The Solar System

The solar system consists of a central star, called the Sun, eight planets, several dwarf planets, dozens of moons or satellites, millions of asteroids and Trans-Neptunian Objects (TNOs), and myriads of comets and meteoroids.

Borders between the categories are not clear. Discoveries of new Solar System bodies caused that in 2006 the International Astronomical Union (IAU) in its General Assembly defined three distinct categories to clarify the situation:

(1)Aplanet is a celestial bodythat: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A dwarf planet or a planetoid is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects orbiting the Sun shall be referred to collectively as Small Solar System Bodies. These include most of the asteroids, Trans-Neptunian Objects, comets, and other small bodies.

A satellite is a body which orbits the primary body so that the centre of mass (barycentre) is inside the primary. If this is not the case, then the system is called a binary system. For example, in the case of the Earth and Moon the barycentre of the system is inside the Earth, and the Moon is Earth's satellite. In the Pluto-Charon system the centre of mass is outside Pluto, and therefore they are called a binary system.

The planets in order from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

According to the IAU 2006 definition, Pluto is a dwarfplanet and the prototype of a new category of Trans-Neptunian objects.

The planets from Mercury to Saturn are bright and well visible with a naked eye. Uranus and Neptune can be seen with a pair of binoculars. In addition to the bright planets, only the brightest comets are visible with a naked eye.

Distances in the solar system are often measured in astronomical units (AU), the mean distance of the Sun and Earth. The semimajor axis of the orbit of Mercury is

0.39 AU, andthedistance of Neptune is 30 AU. Beyondthe orbit of Neptune there is a huge population of small icy bodies extending out to tens of thousands AUs. The Solar System has no obvious outer edge. The distance to the nearest star, Proxima Centauri is over 270,000 AU.

Gravitation controls the motion of the solar system bodies. The planetary orbits around the Sun (Fig. 7.1) are almost coplanar ellipses which deviate only slightly from circles. The orbital planes of asteroids, minor bodies that circle the Sun mainly between the orbits of Mars and Jupiter, are often more tilted than the planes of the planetary orbits. Asteroids and distant Tra ns-Neptunian Objects revolve in the same direction as the major planets; comets, however, may move in the opposite direction. Cometary orbits can be very elongated, even hyperbolic. Most of the satellites circle their parent planets in the same direction as the planet moves around the Sun. Only the motions of the smallest particles, gas and dust are affected by the solar wind, radiation pressure and magnetic fields.

The planets can be divided into physically different groups (see Fig. Fig. 7.2). Mercury, Venus, Earth, and Mars are called terrestrial (Earth-like) planets; they have a solid surface, are of almost equal size (diameters from 5000 to 12,000km), and have quite a high mean density (4000-5000 kg m-3; the density of water is 1000 kg m-3). The planets from Jupiter to Neptune are called Jovian (Jupiter-like) or giant planets. The densities of the giant planets are about 1000-2000 kg m-3, and most of their volume is liquid. Diameters are ten times greater than those of the terrestrial planets.

Dwarf planet Pluto is falling outside this classification. Pluto is the prototype to the family of icy bodies orbiting the Sun at the outer edges of the solar system. The discovery of large objects since early 1990's beyond the orbit of Neptune raised the question of the status of Pluto. The discussion culminated in the General Assembly of the IAU in 2006 when a new planetary definition was accepted. This reduced the number of major planets to eight.

Most and most accurate solar system data are today collected by spacecraft. Many methods used in geosciences are nowadays applied in planetary studies. Landers have been senttoMoon, Venus, Mars, and Saturnian moon Titan and all major planets, their satellites, and many asteroids and comets have been studied with spacecraft.

Hannu Karttunen et al. (Eds.), The Solar System.

In: Hannu Karttunen et al. (Eds.), Fundamental Astronomy, 5th Edition. pp. 131-205 (2007) DOI: 11685739_7 © Springer-Verlag Berlin Heidelberg 2007

Fig. 7.1. (a) Planetary orbits from Mercury to Mars. The dashed line represents the part of the orbit below the ecliptic; the arrows show the distances travelled by the planets during one month (January 2000). (b) Planets from Jupiter to

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