Neutrinos, often called “ghost particles,” are nearly massless subatomic particles that interact so weakly with matter that they can pass through entire planets without being stopped. These elusive particles are produced in abundance by the nuclear fusion reactions powering the sun. Every second, trillions of solar neutrinos pass through every square centimeter of Earth’s surface, yet they rarely interact with atoms, making them incredibly difficult to detect. However, advances in neutrino observatories have allowed scientists to capture and study these particles, providing a potential window into the sun’s inner workings.
The sun, a vast sphere of hydrogen and helium, has been burning for approximately 4.6 billion years. Its immense energy output comes from nuclear fusion, primarily the proton-proton chain reaction, which converts hydrogen into helium. This process releases an enormous amount of energy in the form of electromagnetic radiation, heat, and particles, including neutrinos. Unlike photons, which take hundreds of thousands of years to escape from the sun’s dense core due to constant scattering, neutrinos travel virtually unimpeded, escaping the sun within seconds. This makes them ideal messengers for probing the sun’s interior in real-time.
Scientists rely on the Standard Solar Model (SSM) to predict the sun’s internal structure and its evolution over time. The model describes how nuclear fusion reactions occur within the sun’s core, producing different types of neutrinos. Each fusion process generates neutrinos with unique energy signatures, which can be detected and analyzed to verify theoretical predictions about the sun’s composition and behavior. By studying these neutrinos, scientists can refine their understanding of solar physics and test fundamental principles of particle physics.
The challenge of detecting neutrinos stems from their weak interaction with matter. To overcome this, researchers have built large underground neutrino detectors, often located in deep mines or underwater to shield them from background radiation. These observatories use massive tanks of water, heavy water, or liquid scintillator to capture rare neutrino interactions. When a neutrino collides with an atomic nucleus or electron inside these detectors, it produces a faint flash of light, which can be recorded and analyzed.
One of the most significant breakthroughs in neutrino research came from the Sudbury Neutrino Observatory (SNO) in Canada. In the early 2000s, SNO provided definitive evidence that neutrinos oscillate between different types—or “flavors”—as they travel through space. This discovery resolved a long-standing mystery known as the solar neutrino problem, in which previous experiments had detected fewer neutrinos than predicted by the SSM. The realization that neutrinos change their type (from electron neutrinos to muon or tau neutrinos) confirmed that they have a tiny but nonzero mass, a finding that challenged the Standard Model of particle physics.
The ability of neutrinos to oscillate is described by the Mikheyev-Smirnov-Wolfenstein (MSW) effect, which explains how neutrino flavor conversion is influenced by the density of the medium through which they travel. By measuring how neutrinos change as they pass through the sun, scientists can infer details about the sun’s internal density structure. This provides a direct test of the SSM and helps refine models of stellar evolution.
A recent study by Peter B. Denton (Brookhaven National Laboratory) and Charles Gourley (Rensselaer Polytechnic Institute) explores how solar neutrinos can be used to map the sun’s internal density profile. Unlike photons, which only reveal information about the sun’s surface due to their long escape time, neutrinos provide immediate insight into the core’s current state. By analyzing the detected flux of neutrinos on Earth and comparing it to theoretical predictions, researchers can deduce density variations deep within the sun.
The study highlights both the potential and the limitations of using neutrinos to probe the sun’s interior. While current neutrino observations align well with SSM predictions, there are constraints on how much detail can be extracted from existing data. To refine these measurements, scientists are looking to new experiments, such as the Jiangmen Underground Neutrino Observatory (JUNO) in China and the Deep Underground Neutrino Experiment (DUNE) in the U.S. These projects will provide more precise measurements of neutrino oscillation parameters, improving our ability to reconstruct the solar density profile.
Chinese researchers are also developing TRIDENT, a next-generation neutrino observatory designed to enhance solar neutrino studies. TRIDENT aims to achieve greater sensitivity by leveraging advanced detection techniques. To refine its design, scientists have constructed an underwater simulator to model the detector’s response to different neutrino interactions.
Beyond studying the sun, neutrino research has broader implications for astrophysics and cosmology. Neutrinos play a key role in understanding supernovae, neutron stars, and the early universe. Their weak interactions make them valuable probes of extreme environments that are otherwise inaccessible. For example, when a massive star explodes as a supernova, it releases an enormous burst of neutrinos, carrying crucial information about the explosion’s dynamics. The detection of neutrinos from Supernova 1987A provided direct observational evidence of core-collapse physics, supporting theoretical models of supernova explosions.
Furthermore, neutrino studies contribute to our understanding of dark matter, one of the greatest mysteries in modern physics. Some theories suggest that sterile neutrinos—hypothetical neutrinos that do not interact via the weak nuclear force—could account for a portion of dark matter. Experiments like IceCube in Antarctica and KamLAND in Japan are searching for signs of these elusive particles.
In addition to astrophysical applications, solar neutrino research has practical benefits for fundamental physics. By studying neutrino interactions, scientists can test the limits of the Standard Model, search for new physics beyond the known particles, and refine our understanding of quantum mechanics and particle interactions.
The future of neutrino research looks promising, with next-generation detectors poised to revolutionize our knowledge of both the sun and the wider universe. The synergy between solar neutrino studies and artificial neutrino experiments from nuclear reactors will provide deeper insights into neutrino properties and fundamental forces.
Ultimately, solar neutrinos are more than just byproducts of nuclear fusion; they are cosmic messengers carrying real-time information about the heart of our closest star. With ongoing advancements in detection technology, we are inching closer to a detailed, direct understanding of the sun’s inner structure—something that was once thought impossible. As new observatories come online, the potential for discovery grows, promising to shed light on the fundamental workings of both our sun and the universe itself.
More information: Peter B. Denton et al, Determining the Density of the Sun with Neutrinos, arXiv (2025). DOI: 10.48550/arxiv.2502.17546