The recent discoveries of extrasolar planets have stimulated new research on the formation of planets which is intimately linked to the evolution of circumstellar disks.
Molecular lines constitute excellent diagnostics of the physical conditions of disks.
Observations of their rotational transitions using the IRAM-30m and the James Clerk Maxwell Telescope allow us to infer density, temperature and chemical abundances. The results suggest that the emission comes from a dense and cold region. Comets are considered as the most primitive objects in our Solar System. Because they remain for most of their time at great distances from the Sun, they are unlikely to have suffered any significant modification since the moment of their formation in the outer cold part of the Solar Nebula some 4.5 billions years ago. One of the question about their formation is how much interstellar material is incorporated compared with processed in the Solar Nebula.