'Forbidden' light proposed 40 years ago detected for first time in the universe

The light from a molecule known to be behind the creation of the universe has finally been revealed for the first time. Trihydrogen cation (H3+) has been detected in interstellar clouds thousands of light-years away. It is partly what lends Jupiter its auroral glow. But as the planet rotates, this molecule should emit a light. It was first predicted in 1986 but has eluded scientists till now. The 40-year search has finally borne fruit, and this light has now been seen. However, it is coming from deep interstellar space and not Jupiter. A team of astronomers led by David Neufeld of Johns Hopkins University detected the light using the James Webb Space Telescope (JWST).

The discovery is important because of the unique shape of this molecule. H3+ consists of three hydrogen nuclei fused, with just two electrons, causing it to be shaped like an equilateral triangle. This means the molecule has no permanent dipole moment. As the molecule spins, electrical charge remains evenly distributed. There is no shifting imbalance which can generate the electromagnetic radiation that molecules ordinarily emit as they rotate.

Astrophysicists Fu-Shih Pan and Takeshi Oka of the University of Chicago said in 1986 that the molecule should emit a faint light. When it rotates, centrifugal forces slightly distort its perfectly symmetrical shape, creating a tiny electric dipole moment which causes it to emit radiation. The transitions are extremely weak and considered "forbidden" because they should not exist as per the usual rules governing molecular radiation.

JWST peered through a stellar nursery

The signal has eluded detection even in the laboratory all these years. But JWST seems to have done the trick when it looked at a region making new stars - HH 168. It can penetrate through the dust and see what is happening inside turbulent stellar nurseries, or Herbig-Haro objects. It is feeding on material around it, but powerful magnetic fields are flinging some of it back into space. These jets are travelling at speeds of hundreds of kilometers per second. As they slam into the molecular cloud around it, shock waves heat the gas to thousands of degrees, triggering a blaze.

When the researchers started looking at HH 168, they saw extraordinarily faint spectral lines inside the infrared emissions. The wavelengths were similar to what Pan and Oka had calculated for the forbidden rotational transitions of the trihydrogen cation. There were a total of 26 distinct lines, with one of those registering a staggering 72 sigma. Scientists were now sure that they were looking at a light that had never been seen before in the universe.

Another surprise came from archival data from JWST, which showed another Herbig-Haro object, HH 211, with the same emissions. This means the universe does it relatively commonly and that Pan and Oka were right all along. The study will be published in The Astrophysical Journal Letters.