Circumstellar disks around young low-mass stars: Observed properties and lifetime

Unveiling the physical processes leading to planet formation is one of the major challenges of modern astrophysics. Until the last fifteen years, the origin of planetary systems was mostly investigated from observations and modelling of our own Solar System. However, the new generation of astronomical facilities is slowly but surely changing this. Observations of circumstellar disks around young stars similar to the Sun in its infancy are now routinely provided by large astronomical facilities, from the optical wavelengths to the millimeter range. These observations begin to have the sensitivity and angular resolution needed to provide invaluable information inside the regions of disks where planets should form. In this review, I show some new results which illustrate how fast this new domain is evolving and which kind of physical/chemical parameters can be explored. In particular, I present what recent observations tell us about disk lifetime and evolution.I conclude by some open questions which represent important astrophysical challenges for tomorrow in order to understand how planetary systems form.