Nasa says galactic cosmic rays flowing into our solar system originate from clusters such as this one NASA/U. Virginia/INAF, Bologna, Italy/USRA/Ames/STScI/AURA

A 50-year scientific debate about the origin of cosmic rays, dangerous high-energy particles that zoom through space and hit our planet’s atmosphere, may have finally been answered.

In a new research paper, published in Science, a group of scientists from 18 different countries has presented the results of a 12-year study, and conclude that the cosmic rays with the highest energies come from beyond our own galaxy.


Understanding cosmic rays and where they originate can help us answer fundamental questions about the origins of the universe, our galaxy and ourselves.

But cosmic rays are not ‘rays’ at all. They are named in this way because it was first thought they were electromagnetic radiation, but in fact they are high-energy particles – the nuclei of elements from hydrogen up to iron - that bombard our Earth from somewhere beyond our solar system.

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Exactly where they come from has been a source of mystery for a long time.

The new results looked into the distribution of these particles, mapping the direction from which they arrive at Earth, using the biggest cosmic ray observatory ever built – the Pierre Auger Observatory in Argentina.


By studying more than 30,000 particles, the group discovered anisotropy; meaning the rate of cosmic ray arrivals is different, depending on which direction you look.

"We are now considerably closer to solving the mystery of where and how these extraordinary particles are created, a question of great interest to astrophysicists," says Karl-Heinz Kampert, a professor at the University of Wuppertal in Germany and spokesperson for the Auger Collaboration.

"There have been other pieces of evidence, but I would say this paper really confirms that most of the highest energy cosmic ray particles are not coming from the Milky Way galaxy," said Gregory Snow, professor of physics at the University of Nebraska-Lincoln.

Cosmic rays can help to give us clues to the structure of the universe. "By understanding the origins of these particles, we hope to understand more about the origin of the Universe, the Big Bang, how galaxies and black holes formed and things like that," he said. "These are some of the most important questions in astrophysics."


The high-energy cosmic rays detected during this experiment may have started out life in the explosive centres of distant galaxies.

"The sun emits low-energy cosmic ray particles that are detected here on Earth, but they are nowhere near as high energy as the particles detected at the Auger Observatory," Snow said.

"The particles we detected are so energetic they have to come from astrophysical phenomena that are extremely violent. Some galaxies have an explosive, massive black hole in their centers and there are theories that these very violent centers accelerate particles of very high energy that eventually reach Earth."