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Like the helium hydride cation (HeH⁺), the Molecule of the Week for May 20, 2019, the trihydrogen cation (H₃⁺) looks as though it shouldn’t exist. Also like HeH⁺, H₃⁺ was discovered a long time ago and is abundant in outer space.

In 1911, the British physicist and 1906 Nobel Prize winner J. J. Thomson observed the presence of a molecular ion with a mass/charge ratio of 3:1 in the discharge from a plasma tube. He deduced that the only possible formula for the ion could be H₃⁺.

Fifty years later, D. W. Martin, E. W. McDaniel, and M. L. Meeks at Georgia Tech (Atlanta) speculated the existence of H₃⁺ in space. Now, almost 30 years later, relying on a prediction of the ion’s spectrum, Pierre Drossart at the Paris Observatory, Steve Miller at University College London, and their colleagues

H₃⁺ was subsequently found in the atmospheres of Saturn and Uranus as well as Jupiter. It is also plentiful in interstellar space.

Because it exists only at very low pressure and temperature, there are few physical and chemical data and no hazard information available for H₃⁺. But astrochemists and other space scientists find it a convenient tool for examining chemical bonding and reactions in the observable universe.