A new study has conclusively confirmed that the highest-energy cosmic rays that hit Earth from time to time do not come from within the Milky Way. While we still do not know exactly where in the universe these mysterious and powerful particles originate, now we at least know where not to look.

Calling them rays is a misnomer, based on decades-old misunderstanding of the phenomenon that identified it as electromagnetic radiation, but the name has stuck. Actually, cosmic rays are high-energy sub-atomic particles — nuclei of elements from hydrogen to iron, which means they are positively charged. If taken together, the number of cosmic rays that actually hit Earth is several hundred thousand every second (about 30 pass through everybody each second), but the number of high-energy cosmic rays that reach Earth is much smaller.

To illustrate that in terms that are relatively easy for laypersons to understand, Michigan Technological University (MTU) said in a statement Thursday: “It’s extremely rare for cosmic rays with energy greater than two joules to reach Earth; the rate of their arrival at the top of the atmosphere is only about one per square kilometer per year, the equivalent to one cosmic ray hitting an area the size of a soccer field about once per century.”

Figuring out where these particles — whose existence has been known for over 50 years now — originate is important for astrophysics, since it can tell us a lot about how matter forms, the nature of matter beyond the solar system, and offer clues about the origin of the universe itself. And the best way we currently have to identify their place of origin is to look at the direction they approach Earth from.

Cosmic Rays
Showers of high energy particles occur when energetic cosmic rays strike the top of the Earth's atmosphere. Cosmic rays were discovered unexpectedly in 1912. It is now known that most cosmic rays are atomic nuclei. Most are hydrogen nuclei, some are helium nuclei, and the rest are heavier elements. The relative abundance changes with cosmic ray energy — the highest energy cosmic rays tend to be heavier nuclei. Although many of the low energy cosmic rays come from the sun, the origins of the highest energy cosmic rays remains unknown and a topic of much research. This drawing illustrates air showers from very high energy cosmic rays. Simon Swordy (U. Chicago), NASA

That is easier said than done, though. The highest-energy cosmic rays are rare, and given their charged nature — which makes them interact with the magnetic fields of large objects in space they encounter, such as galaxies — they deviate somewhat from their original direction of travel. Scientists had previously speculated that the center of the Milky Way could be where these particles were originating, but the new study rules that out.

A group of over 400 scientists from 18 countries describe in the paper, published Friday in the journal Science, that they detected an asymmetry in the distribution of arrival directions of cosmic particles as they approached Earth. The scientists found the most prominent arrival direction to be about 120 degrees away from the direction that points toward the center of the Milky Way.

“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,” Gregory Snow, a University of Nebraska-Lincoln (UNL) physics professor, who was a co-author on the paper and is education and outreach coordinator for the Pierre Auger Observatory project, said in a statement Thursday.

The Pierre Auger Observatory in Argentina was built in 2001, with the specific purpose of studying cosmic rays. It is made up of 1,600 particle detectors spread across an 1,160-square-mile area in a hexagonal grid. The research team that published the Science paper had been collecting data for 12 years — from 2004 to 2016 — and that formed the basis for their results.

Pierre Augur Observatory
At the Pierre Auger Observatory in Argentina, cosmic rays have been detected from far off galaxies. Pierre Auger Observatory

Low-energy cosmic rays are produced much closer home, by the sun, but they are not of much interest to astrophysicists, as opposed to the particles emerging from outside our home galaxy which are perhaps the highest-energy particles in nature, several times more energetic than those that can be produced in labs and particle accelerators on Earth.

“The particles we detect 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,” Snow said about the abilities of the Pierre Auger Observatory.

“"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,” Karl-Heinz Kampert, a professor at the University of Wuppertal in Germany and spokesperson for the Auger collaboration, said in the MTU statement.

The paper titled “Observation of a large-scale anisotropy in the arrival directions of cosmic rays above 8 × 1018 eV” concludes: “The direction of the rays indicates that the particles originated in other galaxies and not from nearby sources within our own Milky Way Galaxy.”