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Recent ALMA observations present mounting evidence for the presence ofexocometary gas released within Kuiper belt analogues around nearby mainsequence stars. This represents a unique opportunity to study their icereservoir at the younger ages when volatile delivery to planets is most likelyto occur. We here present the detection of CO J=2-1 emission co-located withdust emission from the cometary belt in the 440 Myr-old Fomalhaut system.Through spectro-spatial filtering, we achieve a 5.4$\sigma$ detection anddetermine that the ring's sky-projected rotation axis matches that of the star.The CO mass derived ($0.65-42 \times10^{-7}$ M$_{\oplus}$) is the lowest of anycircumstellar disk detected to date, and must be of exocometary origin. Using asteady state model, we estimate the CO+CO$_2$ mass fraction of exocomets aroundFomalhaut to be between 4.6-76%, consistent with Solar System comets and thetwo other belts known to host exocometary gas. This is the first indication ofa similarity in cometary compositions across planetary systems that may belinked to their formation scenario and is consistent with direct ISMinheritance. In addition, we find tentative evidence that $(49\pm 27)$% of thedetected flux originates from a region near the eccentric belt's pericentre. Ifconfirmed, the latter may be explained through a recent impact event or COpericentre glow due to exocometary release within a steady state collisionalcascade. In the latter scenario, we show how the azimuthal dependence of the COrelease rate leads to asymmetries in gas observations of eccentric exocometarybelts.

Detection of exocometary CO within the 440 Myr-old Fomalhaut belt: a similar CO+CO$_2$ ice abundance in exocomets and Solar System comets