10-Trillionths of Your Suntan Comes From Galaxies Beyond The Milky Way

12 AUGUST 2016

Astronomers have accurately measured the amount of light that makes it to Earth’s surface from outside of the Milky Way, and have found that we are bombarded with about 10 billion intergalactic photons per second every time we go outside - no matter if it's day or night.


While that might sound like a lot, most of the photons we absorb come from the Sun, with about 10-trillionths of your summer suntan being the result of radiation from elsewhere in the Universe.

"Most of the photons of light hitting us originate from the Sun, whether directly, scattered by the sky, or reflected off dust in the Solar System," said lead researcher Simon Driver, from the International Centre for Radio Astronomy Research (ICRAR).

"However, we're also bathed in radiation from beyond our galaxy, called the extragalactic background light. These photons are minted in the cores of stars in distant galaxies, and from matter as it spirals into supermassive black holes."

GalaxyTanBodyICRAR/Dan Hutton

The team - consisting of astronomers from the University of Western Australia, Arizona State University, and Cardiff University - examined data collected by NASA’s Galaxy Evolution Explorer and Wide-field Infrared Survey Explorer telescopes alongside the Spitzer and Hubble space telescopes, the ESA’s Herschel observatory, and Australia’s Galaxy and Mass Assembly survey.

Using data from so many high-profile telescopes and observatories meant that the team was able to accurately measure the amount of light – in the form of photons – hitting Earth at various wavelengths from microns to millimetres.


In the end, they found that, at any given moment, Earth is bombarded with about 10 billion photons of intergalactic light.

That sounds pretty intimidating, but the team says it would take you trillions of years of exposure for that amount of light to harm you, because dust clouds throughout the Universe provide protection from most of the harmful rays.

"The galaxies themselves provide us with a natural suntan lotion with an SPF of about two," said one of the team, Rogier Windhorst, from Arizona State University.

Though understanding how much outside light is hitting Earth is cool in itself, the study is only one part of the team’s overall mission: to understand how the Universe’s energy, mass, and structure have evolved.

"The processes which shape and shuffle mass generate vast quantities of energy, dwarfed only by the vastness of space," Driver said. "The precise physics as to how this energy is released is still not fully understood and work continues to build numerical models capable of explaining the energy that we've now measured."

Hopefully, as telescope technology continues to advance, the team will be able to use their newly collected data to answer some of the biggest mysteries of the Universe. Until then, remember your sunscreen.

The team’s work was published in The Astrophysical Journal.