H₃⁺ (protonated molecular hydrogen) is an important astromolecule — so important that it has been called "... the molecule that made the universe." For example, it is a source of protons in many astrochemical reactions. However, H₃⁺ cannot be detected via rotational or electronic spectroscopy. Its only observable spectroscopic features are vibrational in nature and fall in the infrared. H₃⁺ was not identified in extraterrestrial sources until the late 1980s. After being found in the ionospheres of three gas giants — Jupiter, then Uranus and Saturn — it was observed toward protostars GL2136 and W33A in 1996 by Geballe & Oka and then in the diffuse ISM toward Cynus OB2-12 by McCall et al.

June 2025 - The detection of H₃⁺ in the atmosphere of Neptune has been reported by Melin and co-workers.

H₃⁺ has three vibrational modes that are shown below. The first two are degenerate asymmetric deformation modes, and the last one is the symmetric breathing mode. Only the asymetric motions have non-zero IR intensities.