The Forbidden Light of H3+:
JWST Confirms a 40-Year-Old Prediction
By Thomas Lee Abshier, ND ·
In 1986, two physicists at the University of Chicago, Fu-Shih Pan and Takeshi Oka, calculated that one of the simplest molecules in the universe should give off a very faint kind of light that, by the usual rules, it should not emit at all. For forty years no one could see it, in space or in the laboratory. This month a team led by David Neufeld of Johns Hopkins University reported that the James Webb Space Telescope (JWST) has now seen it, in the shocked gas around a newborn star in Cepheus. I want to explain what they found, why it was so hard to detect, and its significance.
The molecule
The trihydrogen cation, written H3+, is three protons held together by two electrons. It is the simplest molecule that has more than two nuclei. J. J. Thomson first saw it in electrical discharges in 1911, and Takeshi Oka recorded its infrared spectrum in the laboratory in 1980. It was later found in Jupiter’s aurorae, and in 1996 Geballe and Oka detected it in interstellar clouds.
Its importance comes from how readily it gives away a proton. In a cold molecular cloud, cosmic rays knock an electron off an ordinary hydrogen molecule (H2), making H2+. That ion quickly collides with another H2 and becomes H3+. Because H3+ holds its third proton loosely, it hands that proton to nearly any atom or molecule it meets: oxygen, carbon, nitrogen, carbon monoxide. Those protonated species then react further, step by step, into water, hydrocarbons, and eventually the complex organic molecules found in star-forming regions. For that reason chemists often say that most gas-phase chemistry in interstellar space begins with H3+, and some have called it “the molecule that made the universe.” I think the nickname fairly summarizes its role as the first link in the chain, provided we remember it is a catalyst of chemistry, not a creator of matter.
Why its spin should be dark
Most molecules announce themselves to radio and infrared astronomers through their rotation. A molecule like carbon monoxide has a slightly lopsided charge distribution: a permanent electric dipole. When it spins, that lopsided charge swings around, and a swinging charge radiates. Each change in rotation rate produces light at a specific wavelength, and those wavelengths are the molecule’s fingerprint.
In its resting state, H3+ is an equilateral triangle. Its charge is spread evenly, so it has no permanent dipole. When a perfectly balanced triangle spins, nothing swings around, and the standard selection rules say the molecule should have no pure rotational spectrum. That is why astronomers have always had to detect H3+ through its vibrations, not its rotation.
The loophole Pan and Oka found
Pan and Oka noticed that a spinning triangle isn’t perfectly symmetric. As it rotates faster, centrifugal force stretches it slightly, and the stretch isn’t the same in every direction. That small distortion gives the molecule a very small, induced dipole moment, and with it a very weak ability to radiate as it changes its rotation.
Physicists call these transitions “forbidden.” The word does not mean that physics prohibits them. It means that the simple rules we use to predict which transitions happen leave them out, and a closer treatment shows they occur, but extremely rarely. A given molecule in such a state may wait a very long time before it emits. The light is therefore faint, and it can only be seen where there is a great deal of hot H3+ in the right rotational states.
Where JWST found it
The place turned out to be a Herbig-Haro object. When a young star forms, it draws in gas from a surrounding disk, but magnetic fields throw part of that gas back out in narrow jets moving at hundreds of kilometers per second. Where the jets strike the surrounding cloud, they drive shock waves that heat the gas to thousands of degrees. The glowing knots that result are called Herbig-Haro objects. They are usually buried in dust, which is why an infrared telescope such as JWST is the right instrument to study them.
Neufeld’s team was studying HH 168, which lies in the massive star-forming region Cepheus A. In the infrared spectrum of the shocked gas, they found a set of very weak lines whose wavelengths matched Pan and Oka’s calculated forbidden rotational transitions. They identified 26 separate lines, each at high statistical confidence; the strongest stands at about 72 sigma, which in practical terms leaves no reasonable doubt that it is real. They then searched archival JWST data for another outflow, HH 211, and found the same emission there. Seeing it in two different objects tells me this isn’t a peculiarity of one unusual place; ordinary star-forming shocks can produce it.
What the maps show
The team mapped where the H3+ emission comes from. It closely follows the emission of hot, ordinary molecular hydrogen (H2). That makes sense chemically: H3+ is made from H2, and both are being excited by the same shock. It also means H3+ can be used with H2 as a probe of conditions inside these shocks.
What is still unexplained
The emission is brighter than the models expected. The researchers tried two kinds of shock model. One reproduced the H2 emission well but predicted only about a fifth or sixth of the H3+ light that was observed. The other came closer to the H3+ brightness, but only by using conditions outside the range in which that model had been tested. So the detection is secure, but its explanation is not yet settled. Something in the shock is either making more H3+, keeping it from being destroyed, or exciting it more efficiently than current models assume. Working out which will tell us something real about how these shocks work.
Why I find this worth writing about
Three things stand out to me.
First, a careful calculation made in 1986 has been confirmed by an instrument that didn’t exist then. Pan and Oka were not guessing; they worked out what a well-understood molecule must do when its symmetry is slightly broken by its own motion. The prediction waited forty years for a telescope sensitive enough to test it, and when the test came, the lines appeared where the calculation predicted.
Second, it is a clean example of what “forbidden” means in physics. Rules of thumb approximate deeper laws. When nature does something the rule of thumb says it cannot, the deeper law usually still holds, and the exception teaches us where the approximation ends.
Third, the molecule that starts so much of the chemistry of space can now be watched in a new way, through its rotation, in the hottest and most violent places where stars are born. The mismatch with the models is not a failure; it is the next question.
The paper by Neufeld and colleagues has been accepted by The Astrophysical Journal Letters and is available as a preprint on arXiv.
Sources
- ScienceAlert, “JWST Detects a ‘Forbidden’ Signal Never Before Seen in The Universe – Not Even in a Lab” (12 November 2026): https://www.sciencealert.com/jwst-detects-a-forbidden-signal-never-before-seen-in-the-universe-not-even-in-a-lab
- Neufeld, D. A., et al. (2026), detection of H3+ pure rotational emission in HH 168 and HH 211, accepted to The Astrophysical Journal Letters; preprint on arXiv.
- Pan, F.-S., & Oka, T. (1986), calculation of the forbidden rotational spectrum of H3+, The Astrophysical Journal.
- Background on H3+ in the interstellar medium: “H3+ in irradiated protoplanetary disks,” arXiv:2506.05189: https://arxiv.org/abs/2506.05189
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