The two families of planets in the solar system

Copyright : NASA / JPL and Paris Observatory / UFE

The planets of the solar system

The solar system refers to the set of celestial bodies bound by the gravitational attraction of the Sun. Among these bodies, we can distinguish :

• the five planets visible to the naked eye and known since antiquity : Mercury, Venus, Mars, Jupiter and Saturn;

• two planets discovered more recently through the use of a telescope : Uranus (1781) and Neptune (1846);

• and several dwarf planets, such as Ceres (1801), Pluto (1930) and Eris (2003).

Altogether there is a total of eight planets (Earth included) orbiting the Sun.

The planets of the solar system can be divided into two groups, the telluric planets and the gas giant (or "Jovian") planets. The telluric planets are spherical bodies with a crust of rock, and the gas giant planets are spheres composed of gas and ice (Jupiter, Saturn, Uranus and Neptune). Pluto is a solid body, whose composition consists of a variety of rocks and ices.

Contrary to the Sun and the stars, thermonuclear fusion reactions do not occur in planets (nor do they occur in other bodies in the solar system). The planets are visible because they reflect sunlight.

Exoplanets : planets outside the solar system

An exoplanet (or extrasolar planet) is a planet orbiting a star different from the Sun (the "exo" prefix means "outside" in Greek). Up until now, one has found mainly gas giant planets, which are easier to detect than telluric planets. However, due to the increasing sensitivity of the detection methods, one already begins to observe the first planets of sizes comparable to the Earth.