Scientists Find Ghostly Neutrino Particles From the Milky Way

It’s no surprise that neutrinos come from within our galaxy, but the tiny, chargeless particles are very hard to detect

IceCube Lab with a starry night sky showing the Milky Way and faint green auroras
The IceCube Neutrino Observatory first detected evidence of neutrinos originating from outside the Milky Way a decade ago. Benjamin Eberhardt, IceCube / NSF

About a mile below the icy surface of the South Pole lies a detector that searches for tiny, evasive particles called neutrinos, which move across the universe but hardly interact with matter at all.

For the first time, this detector, called the IceCube Neutrino Observatory, has observed neutrinos that originated within our own galaxy, implying that sources within the Milky Way can generate these elusive particles. The findings, which could unlock a new way to observe our cosmic surroundings, were published Thursday in the journal Science.

“It was accepted that there would be neutrinos coming from our galaxy, but we didn’t have any evidence until now,” Ignacio Taboada, physicist at the Georgia Institute of Technology and IceCube’s spokesperson, tells NPR’s Nell Greenfieldboyce.

“This is a very important discovery,” Luigi Antonio Fusco, an astronomer at the University of Salerno in Italy who wrote a perspective accompanying the paper but was not involved in the research, tells Popular Science’s Rahul Rao.

Neutrinos are the most abundant particle in the universe that has mass—tens of trillions of neutrinos from the sun pass through your body every second, according to the Department of Energy. But neutrinos have earned a reputation for being “ghostly”—they have a tiny fraction of an electron’s mass, carry no charge and often pass through objects without interacting with them. Taken together, these traits make neutrinos notoriously hard to detect.

But the IceCube Neutrino Observatory is up to the task. The aptly named cube is about a kilometer long on each side and contains 5,160 sensors. It detects light signals created during the occasions when neutrinos do interact with the ice around the observatory, according to National Geographic’s Liz Kruesi.

IceCube first found evidence of neutrinos from other galaxies a decade ago, per NPR. But pinpointing neutrinos from our own galaxy was more challenging, because the detector had to look to the southern skies, toward the Milky Way’s disk. From its location deep in Antarctic ice, IceCube’s observations of the northern skies would be filtered through the mass of the Earth, which blocks out extraneous particles. But looking to the south, less of Earth is in the way, making it difficult to focus in on the right signal.

To tackle this problem, the team developed machine learning methods to identify specific, sphere-like bursts of light that come from neutrinos interacting with ice in what are known as “cascade” events. By scouring a decade of observational data for these incidents, the program identified neutrinos from the Milky Way.

“This is like a quantum leap to be able to say this has finally happened,” Kathrin Valerius, a physicist at the Karlsruhe Institute of Technology in Germany who did not contribute to the research, tells Scientific American’s Stephanie Pappas. “People a few years ago cannot have imagined it would be done.”

an artist's rendering of the milky way, with a blue glow depicting neutrinos
An artist's composition of the Milky Way, with neutrinos appearing in blue. IceCube Collaboration / U.S. National Science Foundation (Lily Le & Shawn Johnson) / ESO (S. Brunier)

The researchers don’t yet know exactly where the neutrinos are coming from, per National Geographic, though they want to learn that soon. “Now, the next step is to identify specific sources within the galaxy,” Taboada says in a statement from the observatory.

Neutrinos are produced by interactions between high-energy particles called cosmic rays and galactic gas and dust, according to the statement. The new findings could help researchers better understand the origin of these cosmic rays, Taboada tells NPR.

“Only cosmic rays make neutrinos, so if you see neutrinos, you see cosmic ray sources,” Francis Halzen, a member of the IceCube team and physicist at the University of Wisconsin, tells Popular Science. “The goal of neutrino physics, the prime goal, is to solve the 100-year-old cosmic ray problem.”

The researchers have used the data to make the first image of the Milky Way that doesn’t involve light, writes Science News’ James R. Riordon. And in the future, astronomers might be able to observe the universe using particles like neutrinos, revealing hidden phenomena that cannot be picked up by light-based telescopes, writes NPR.

“Now we see, for the first time, our galaxy in something other than light,” Naoko Kurahashi Neilson, a physicist at Drexel University who developed the machine learning method, tells Scientific American.

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