What we Learn from the Solar System

As we have mentioned earlier (Sect. 1.4.3), it has been the study of the Solar System that has led us to favour the model of star formation that envisages the collapse of an interstellar nebula. In this chapter we intend to examine in more detail how a coherent scenario for stellar and planetary formation has been reached. This takes overall account of the observations that have been accumulated about the objects around us. These observations concern not only the dynamics of the objects (their orbits, and the way in which their paths have evolved), but also their physical properties (mass, density, atmospheric composition, nature of their surfaces, etc.). More comprehensive discussions of the Solar System may be found in Lewis (1997), Pater and Lissauer (2001), and Encrenaz et al. (2004).

4.1 Observational Methods

For several centuries, observation of the planets was limited to visual observation, which allowed the orbits of the brightest planets to be determined. At the beginning of the 17th century, Galileo's use of the refractor that has since carried his name brought a decisive advance in the observation of planets and their systems (the Galilean satellites being discovered in 1610). Planetary exploration was carried out and advanced by the observations of Cassini and Huygens, in particular, and by the construction of major observatories. A century later (1781) with Herschel, the arrival of large telescopes brought the discovery of a new planet, Uranus, invisible to the naked eye, at a distance of about 20 AU from the Sun. In 1846, Le Verrier, based on perturbations in the orbit of Uranus, deduced the existence of an eighth planet, and calculated its orbit. Neptune was discovered near the position indicated, at a distance of nearly 30 AU from the Sun. In 1930, Pluto was discovered by C. Tombaugh; its semi-major axis is approximately 40 AU. It was classed as a planet, and remained one until 2006.

Do other planets exist beyond Pluto? This question, which remained open throughout the 20th century, took on a new significance in the 1990s with the discovery by D. Jewitt and J. Luu of a new population of objects, the TNOs (Trans-Neptunian Objects). These occupy the Kuiper Belt, most lying between 30 and

M. Ollivier et al., Planetary Systems. Astronomy and Astrophysics Library, DOI 978-3-540-75748-1.4, © Springer-Verlag Berlin Heidelberg 2009

Fig. 4.1 Artist' view of the Cassini probe (© NASA/ESA)

50 AU. Nowadays, it seems that Pluto is one of the largest representatives of this new class of objects. In 2003, another TNO (2003 UB 313) was discovered, with a larger diameter. Other more distant objects will probably be discovered, thanks for the use to ever more powerful telescopes. In parallel with imaging and photometric observations, spectroscopy, which has been carried out since the beginning of the 20th century, has provided information about the chemical composition of the planets and their atmospheres.

Alongside telescopic observations, the exploration of space, which started at the beginning of the 1960s, has revolutionized our knowledge of the planets in the Solar System. After the Moon, which was explored in-situ by the Apollo missions, and whose success is well-known, Venus (with the Venera, Pioneer Venus and Magellan probes), and Mars (with Mariner 9, Vikings 1 and 2, Mars Global Surveyor, Mars Odyssey, Mars Express, and the Spirit and Opportunity rovers) were the various space agencies' favoured targets. Despite several failures, the various space missions have returned a rich harvest of data on the nature of the surface and atmosphere of the terrestrial planets (Fig. 4.1). As for the giant planets, they have also greatly benefited from space exploration. After the fly-bys of Jupiter and Saturn by Pioneers 10 and 11, the Voyager 1 and 2 probes flew past the four giant planets between 1979 and 1989, completely altering our knowledge of these objects. More recently, the Galileo and Cassini missions, orbiting Jupiter (1995-2003) and Saturn (from 2004, Fig. 4.2), have carried out an in-depth investigation into the properties of the two planets, with that for Saturn still in progress.

Fig. 4.2 The planet Saturn as imaged by the Cassini probe (image credit: courtesy NASA, JPL Space Science Institute)

4.2 The Observational Data

In this section we intend to summarize the observational features that form the basis for the model of the Solar System's formation. These relate to the orbital properties of the planets; their physical properties; the properties of the small bodies; and the Solar System's age.

4.2.1 Orbits that are Essentially Co-Planar and Concentric

The first, and most obvious, feature, is the regularity of the orbits of the planets: most of them are co-planar, quasi-circular, and close to the plane of the ecliptic (defined as the plane of the Earth's orbit). The only exception is the eccentricity of Mercury (which exceeds 0.2, whereas all the others are less than 0.1), and its inclination (7°, whereas all the others are less than 4° - see Table 1.1 in Chap. 1). With the exception of Venus and Uranus, all rotate in a direct sense, which is also the direction of rotation of the Sun itself, as seen from the north pole of the ecliptic. This regularity of the orbits is also found in many of the asteroids and TNOs. It also occurs among the orbits of the regular satellites of the giant planets. We shall see that this feature is also relevant to the formation of these planets.

4.2.2 Terrestrial Planets and Giant Planets

The planets in the Solar System fall into two categories, the terrestrial planets and the giant planets. The terrestrial planets (Mercury, Venus, the Earth, and Mars) are dense and small in diameter (Table 1.2, Chap. 1); they either have no satellites or very few. The giant planets (Jupiter, Saturn, Uranus, and Neptune) by contrast are very large in size, have low densities, have systems of rings, and a large number of satellites, many of which occupy circular orbits in the planets' equatorial planes. The giant planets therefore appear to be Solar Systems in miniature.

Fig. 4.3 The planet Mercury, photographed by the Mariner 10 probe (© NASA)

Mercury (Fig. 4.3), close to the Sun and of low mass, does not have a sufficiently strong gravitational field to preserve a stable, neutral atmosphere. The atmospheres of the other terrestrial planets basically consists of CO2 and N2, with H2O being also present on the Earth and Mars. In the case of Earth, the CO2 is held in the oceans, and O2 has appeared following the development of life. In contrast, the atmospheres of the giant planets (Fig. 4.4) is dominated by hydrogen and helium. Other elements are primarily present in reduced form (CH4, NH3, PH3, H2S, etc.). We shall see how measurements of the abundances of the elements provides constraints on models for the formation of the giant planets.

4.2.3 The Small Bodies

The main concentration of asteroids (or minor planets) extends between the orbits of Mars and Jupiter (Fig. 4.5). With diameters of less than 1000 km, they main occur

Fig. 4.4 The planet Jupiter, photographed by the Cassini probe during its fly-past in December 2000 (image credit: courtesy NASA)

Fig. 4.5 The asteroid Gaspra, photographed by the Galileo probe (© NASA)

within the Main Belt, which lies between 2.0 and 3.5 AU in heliocentric distance. We will see that, in the model for the formation of the Solar System, their presence is interpreted as being the residue of a disk of planetesimals, that has been dispersed by direct or indirect gravitational perturbations by Jupiter. The chemical composition of the asteroids varies with their heliocentric distance, with the most dense closer to the centre. Simulations of the dynamical history of the asteroids provide constraints on the age of the Main Belt and the date of Jupiter's formation.

A wide range of small bodies populate the outer reaches of the Solar System. Some have been known since antiquity: these are the comets, icy nuclei, whose diameters do not exceed a few tens of kilometres, with extremely eccentric orbits. When their paths approach perihelion, the surface sublimes, resulting in the emission of gas and dust in a coma that may occasionally be extremely spectacular. Comets may have two different origins. Some, with a wide range of inclinations, originate in the Oort Cloud, a vast shell lying at the limits of the Solar System, at a heliocentric distance of some 40 000 AU. The gravitational perturbations of the giant planets, particularly Jupiter, have ejected them into this vast reservoir (the existence of which was established by the work of Jan Oort and, subsequently, of Brian Marsden), and which may contain some 1011 comets. A tiny fraction of these may, by chance, be returned towards the inner Solar System, and eventually established in stable orbits, once again through planetary gravitational perturbations. In particular, this is the case with Comet Halley, which has been known since antiquity, which has a period of 76 years. The second class of comets is distinguished by a low inclination to the ecliptic and a shorter period. These are believed to originate in the Kuiper Belt, the reservoir of trans-Neptunian objects that has recently been discovered.

The existence of comets, of TNOs, of the Kuiper Belt, and of the Oort Cloud are all simply explained in the planetary formation model in which the planets formed within a protoplanetary disk, by accretion of solid particles subject to multiple collisions and mutual gravitational interactions.

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