The highest energy cosmic rays striking our planet don’t come from the Milky Way – they actually come from other galaxies far away, according to a new study.

Published in Science, the study was led by the Pierre Auger Collaboration. They managed to pinpoint the location of cosmic rays that have energies a million times greater than protons accelerated in the Large Hadron Collider (LHC). This solves a 100-year-old mystery.

“The ultra-high energy cosmic rays are very rare,” Professor Karl-Heinz Kampert from the University of Wuppertal in Germany, a co-author on the study, told IFLScience. “Lower energy cosmic rays are for sure produced in our own galaxy. But there’s something different with high energies. No object in our galaxy can produce them.”

Cosmic rays are energetic particles, usually atomic nuclei, that traverse through the galaxy. The lowest energy rays, 100 times the energy of protons in the LHC, are produced by ordinary stars, while higher energies can be produced by supernovae or other supermassive black holes. However, we don’t yet know what causes the ultra-high energy rays.

They are known to cause some health problems for astronauts, such as their chance of developing cancer by breaking and mutating DNA as they ionize atoms in our bodies. Astronauts can even see cosmic rays when they sleep, as they fly through their eyes.

Fortunately, ultra-high energy cosmic rays are so rare that they don’t pose much of a threat. You’ll probably only interact with one about once in your lifetime.

No matter the energy, they are unable to make it to the surface of Earth. But when they hit our atmosphere, they produce a cascade of electrons that shower down on Earth. These we can detect, and pinpoint where the initial rays were coming from.

We haven’t been able to pinpoint the location of ultra-high energy cosmic rays before, though, because they are extremely rare. The rate they hit our planet is about one per square kilometer per year. That’s equivalent to an area the size of a football pitch being hit once per century.

The showers, however, are much larger, and they can spread out like a dinner plate across several kilometers. Using the Pierre Auger Observatory in Argentina, the largest cosmic-ray observatory ever built, this team were able to detect some of these showers and work out they were coming from a portion of the sky away from the center of our Milky Way. Thus, they must be produced in other galaxies.

The observatory found the particles by observing their Cherenkov light, the moment that the particles slowed down as they entered our atmosphere and “broke” the light-speed barrier. About 1,600 detectors spread over 3,000 square kilometers (1,160 square miles) were able to detect these particles.

“Most of the high-energy cosmic rays come from a direction where there is a large density of galaxies,” said Kampert.

The next step is to find out what’s causing them. At the moment, we don’t have a good theory for their source. The team are now planning to study these galaxies and try and pinpoint if there’s anything unusual about them.