An international team of scientists reported on Thursday that over a two-year period they had detected 28 of these particles, known as neutrinos, that arrived from outside the solar system and possibly from across the universe.
"This gives us a new way to do astronomy," said Francis Halzen, a physics professor at the University of Wisconsin who is the principal investigator for the project, the IceCube Neutrino Observatory. The findings appear in the journal Science.
In 1987, detectors in the United States, Japan and Russia noted two dozen neutrinos that originated from a supernova explosion of a star about 165,000 light-years away. That was the first and last time distant neutrinos had been detected until the IceCube observatory, largely financed by the National Science Foundation, started its observations in 2010.
More than 5,000 sensors have been lowered and frozen into a cubic kilometer (about one-quarter cubic mile) of Antarctic ice, looking for flashes of blue light that are given off by the cascades of debris generated by a neutrino.
Neutrinos are ghostlike particles that interact only very rarely with the rest of the universe. The fusion reactions that power the sun give off a flood of neutrinos, but almost all of them go undetected and unfelt: Every second, trillions of them pass through every person on earth.
IceCube, built over a decade at a cost of $271 million, ignores the solar neutrinos, but is instead searching for those coming from cataclysmic events elsewhere in the galaxy or even farther away. Until they looked, scientists did not know what they would find.
"I actually think the most surprising thing is that they show up when they were expected," Dr Halzen said.
The scientists named the newly discovered neutrinos after " Sesame Street" characters. The two most energetic of the 28 are Bert and Ernie; two others are called Miss Piggy and Snuffleupagus.
In the third year of data, not reported in the Science paper, an even more energetic neutrino appeared. That one is called Big Bird.
Dr Halzen said some of the 28 might have come from our own Milky Way galaxy, but others originated farther away. "Some of them are certainly extragalactic," he said. IceCube detects only one out of a million neutrinos, which means about 28 million neutrinos from outside the solar system passed through during that time.
Because neutrinos so rarely collide with anything, detecting them requires a great deal of material — in the case of IceCube, the vast expanse of ice readily available at the South Pole.
One in a great while, a neutrino does collide with something, setting off a cascade of electrons and other subatomic debris. Charged particles in a transparent material like ice give off blue light. Phototubes record the bursts of light, and from the patterns, scientists can determine the direction and energy of the incoming neutrinos.
Although IceCube is not tuned to find the low-energy neutrinos from supernovas in the galactic neighborhood, another explosion like the one observed in 1987 would be a bonanza for IceCube. It would register about 100,000 neutrinos, Dr. Halzen said.
Until now, most telescopes have looked at the universe by gathering photons, or particles of light, including lower-energy radio waves, visible light, X-rays and gamma rays.
The very-high-energy neutrinos detected by IceCube open a new spectrum for observing the universe.
With more observations, the scientists hope to be able to determine where the neutrinos originated, whether from black holes or the rapidly rotating burned-out stars known as pulsars.
John G Learned, a professor of physics at the University of Hawaii who is not part of the IceCube team, said its conclusion that the 28 neutrinos came from beyond the solar system was convincing.
But what was surprising is that IceCube did not detect neutrinos of even higher energies than Bert, Ernie and Big Bird, which was expected given current ideas of the cosmic processes thought to generate them. "They're not there," Dr Learned said. "We don't understand that."
But he added, "That's the kind of situation we love. A mystery may mean we may learn some fundamentally new science."
There is also a tantalizing hint that some of the neutrinos are coming from the center of our own galaxy. That too would be surprising, because anything there creating neutrinos should also be creating very-high-energy cosmic rays and gamma rays, which are not seen.
Dr Halzen said it was too early to tell for sure. "We now know what we're looking for, and we'll figure it out," he said.
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