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General Presentation of the Solar System

Thérèse ENCRENAZ, Françoise ROQUES and Laurent LAMY

LESIA, Paris Observatory, PSL University, Paris, France

1.1. Introduction

The Solar System is defined as the set of objects subjected to the gravitational field of the Sun. According to this definition, it extends up to about two light-years (about 2 1013 km), halfway from the nearest star, Proxima Centauri. The outermost boundary of the Solar System is the Oort cloud, a vast spherical reservoir of comets located at about 20,000 to 40,000 astronomical units (au) and whose outer edge could extend to a few hundred thousand au, or a third to half a light-year. Apart from comets, all the objects in the Solar System that we know are located at distances not exceeding a few thousand au. Most of them are located close to the plane of the equator of the Sun, itself very close to the plane of the Earth's orbit; it is this plane, called "ecliptic", which is adopted as a reference. The Solar System thus has the structure of a disk (resulting from its formation scenario - see Chapters 2 and 4 of Volume 2), in which the planets, their satellites, asteroids and comets evolve at great distances from each other. The mass of this disk is very low (about 0.15%) with respect to that of the Sun: it is essentially distributed in the planets.

There are eight planets in the Solar System and they are divided into two categories: the telluric planets (Mercury, Venus, Earth and Mars), close to the Sun; and the giant planets (Jupiter, Saturn, Uranus and Neptune), that are further away. The brightest (Mercury, Venus, Mars, Jupiter and Saturn), observable with the naked eye, have been known since antiquity. Their number has varied over the centuries, depending on successive discoveries. Following the discovery of Pluto in 1930, this number reached 9, until in 2006, the International Astronomical Union (IAU) reviewed the definition of a planet (see Box 1.1) and reduced it to 8, Pluto now belonging to the category of Trans-Neptunian objects (TNO). With the exception of Mercury, they all have an atmosphere. The telluric planets are relatively small (the largest, Earth, has a radius of 6,400 km) and dense, and have a small number of satellites. On the other hand, the four giant planets are large (with a radius greater than 20,000 km) and not very dense; all of them have a system of rings (formed of particles whose size can vary from a micrometer to a meter) and a procession of satellites (also of very variable size, the largest being comparable to Mercury); we will see further on that these characteristics are the consequence of their formation scenario. With a few rare exceptions (including Titan, Saturn's largest satellite), satellites have no atmosphere.

The list of the planets of the Solar System is, however, not definitively closed. In 2016, astronomers K. Batygin and M. Brown (Batygin and Brown 2016) suggested, based on numerical simulations, the existence of a ninth planet of about 10 Earth masses that would have acted as a massive disturbance of the outer Solar System, and whose existence would explain the very asymmetrical distribution of the orbits of the most distant TNO. This planet, formed in the current environment of the giant planets, would have been ejected towards the outside by the play of the gravitational disturbances, for which they are responsible. If it was not definitively ejected from the Solar System, it would evolve between some 200 and 1,200 au, on an inclined orbit of about 30° on the ecliptic plane. Given its distance, this planet will be extremely difficult to observe. Other studies suggest that the particular configuration of the distant TNO orbits could also be caused by the combined gravitational forces of several small trans-Neptunian objects.

Box 1.1. How do you define a planet?

Historically, the planets of the Solar System have been defined as the "wandering objects" ( in Greek) of the sky. In the 16th century, the Earth was added to this list, then Uranus and Neptune. The asteroids, discovered in the 19th century, were classified rather quickly in a category apart because of their ever-increasing number. Pluto, discovered in 1930, was first considered as a planet. In 2003, the discovery of the TNO Eris, of a size comparable to that of Pluto, needed a new reflection on the specificity of a planet. After much debate, the IAU adopted the following definition in 2006: a planet is a celestial body that (1) is in orbit around the Sun, (2) has sufficient mass to be in hydrostatic equilibrium (in other words, spherical in shape), and (3) has eliminated the neighborhood around its orbit. According to this definition, there are eight confirmed planets of the Solar System; the four telluric ones and the four giants. There is perhaps a ninth planet at the confines of the Solar System, whose existence remains to be confirmed. The IAU has also introduced the concept of the "dwarf planet" which includes the first two criteria, but not the third. There are currently five objects in this category, one asteroid (Ceres) and four TNOs (Pluto, Haumea, Makemake and Eris) (see section 2.4, Volume 2).

The discovery of exoplanets, since 1995, has forced astronomers to redefine the border between planet and star. Unlike planets, stars have the ability to trigger nuclear reactions within them. Brown dwarfs are intermediate objects between stars and planets, which are the seat of the first nuclear reaction cycle causing the fusion of deuterium, but not of the next, responsible for the fusion of helium. The brown dwarfs have a mass ranging between 13 MJ (mass of Jupiter) and 0.07 MS (mass of the Sun), in other words, 75 MJ. This definition was adopted by the IAU in 2003.

The following books present general studies on the physics of Solar System objects: (Kivelson and Russell 1995; Lewis 1995; Encrenaz et al. 2003; Hanel et al. 2003; Lissauer and de Pater 2013; Spohn et al. 2014; Catlin and Kasting 2017). Figure 1.1 presents a simplified diagram of the Solar System.

Figure 1.1. Simplified diagram of the structure of the Solar System. The dimensions are not respected on the figure and the scale of the distances to the Sun indicates an order of magnitude. The outer limits of the Kuiper belt, the trans-Neptunian object region (TNO) and the Oort cloud are very imprecise. For a color version of this figure, see www.iste.co.uk/encrenaz/solar1.zip

Asteroids are small objects (with a radius of less than 500 km), also devoid of atmosphere. Most of them are located in a torus called the "main asteroid belt", between the orbits of Mars and Jupiter. Beyond the orbit of Neptune extends the "Kuiper Belt", occupied by the trans-Neptunian objects. These objects, discovered since 1992, have a diameter of a few hundred kilometers, on highly different orbits; Pluto, discovered in 1930, is one of the largest objects of this family. Lastly, comets are even smaller objects (less than 10 kilometers). Unlike all the preceding objects, they evolve in very elliptical orbits that can take them from the farthest reaches of the Solar System to near the Sun. The extraterrestrial matter received on Earth mainly comes from asteroids, in the case of meteorites, and from comets, in the case of shooting stars and micrometeorites.

Table 1.1. Orbital characteristics of the planets of the Solar System

Table 1.2. Physical characteristics of the planets of the Solar...