for the indirect detection of extrasolar planets, their sensitivities, as well as their bias, either intrinsic, or linked to the choice of observational sample. Figure 2.14 summarizes the best observational domains for the different methods, with a diagram that plots the mass of the object against its distance from the central star (assuming the star to be of solar type).
In this section we intend to discuss direct detection of an exoplanet, in the sense of being able to detect directly photons arising from the planet, and not the effects of the latter on the central star. The objective of direct detection is to be able to separate the photons from the star from those from the planet, to enable us to make a spec-troscopic analysis and to deduce details of the planet's composition, or at least that of its atmosphere. Strictly speaking, however, the transit method, when carried out in the infrared, is a direct-detection method, in particular when the secondary transit is observed (the passage of the planet behind the star). By measuring the thermal flux before and during this secondary transit, we obtain, by subtraction, a measure of the flux from the planet. This method was used on HD 209458b and TrEs-1 with the help of the Spitzer space observatory, and enabled spectral information to be gathered in four channels between 5 and 25 |m(Charbonneau et al., 2005).
Here, however, we shall restrict discussion to the collection and analysis of photons from planets.
The choice of a spectral region for direct observation involves a compromise between the contrast between the star and the planet (typically between 104 and 1010, depending on the nature of the companion and the spectral region chosen), and the angular resolution of the instrument, which primarily depends on the size of the collector. The limiting angular resolution is set by diffraction. The image of a point at infinity through a telescope is not a point but a disc, the form and size of which depend on the shape and size of the telescope's aperture. For a circular aperture, the diffraction disc appears as shown in Fig. 2.15.
The radius r of the central diffraction disc is given by:
where X is the observational wavelength and D is the diameter of the aperture (the telescope's primary mirror).
To 'image' an exoplanetary system, taking account of its spatial size, the compromise finally comes down to a choice between two domains:
• The visible and near infrared. Here, the star-and-planet pair is spatially resolved by a telescope a few metres in diameter. The difficulty lies in overcoming the contrast, which is 109-1010 for a terrestrial-type planet. In this instance, corona-graphic methods are used, possibly together with adaptive optics.
• The thermal infrared (around 10 ¡m). Here, the contrast is optimal (107 for a terrestrial planet), but we have to resort to interferometry to obtain the angular resolution of the star-and-planet pair, because it would require a monolithic telescope several tens of metres in diameter to resolve the system.
The luminous flux of the objects also needs to be taken into account. This point is particularly important when we want to consider the question of imaging planetary surfaces. Table 2.5 gives an estimate of this flux.
Integrated flux received on Earth
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