Physicists announced in June 2001 that they had discovered the Sun’s missing neutrinos. These neutrinos are actually subatomic particles without charge and practically without mass. The discovery ended a thirty-year search for tiny packets of energy, but showed that these particles also have some mass.
Nuclear reactions at the heart of the Sun, the process by which the Sun shines, produce billions of neutrinos, half of which are estimated to travel 150 million kilometers to Earth. However, experiments in the late 1960s discovered a small number of these ghostly particles, which later came to be known as the solar neutrino problem.
Experiments by Canada’s Sudbury Neutrino Observatory in 2001 showed that the missing neutrinos actually transform into two types of neutrinos on their way to Earth, making them difficult to detect. This conversion requires that the neutrinos have a mass that is hundreds of thousands of times less than the mass of the second lightest particle, the electron.
2001: Discovery of the first exoplanet with an atmosphere
