Imagine walking into a room at night, turning off all the lights, and drawing the shades. However, the walls, ceiling and floor glowed eerily. The low light is barely enough to see your hands before your face, but it’s there nonetheless. Sounds like a scene from Ghost Hunters? No, for astronomers, this is the real deal. But it’s not easy to find something that’s barely there.
One possible explanation is that a crust of dust surrounds our solar system all the way to Pluto and reflects sunlight. It’s not surprising to see the dust in the air hit by the sun when you clean your house. But it must have had more exotic origins. Because the glow is so evenly distributed, the likely source is a myriad of comets—free-flying dusty snowballs of ice. They fall toward the sun from all directions, spewing clouds of dust as the sun’s heat sublimates the ice. If true, it would be a newly discovered architectural element of the solar system. It remained invisible until very imaginative and curious astronomers and the power of Hubble came along.
Save for the shimmering tapestry of stars and the glow of the waxing and waning moon, the night sky appears pitch black to the casual observer. But just how dark is the darkness?
To find out, astronomers decided to sort through 200,000 images from NASA’s Hubble Space Telescope and take tens of thousands of measurements on them in an ambitious project called SKYSURF to find the sky any residual background light in the This would be any remaining light after subtracting the glow from the planets, stars, galaxies, and dust in the plane of our solar system (known as the zodiacal light).
When the researchers went through the list, they found minuscule amounts of excess light, equivalent to the steady glow of 10 fireflies across the sky. It’s like turning off all the lights in a room with blinds and still finding the walls, ceiling and floor glowing eerily.
One possible explanation for this residual glow, the researchers say, is that our inner solar system contains a fragile sphere of cometary dust falling into the solar system from all directions, and that this glow is sunlight bouncing off of that dust. If true, this dust shell could be a new addition to the known structure of the solar system.
This idea gained support because in 2021, another team of astronomers also measured the sky background using data from NASA’s New Horizons spacecraft. New Horizons flew by Pluto in 2015 and a small Kuiper Belt object in 2018, and is now heading into interstellar space. New Horizons’ measurements were made at a distance of 4 billion to 5 billion miles from the sun. This is well beyond the range of planets and asteroids that are not polluted by interplanetary dust.
New Horizons found something fainter than what Hubble detected, which clearly came from a more distant source. The source of the background light seen by New Horizons also remains unexplained. There are many theories, from the decay of dark matter to the abundance of unseen distant galaxies.
Tim Carleton of Arizona State University (ASU) said: “If our analysis is correct, there is another dust component between us and the distance New Horizons is measuring. This means that It’s some kind of extra light from the inner solar system.”
“Because our measurements of peripheral light are higher than New Horizons’, we think it is a local phenomenon, not far from the solar system. It may be a new element of the solar system’s content, which has been hypothesized but not quantitatively measured until now,” Carlton said .
Hubble senior astronomer Rogier Windhorst, also of Arizona State University, first came up with the idea of collecting Hubble data to look for any “ghost lights.” “More than 95 percent of the photons in the Hubble archive come from a distance of less than 3 billion miles from Earth. Since the early days of Hubble, most Hubble users have discarded these sky photons because they are interested in faint discrete objects. in vibrant images, such as stars and galaxies,” Windhorst said. “But these sky photons contain important information, which can be extracted thanks to Hubble’s unique ability to measure faint brightness levels with high precision during its three-decade lifetime.”
Many graduate and undergraduate students have contributed to the SKYSURF project, including Rosalia O’Brien, Delondrae Carter and Darby Kramer from ASU, Scott Tompkins from the University of Western Australia, Sarah Caddy from Macquarie University in Australia, and others.
