The detection of argonium (ArH⁺ or protonated argon) was reported in 2013 by Barlow et al. toward the Crab Nebula. On the basis the new experimental data of Cueto et al., two rotational lines of ArH⁺ were identified in spectra collected by the Herschel space observatory. The argonium detection has already been confirmed by the observations of Schilke et al. It has also been observed in the extragalactic source PKS 1830-211 a quasar, by Müller et al..

Argonium is the only protonated rare gas molecule to be detected to date in space: neither protonated helium nor protonated neon have been observed. The animation depicts the bond formation of ArH+ via dative bonding: the bare proton (with an empty 1s orbital) draws some of the charge of one of the three Ar 3p2 orbitals onto itself at short separations.