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About 74% of mass in the Universe is in the form of hydrogen. In hot environments or those dominated by high energy radiation fields, such as stars and HII regions, hydrogen is found in atomic form. However, for weaker radiation fields and kinetic temperatures below a few thousand K hydrogen adopts a molecular form. Measuring the amount of molecular hydrogen in warm to cold environments is key to estimate their masses unaffected by chemistry. This is particularly important for circumstellar envelopes of asymptotic giant branch (AGB) stars, which eject large amounts of matter to the interstellar medium. Their mass-loss rates are usually calculated using CO rotational lines but the CO/H2 abundance ratio is typically adopted and its uncertainty can be as high as a factor of 2 to 3. H2 was detected in many AGB stars in the past by means of its rotation-vibration band around 2µm. Unfortunately, part of the opacity of this band is produced by H2 in their photosphere and the close gravitationally bound matter. The use of the rotational spectrum of H2 could allow us to calculate the right mass-loss rates but no rotational line has been detected in AGB stars so far. In this talk, we will present the detection of the H2 S(1) rotational line at 17µm for the first time in an AGB star (IRC+10216) with the high spectral resolution spectrograph EXES mounted on SOFIA. We will model and analyze the observations to simultaneously derive the mass-loss rate of IRC+10216, the CO/H2 abundance ratio, and the physical conditions throughout its envelope.