JWST Detects a 'Forbidden' Signal Never Before Seen in The Universe – Not Even in a Lab

One of the most important molecules in the Universe has been hiding its light for decades.

It's the foundation for almost all gas-phase interstellar chemistry  – including the formation of complex organic molecules that make up the ingredients for life.

Its name is the trihydrogen cation (H₃⁺), and it's so important that it's often referred to as "the molecule that made the Universe."

It's been detected lurking in interstellar clouds and is partially responsible for Jupiter's auroral glow – but a particular kind of light it should emit as it rotates, predicted since 1986, has eluded every search – including attempts to observe it in laboratory settings.

Now, using the the JWST, a team of astronomers led by David Neufeld of Johns Hopkins University has finally detected that elusive light, shining from gas thousands of light-years away.

And the discovery could help us understand the physics of the giant shock waves generated by newborn stars – as well as what happens to this critical molecule amid all that fire and fury.

To understand why this discovery is so incredible, we need to look at the peculiar shape of the trihydrogen molecule itself.

It consists of three hydrogen nuclei chunked together, sharing just two electrons between them. The shape they form is an equilateral triangle – which means the molecule has no permanent dipole moment and no allowed pure rotational spectrum.

That means that, as the molecule spins, its electrical charge remains evenly distributed, with no shifting imbalance to generate the electromagnetic radiation that molecules ordinarily emit as they rotate.

Even so, in 1986, astrophysicists Fu-Shih Pan and Takeshi Oka of the University of Chicago calculated that trihydrogen should be able to emit a very faint amount of light as it rotates.

Here's the loophole. As the molecule spins, centrifugal forces distort its perfectly symmetrical shape ever so slightly. This creates a tiny electric dipole moment, allowing it to emit radiation after all.

The resulting transitions are so extraordinarily weak that they're considered "forbidden" – not because they violate the laws of physics, but because they are prohibited by the usual rules governing molecular radiation.

For four decades, that prediction remained unconfirmed. The signal was simply too faint to detect, even in laboratory experiments.

But what Pan and Oka's paper did not predict was the killer combination of JWST – and a newborn star throwing a tempestuous tantrum of cosmic proportions.

Herbig-Haro objects are some of the most spectacular phenomena in the galaxy, born from the violent outbursts of baby stars.

As a star forms, it feeds on material swirling around it. But not all of that material ends up in the star. Powerful magnetic fields help fling some of it back out into space in tightly focused jets, traveling at hundreds of kilometers per second.

When those jets slam into the surrounding molecular cloud, they generate immense shock waves, heating the gas to thousands of degrees and causing it to blaze with light.

It's a pretty short-lived but important phase of a star's lifespan, and one that astronomers are keen to learn more about. However, because it often unfolds inside the thick nebula from which the star is born, it can be really hard to see.

This makes Herbig-Haro objects particularly interesting targets for JWST. The infrared light that the telescope is attuned to penetrates obscuring dust more effectively than visible light, allowing it to peer into these turbulent stellar nurseries.

But when Neufeld and his colleagues started examining JWST's observations of an object named Herbig-Haro 168, or HH 168, something was a little peculiar.

Buried among the infrared emissions from the hot, shocked gas were a series of extraordinarily faint spectral lines.

And their wavelengths matched those of the forbidden rotational transitions of the trihydrogen cation, calculated by Pan and Oka 40 years earlier.

Not just one line, either. The team identified 26 distinct lines, all detected with high statistical confidence. The strongest of those lines registered a staggering 72 sigma – leaving virtually no doubt about the detection.

For the first time, astronomers had caught the molecule emitting light that had never been observed anywhere in the Universe – and not just a small amount. Compared to what their models later predicted, it was positively incandescent.

But the presence of the signal was only part of the story. Equally interesting was where it appeared among the turbulent gas shocked by the baby star's jets.

It closely tracked the emission of ordinary molecular hydrogen (H₂), suggesting that both molecules were responding to the same violent conditions.

But explaining exactly how those conditions produced such bright trihydrogen emission proved a little trickier. When the researchers tried to reproduce the observations using shock wave models, neither approach worked perfectly.

One model did a good job of reproducing the emission from ordinary molecular hydrogen, but underestimated the trihydrogen signal by a factor of five or six.

Another came much closer to matching the trihydrogen emission, but relied on conditions outside the range for which the model had been validated.

So more work needs to be done to figure out why the emission was so bright. But there's one last piece of the puzzle that suggests whatever is going on, the Universe can do it with relative ease.

When the team examined archival JWST observations of another Herbig-Haro object, HH 211, they found the same telltale emission there, too.

That's actually really kind of wonderful. Pan and Oka have been right all this time – the thing lacking was the tool sensitive enough to confirm it.

And now the trihydrogen itself may turn into a tool – one that scientists can use to better understand the chemistry that transforms the simplest element in the Universe into the breathtaking complexity of the cosmos.

The research, which has been accepted for publication in The Astrophysical Journal Letters, is available on preprint server arXiv.