Scientists agree that planets are formed by accretion of asteroids and comet nuclei. At the end of this process, most cometary and asteroid material is thought to be merged into planets, which then undergo profound transformations as they cool.

4.5 billion years ago, the protosolar nebula, subjected to extreme increases in temperature and condensation, produced the 9 planets we know today. On the periphery of the system, the lower temperatures allowed ice and dust to subsist, forming the nuclei of comets. These nuclei were not part of the accretion phase. Sometimes, the gravitational tug of distant stars sends comets towards the inner Solar System,where the Sun’s heat causes material to outgas from their surface, forming the tails we see from Earth.

Since their formation, comets have remained intact. As they orbit far from the Sun, their low temperatures have frozen all chemical evolution. They therefore conserve the memory of the solar system's first moments and may be able to give us a new understanding of how it formed.

In addition to their common origin with the solar system, comets seem to have played an essential role in the biological evolution of the Earth. The organic molecules that they contain are the key elements of amino acids, the essential ingredients for the development of living things. Comets may therefore have released the elementary building blocks of life when they collided with the Earth's surface.

The Rosetta probe is intended to study the composition of a comet, as well as the emergence and development of its activity as it approaches the Sun. The probe carries 21 scientific instruments, to cover 2 main categories of experiments. One category will analyse the composition of gases and dust emitted by the nucleus when the comet degasses. The other instruments will study the composition and structure of the nucleus in situ from the Philae lander.