Every second, trillions upon trillions of incredibly low-mass subatomic particles called neutrinos shoot out from the Sun and slide undetected through your body.

These neutrinos are produced by the two known types of fusion reactions that happen in our Sun, and up until now scientists have only detected one group of them - the neutrinos produced by protons squeezing together to create helium.

But for the first time, physicists working on the Borexino experiment in Italy say they've finally observed the second more elusive group of 'missing' neutrinos.

This tiny handful of neutrinos are the result of a second recipe for helium. They're known as 'CNO neutrinos', named after the Sun's carbon-oxygen-nitrogen (CNO) cycle, and scientists have been looking for them for decades.

If we can find a way to better analyse them, we might be able to solve a major ongoing about our Sun - just how much metal it really contains.

"With this outcome, Borexino has completely unraveled the two processes powering the Sun," said physicist Gioacchino Ranucci from Italy's National Institute for Nuclear Physics in Milan, as ScienceNews reports.

Ranucci presented the results at the Neutrino 2020 conference being hosted virtually this week. It's important to note that the details haven't been peer reviewed as yet, but the possibility is already attracting interest from the physics community.

Experimental particle physicist Lindsey Bignell from ANU in Australia wasn't involved in the study, but was paying close attention to the presentation's results.

"The measurements presented by the Borexino collaboration represent the culmination of a heroic effort to remove backgrounds in their detector enough to measure the CNO neutrinos," Bignell told ScienceAlert.

"These results will be very important for understanding the nature of our Sun and may resolve the solar metallicity problem."

As odd as it seems, we just can't seem to work out how metal our Sun is, with predictions based on two different methods disagreeing by around 28 percent. These CNO neutrinos just might give us a deciding vote on which model, if either, is correct.

The reason why CNO neutrinos have been so difficult to find is because they're hard to distinguish from the other type of neutrino produced by our Sun - particularly with the background currents and temperature changes that result from the Borexino detector.

The experiment's setup consists of a huge stainless steel sphere designed to capture the elusive blink ofneutrinos as they bump into molecules of a solvent called pseudocumene. To avoid flashing with boring old stray particles of radiation, the entire experiment is immersed in a tank of water. And then shielded in rock.

Just under 300 tons of solvent make up the core of Borexino's detector, so you'd imagine the whole thing would be flashing like a Fourth of July celebration.

But neutrinos only make themselves known through the weak nuclear force, requiring them to pass within an insanely small window of 10^-46 square centimetres to have a hope of shaking an atom's nucleus.

This means in spite of being showered with the tiny particles, maybe a few dozen neutrinos per day come close enough to have a chance of triggering a burst of light.

Experiments like this one have been used to measure neutrino emissions since the 1960s, mapping distant cosmic explosions, nearby nuclear reactions, and the bounty of particles shining from the Sun.

Most of the neutrinos emitted by our closest star were made as hydrogen ions merged to produce an alpha particle of helium, so it's no surprise that decades of neutrino hunting have given us a good grasp of that particular stellar reaction.

Borexino's choice of solvent was fine-tuned to pick up neutrinos that packed a specific punch, one that would allow it to focus on some other kinds of nuclear reaction.

Since 2007 the experiment has recorded flashes that can be traced to the production of isotopes like beryllium-7 and boron-8, for example, helping researchers confirm the mathematics behind the Standard Model 'zoo' of particles.

But CNO neutrinos have been a little harder to detect than the rest.

The CNO cycle is a convoluted alternative to the 'proton plus proton' method for helium manufacture that uses carbon, oxygen, and nitrogen as a series of stepping stones.

Finding traces of the CNO cycle will help us better determine its limits, and the Sun's confusing mix of elements, especially when it comes to its role in the nucleosynthesis of other parts of the periodic table.

"Using neutrinos is advantageous because it is not only an independent way of measuring the solar metallicity, but it is the only technique that is directly sensitive to the metallicity of the core of the Sun," says Bignell.

This study was presented at the Neutrino 2020 virtual conference.