Francoise Roques

Structure and Evolution of an Exoplanet

We have seen that the process of formation (or evolution) of exoplanets seems to be very different from the planets in the Solar System, because giant exoplanets are discovered in close proximity to their stars. What about their internal structure and their atmospheres? We know the principal external factors responsible for the structure of planetary atmospheres (solar radiation, magnetic field, and interaction with the surface). In addition, the temperature and cloud structures of planetary atmospheres depend upon their composition, which determines, at each level, the opacities of the gaseous and solid phases (Sect. 4.4.2.2). We can also model a synthetic exoplanet spectrum that corresponds to each model in terms of comparing it with experimental data, when instrumental methods allow us to observe the spectra. Modelling the internal structure of a sub-stellar object requires determining the temperature gradient and thus knowing the energy-transfer mechanism. Three mechanisms may be involved: radiation, conduction, and convection. A complete description of these processes may be found, in particular, in de Pater and Lissauer (2001) and Guillot (2006). A key parameter involved in all cases is the opacity, which depends on the atmospheric composition, pressure, and temperature, and determines how much energy is absorbed at each level. At pressures higher than a few bars, the collision-induced absorption caused by H2–H2 and H2–He collisions must be taken into account. In addition, molecules present in planetary atmospheres (CH4, NH3, H2O . . .) contribute to the opacities through their rotational and vibration-rotation bands. As a result, the spectrum of a planetary atmosphere is strongly dependent upon the wavelength (see Sect. 4.4.2.2.). Finally, condensates may play an important role by contribution to the opacity; their abundances mostly depend on the temperature (see e.g., Pollack et al., 1994). Radiation transfer typically dominates in planetary stratospheres and upper tropospheres, at pressures lower than 0.1 bar. In the conduction regime, energy is transferred by collisions between particles. This mechanism is efficient, in particular, near the surface of the terrestrial planets. Finally convection is the energy transport caused by large-scale motions induced by density gradients resulting from temperature differences. It may be shown (Guillot, 2006) that convection dominates in isolated sub-stellar objects (weakly irradiated exoplanets or brown dwarfs). It is

New Planetary Systems

‘An infinite number of Suns exist, an infinite number of Earths orbit around these suns as the seven planets orbit our Sun. Living beings inhabit these worlds.’ Four centuries ago, this was Giordano Bruno’s brilliant intuition. Nowadays, we know that the Sun is a star, and that an extremely large number of stars in the Galaxy are accompanied by planets. Our knowledge is, however, still too fragmentary to know whether conditions for the appearance of life exist, or have existed, on extrasolar planets.

The Solar System Within the Universe

To try to understand the nature of the Solar System, its origin, and its evolution, it is indispensable to consider it in its context, that of a universe in expansion, consisting of stars and galaxies. First, it is necessary to establish the distance scale that allows us to place the Solar System within its immediate surroundings: the nearby stars and galaxies, and the Local Group. Second, we briefly describe the universe at we view it today: the Big-Bang model and primordial nucleosynthesis; galaxies; stars and stellar nucleosynthesis; stellar formation; and interstellar medium.

The Search for Life in the Universe

The recent discovery of planets around neighbouring stars has given new topicality for a question that human beings have been asking ever since the origins of conscious thought. Are we alone in the universe? We find discussions in ancient text, notably those by the Greeks Democritus and Epicurus, then by the Roman poet Lucretius, about the question of the multiplicity of inhabited worlds. The idea of extraterrestrial life was reopened by Giordano Bruno in the 16th century. A victim of the Inquisition, he paid for his speculations with his life. Galileo, and then numerous astronomers and philosophers (Kepler, Kant, Huygens, Fontenelle, etc.) all picked up in their own time this doctrine which was held to be heretical by the Catholic church.

The Terrestrial Planets and Their Satellites

The four planets closest to the Sun; Mercury, Venus, Earth, and Mars (and to which one can add the Moon), have many similarities that justify their being considered together. These planets are sometimes called the inner planets, because of their proximity to the Sun, or terrestrial planets, with the Earth serving as a convenient reference.

The Interaction of Solar-System Bodies with the Interplanetary Medium

The external envelopes of the different bodies in the Solar System are directly in contact with the interplanetary medium. This chapter describes the different physical processes that are involved, and which govern the exchange of matter, energy, and momentum between the bodies and the interplanetary medium.

General Features of the Solar System

The Solar System may be defined as consisting of those objects that are governed by the Sun’s gravitational field. Other effects arising from the proximity of the Sun could equally well be used as criteria, such as radiation pressure or interaction with the solar wind. With any of these definitions the Solar System extends out to about two light-years; the closest star, Proxima Centauri, lies at a distance of slightly more than four light-years. Our knowledge of this region of space certainly does not reach as far as this, however, because the most distant Solar-System objects that we know about, the comets, seem to originate no more than 50 000 astronomical units1 away, or less than a third of the total distance, whilst the other Solar-System bodies known to us lie within 50 AU. Our study is therefore confined to what is primarily the central region of the Solar System.

Methods of Studying the Solar System

Because of the brilliance of certain of its objects, the Solar System has been studied since antiquity. For centuries these observations were restricted to what was visible with the naked eye. Successive advances have been made, since the beginning of the 17th century, by the use of larger and larger telescopes, then by photographic observations taking over from visual ones. During the 20th century, astronomical observational techniques have undergone a veritable revolution. First of all, observations from space allowed access to ultraviolet and infrared regions of the spectrum, as well as to X- and γ-rays. In addition, the century has seen the beginning of radio astronomy and, above all, beginning in the sixties, the start of “in situ” observations of the Solar System, with the launch of spaceprobes to the Moon and planets. This chapter aims to review the observational techniques that have so far been used to study the Solar System. A detailed discussion of astrophysical instrumentation can be found in the book by P. Lena: Methodes physiques de l’observation [InterEditions/Editions du CNRS (Paris 1986); English edition: Observational Astrophysics (Springer-Verlag, Berlin, Heidelberg 1988)].

Interplanetary Dust, Micrometeorites and Meteorites

Apart from radiation and particles of solar or galactic origin, interplanetary space contains dust and rocky bodies of all sizes. Their collisions with the Earth cause meteorite falls and showers of shooting stars. Scientific study of this extraterrestrial material, which has gone on for centuries, has still not provided a definite answer to the question of their origin: are they derived from cometary nuclei or from minor planets?

The Formation of the Solar System

Although the observation of objects in the Solar System has been practiced, to a high degree of precision, since ancient times, the problem of its origin was not really considered until after the Copernican revolution. This, which merely repeated the theory first advanced by Aristarchos of Samos, located the Sun at the centre of the System. The first models for the system’s formation tried initially to explain, more or less in qualitative terms, the observed movements. Various factors had to be taken into account: (1) the orbits of the planets are close to the plane of the Earth’s orbit; (2) the orbits are essentially circular (with the exception of that of Pluto, which was discovered in 1930); (3) the planets all rotate in the same sense, which is the same as that of the Sun; (4) the heliocentric distances of the planets obey the Titius-Bode law (see Sect. 1.1.2). These particular constraints applied to all theories developed up to the 19th century. At the end of the 19th century, and the beginning of the 20th, theoreticians started to pay attention to the problem of angular momentum. The angular momentum of the Sun, which contains 99.8 % of the mass of the Solar System, only has 2 % of the angular momentum possessed by all of the planets. Finally, the second half of the 20th century benefited from the contribution of new theories concerning stellar formation, as well as new data, on the one hand about the age of the various bodies, and on the other about their chemical, isotopic and crystallographic composition. All these new elements have allowed a choice to be made amongst all the different models proposed, and for the most likely one to be selected. The problem is far from being solved, however, and even if a coherent description of the system’s formation is now becoming clear, all the various physical and chemical mechanisms are still not understood.