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star (Fig. 2.1), the reflected component is dominant in the visible and near-infrared regions, while the planet's own emission component dominates in the thermal infrared (beyond 4 |m).

(b) the nature of the stellar and planetary objects: The two spectral components just mentioned obviously depend on the nature of the various objects forming the planetary system. The spectral distribution and the amplitude of the reflected component depend on the star's effective temperature (and thus on the type of star), but also on the albedo, the size, and the distance of the planet from its star. The planetary emission itself depends exclusively on the nature of the planet - whether terrestrial or giant - and primarily on its effective temperature, but also on the atmospheric composition, which is indicated by the presence of absorption lines in the emission spectrum (Fig. 2.1).

(c) the distance between the objects: The distance between the star and the planet is one of the essential factors involved in the calculation of the radiative equilibrium temperature of the planet. For example, an object of the size of Jupiter has an effective temperature of about 1500 K at 0.05 AU (i.e., as is the case with a hot Jupiter), whereas it is only about 110 K at 5.2 AU (Jupiter's location in the Solar System). The contrast is directly affected: at a wavelength of 10 |m it is about 104 for a hot Jupiter, but it rises to 109 for a true Jupiter-like planet.

2.1.2 Angular Separation Between the Objects

Because of the significant distance between the Sun and the nearest stars - the closest stars are several parsecs from us - exoplanetary systems subtend very small angular distances. A Sun-Earth system, for example, at a distance of 10 parsecs would subtend an angle of one tenth of a second of arc. (One second of arc, abbreviated 'arcsec', is 1/3600th of a degree, one milliarcsec, abbreviated 'mas', is 1/1000th of an arcsec, one microsecond of arc abbreviated '|as', is 1/1000th of a mas.) This tiny angular separation means that methods with high angular resolution must be used to carry out direct observation (see Sect. 2.3). An illustration of these problems of contrast and angular separation may be framed as follows: Trying to observe a planet like the Earth orbiting a star like the Sun, the whole system being at a distance of 10 parsecs from us, resembles trying to observe a glow-worm 30 cm from a lighthouse in Marseille, when the observer is in Paris (at a distance of about 700 km).

2.1.3 Environment of the Earth and Exoplanets

Apart from its suite of planets, the Sun is surrounded by a disk of dust resulting from collisions between asteroids as well as cometary dust. This disk of dust lies in the plane of the ecliptic. The dust, both illuminated and heated by the Sun, has its own emission, which (in the visible region) is known as the zodiacal light. An observer in the Solar System who is examining the sky in the infrared therefore records a signal from this zodiacal emission. This emission is not negligible because, at a wavelength of 10 |um, the spatially integrated emission from the whole disk of dust is 300 times the Earth's emission.

In the same manner, it is not unreasonable to assume that exoplanetary systems also contain a disk of debris (see Chap. 5), whose emission is certainly not negligible, and may even be greater than that of the Solar System. The presence of this disk therefore is a source of a parasitic signal in any radiation that is detected. To illustrate this last point, we may return to our earlier analogy by imagining that this time our glow-worm and our observer are both bathed in the light from Marseille and Paris, which restricts the visibility of faint objects. If you need to be persuaded of this, all you need do is compare the number of stars visible from the centre of Paris with those visible on a clear night in the wide-open country.

Table 2.1 Properties of certain typical objects which are assumed to be orbiting a solar-type star, lying at a distance of 10 parsecs

Object

Radius

Mass

Dist. (AU)

Angular

Contrast

Contrast

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