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The Sun is a giant nuclear reactor that continuously sends us light and heat. But it also has another, almost silent postal service: every second, tens of billions of neutrinos, ghostly particles born in the thermonuclear furnace, fly through a square centimeter of your palm. Most of them leave no trace. The slowest, thermal neutrinos are like a whisper in a roar: their energy is so low that no instrument has been able to hear them until now.

A group of physicists decided to tune an existing "microphone"—the KATRIN setup in Germany, originally built to weigh neutrinos. Its heart is gaseous tritium (superheavy hydrogen), whose nuclei can capture neutrinos like a net. Analyzing the accumulated data, the scientists for the first time set an upper limit on the flux of thermal solar neutrinos: it does not exceed 1.86×10¹⁸ of the predictions of the standard solar model. This is like trying to hear a mosquito at a rock concert, and saying that it is definitely not louder than a certain threshold.

But the real surprise was yet to come. It turned out that even with a modest data set (100 kg·yr of exposure), KATRIN will not only refine the limit for the thermal component but also, for the first time, catch neutrinos from the proton-proton cycle — the main energy source of the Sun. There is a catch, though: the elastic scattering of the same neutrinos on electrons creates an unavoidable background that masks the signal. This is like a translator trying to make out a quiet speech while the audience keeps rustling, and that rustling, unfortunately, cannot be turned off.

So the quiet whisper of solar neutrinos is gradually becoming more audible. And when detector sensitivity increases, we will be able not just to register these particles but to read the history of stars from them — from their first minutes of life to their deep old age.