-0.52 i 0.04
form, as ices of H2O, CH4, NH3, etc. The large amount of material available to form planetesimals allowed the formation of nuclei that were far larger than the terrestrial planets. Planetary formation models predict that when the mass of a nucleus reaches some ten Earth masses, the gravitational field becomes sufficient to cause the surrounding protosolar material to collapse into a disk, one which largely consists of hydrogen and helium. Within this disk, which lies in the planet's equatorial plane, a series of satellites will be formed in its turn. The planets that have formed in this manner are very large and low in density because of the contribution from protosolar gas: they are the giant planets. The heliocentric distance beyond which the gases become frozen is known as the ice line. At the time the planets were formed, it was probably located around 4 AU.
In summary, the current scenario of Solar-System formation is able to account for the overall properties of the Solar System. In this model, based on accretion around solid particles, the small, dense planets are expected to form close to the Sun, whereas the giant planets form at greater distances.
The challenge is therefore the following: to create a model for star formation that would take account of the properties of the newly discovered planetary systems, but also to ensure that the Solar System could be incorporated as a specific case within the overall model.
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Surprising though it may seem, practically all of the exoplanets that have been currently detected have never been 'seen' directly, in the sense that no images obtained with a telescope exist for these objects. The reason is very simple, and lies in the extreme difficulty of detecting these objects. This also explains why it was necessary to wait for the beginning of the 1990s for the first systems to be discovered, when far more exotic astrophysical objects (quasars, pulsars, etc.) had been imaged for several decades. In this chapter, we discuss the different techniques employed to detect and determine the properties of exoplanets and their environment.
The direct observation of an exoplanet - in the sense of being able to separate physically the photons from the planet from those of the central star, sufficient to obtain an image of the two objects - is a problem that is as simple to state as it is difficult to achieve in practice. It may be largely summarized by three critical points which strongly influence the detection methods that may be envisaged:
• the contrast between the star and the planet,
• the angular distance between the two objects,
• the environment of the Earth and of the exoplanet (and more generally, of the exosystem as a whole).
2.1.1 Contrast Between Star and Planet
The contrast between an exoplanet and its parent star depends on several factors:
(a) the spectral region in which observations are made: The spectrum of an exoplanet primarily consists of two components (see Chap. 7): one component arising from the reflection of the star's light by the planet, and the other being its own emission component. In the case of a planet like the Earth, orbiting a solar-type
M. Ollivier et al., Planetary Systems. Astronomy and Astrophysics Library, DOI 978-3-540-75748-1.2, © Springer-Verlag Berlin Heidelberg 2009
Fig. 2.1 Comparative spectra of the Sun and the Earth as they would be seen from a distance of 10 parsecs. The Earth's spectrum clearly shows the component consisting of reflected sunlight (0.1-4 |m) and the component linked to the planet's own emission (beyond 4 | m)
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