Minor planets move around the Sun in the same way as other planets do (picture 2). Most of them are found in an orbit between Mars and Jupiter. A minority has another trajectory: some approach the Earth very closely, while others circle in the orbit of Jupiter or on the outer limits of our solar system, farther than Neptune. Today, thirty thousand minor planets are well known (picture 3), while a few hundred thousands were already observed but their precise orbit is not known yet.

JH Oort, an astronomer from the Netherlands formulated the hypothesis of the existence of a loose cloud formed by dust accumulation and frozen gasses on the edge of our solar system. Because of small disturbances, some of these accumulations are sent to the inner part of our solar system and form comets. (picture 4).

When a comet approaches the Sun, the volatile materials (mainly dirty ice) evaporate because of the increased solar radiation and a sort of momentary atmosphere (“coma”). (video 1) is formed. During this process, the body of the comet (“core”) frees a lot of dust. However, as comet bodies are small, their gravity cannot hold the gas and the freed dust. They escape and form two long tails, in the direction opposite to the Sun. (picture 5 & 6)

The study of asteroids and the composition of comet bodies itself — of which one thinks they are remainders of the protoplanetary nebula at the origin of our solar system — can teach us a lot about the origin, the evolution, the structure and the composition of the Sun and the planets.