Pluto probe finds mysterious night light in the universe

How dark is space? Astronomers attempting to answer the question found a surprising answer: not as dark as they thought it would be.

Researchers using imagery from NASA's distant New Horizons space probe have found a very faint glow of visible light suffusing the universe. 

Astronomer Tod Lauer, with the National Science Foundation's National Optical-Infrared Astronomy Research Laboratory (NOIRLab), examined images taken by the probe as it peered out beyond our galaxy. He and his colleagues were looking particularly for fields of view that didn't include stars and galaxies so they could measure how dark space was. 

"What we found when we were done is a little bit more light than we thought was there or should be there," he told Quirks & Quarks host Bob McDonald.

"There is a very faint glow, very faint, but it's more than we can explain from known sources," such as local stars, distant galaxies or interstellar dust, he said.

"So it's a little component of the universe that we've uncovered."

A glowing mystery

To try to understand the source of the faint glow, Lauer and his colleagues systematically eliminated all the sources of light that it could possibly be. The New Horizons images, so far, are the only ones that could be of use in this study, which has been accepted for publication in The Astrophysical Journal.

After New Horizons' Pluto fly-by in the summer of 2015, it continued out through the outer reaches of our solar system and is presently at the edge of the Kuiper belt and on the verge of leaving the solar system, making it only the fifth man-made craft to do so. 

In the inner solar system, space is filled with dust particles that reflect light from the sun, which produces a diffuse glow in images taken by ground-based or orbital telescopes.

But out beyond Pluto, where New Horizons is currently cruising, it is beyond the light "pollution" caused by sunlight bouncing off the dust and debris of our solar system. It's able to see the dark of space clearly in much the way we might see night stars more clearly when we leave a city and go out into the countryside. 

The team combed through the archive of images from the New Horizons mission and subtracted all of the known light sources, including stars in the Milky Way and distant galaxies. 

They also had to eliminate light that might be coming from the spacecraft itself, possible defects in the camera and possible errors in their calculations.

WATCH | New Horizons's journey animated. (NASA)

The mysterious diffuse glow remained. 

So far, the team doesn't have any good explanation for the source of this light, and they're left with speculation. One possibility is rogue stars in intergalactic space that are too dim to see as points of light, said Lauer.  

"Maybe there are stars that have been ripped off of the galaxies when they emerge and interact with each other so that it makes a diffuse background." 

"There are a couple of wilder suggestions," said Lauer. One had to do with interactions between still-hypothetical particles of exotic "dark matter" that's thought to be present throughout the universe. 

"Some people have thought there might be particles that interact and occasionally annihilate each other and that might produce light. That's more speculative."

Lauer has even revisited a previously discounted theory — one that astronomers do not want to be true. 

"New Horizons has an instrument to look for dust just by direct impact, and we don't see very much. So we think it's clean out there," he said. "But maybe we are mistaken and maybe there is a diffuse cloud of dust scattering sunlight even way out in the Kuiper belt."

How bright is this dim glow?

Lauer says this diffuse glow does represent a significant amount of light — roughly the same amount of light as we see from the galaxies in the universe. It's just very widely spread out across the vastness of space.

It's also not very interesting to look at in the images captured by New Horizons. 

"It looks like noise," Lauer said. "It's about as exciting as looking at a blank image on the TV. It's just static."

To give a sense of roughly how dim this glow is, Lauer gave an analogy. 

"Imagine that you're out in the country and it's a moonless night," he said. "You have a neighbour a mile down the road and the neighbour goes into his kitchen at 3 a.m. and opens the door of the refrigerator. Getting that much light from a mile away is about the equivalent to what we're seeing from the distant universe."

While astronomers don't have a satisfying explanation for this light, finding a new phenomenon like this — a new puzzle to solve — is certainly gratifying in and of itself

"If we didn't have our mysteries, we'd have to find something else to do. One of my pleasures of being an astronomer is you can ask questions like, 'How dark is space?'" he said.  

"There will always be mysteries, I hope."

Written and produced by Mark Crawley