Located in the heart of the Sun, physicists have for the first time directly detected neutrinos, revealing a little about our closest star and the sources of its energy, a new study describes.

Scientists at the University of Massachusetts Amherst discovered these neutrinos via the "keystone" proton-proton (pp) fusion process happening at the Sun's core - a reaction responsible for about 99 percent of the Sun's power.

According to researchers, light emitted at the time of reactions at the Sun's center takes nearly tens of thousands of years to reach the surface, which then takes eight minutes to reach Earth. Traveling at almost the speed of light, as many as 420 billion neutrinos hit every square inch of the Earth's surface per second. But these particles pass through matter virtually unaffected, which makes them difficult to detect.

"By comparing the two different types of solar energy radiated, as neutrinos and as surface light, we obtain experimental information about the Sun's thermodynamic equilibrium over about a 100,000-year timescale," Andrea Pocar, who led an international team of scientists in the study, explained in a news release. "If the eyes are the mirror of the soul, with these neutrinos, we are looking not just at its face, but directly into its core. We have glimpsed the sun's soul."

Using the Borexino instrument - the only detector capable of observing a spectrum of solar neutrinos at the same time - UMass physicists located these hard-to-find particles.

According to scientists, when these neutrinos arrive at the Borexino instrument they rarely interact with the organic scintillators at its center, filled with a benzene-like liquid that is one of the oldest petroleum that exists. This type of liquid is essential because they want all the Carbon-14 to have decayed, since decay of carbon-14 covers the neutrino signals that they actually wanted to detect.

Though detecting pp neutrinos was not part of the original National Science Foundation-sponsored Borexino experiment, "it's a little bit of a coup that we could do it," Pocar said. "We pushed the detector sensitivity to a limit that has never been achieved before."

The scientists are planning to make further use of the detector to study the internal composition and structure of the Sun.

The new findings were published in the journal Nature.