Molecular hydrogen in the circumstellar disks of pre-main sequence stars

Circumstellar disks surrounding young forming stars, are likely the location where planets form. While the gaseous phase represents up to ∼!99% of the disk mass and control the dynamics, most of disk properties relies on dust analyses. The main constituent of the gaseous component, molecular hydrogen (H}{2}), remains nearly out of reach and the gas disk is probed through emission lines of minor tracers, such as CO. In this lecture, we will first recall how {H}{2} symmetric molecular structure makes its detection difficult. We will then review the most significant results achieved so far, thanks to new generation of ground and space-based telescopes, with a special emphasize given to Herbig Ae/be, which are pre-main sequence stars of intermediate mass. Though the first direct estimates of circumstellar disk mass have been reported, observation of {H} {2} is still challenging detection.