At the European Southern Observatory at La Silla, in January 1986, Ferlet and collaborators observed weird hourly variations of narrow and deep redshifted absorption lines in the calcium doublet of the Beta Pictoris spectrum. This was the very first observation of exocomet transits and the subsequent detection of calcium in their body. Later on, the far richer UV spectrum of the A-type star Beta Pictoris collected from space telescopes led to observations of atomic absorptions by many other species present in the exocomet transiting clouds. One main result of such an exploration is the detection of highly ionized atoms in exocomets, such as C IV and Al III. It allowed inferring the existence of a bow shock surface ahead of the evaporating nucleus. The exploration of exocomet composition also opens to the question of similarity with solar system comets. What is the dust-to-ice ratio? Water or CO ice? We deeply believe that having a glare into planet-forming systems can teach us about the youth of our own system.
In this talk, I will review the main results and means of the exploration of exocomets composition from the first discovery to present day. I will also review the main questions on hold, and how to address them in the future.