Imagine walking into a room at night, turning out all the lights and closing the shades. Yet an eerie glow comes from the walls, ceiling, and floor. The faint light is barely enough to see your hands before your face, but it persists.
Sounds like a scene out of "Ghost Hunters?" No, for astronomers this is the real deal. But looking for something that's close to nothing is not easy. Astronomers searched through 200,000 archival images from Hubble Space Telescope and made tens of thousands of measurements on these images to look for any residual background glow in the sky. Like turning out the lights in a room, they subtracted the light from stars, galaxies, planets and the zodiacal light. Surprisingly, a ghostly, feeble glow was left over. It's equivalent to the steady light of ten fireflies spread across the entire sky.

Where's that coming from?
One possible explanation is that a shell of dust envelops our solar system all the way out to Pluto, and is reflecting sunlight. Seeing airborne dust caught in sunbeams is no surprise when cleaning the house. But this must have a more exotic origin. Because the glow is so smoothy distributed, the likely source is innumerable comets – free-flying dusty snowballs of ice. They fall in toward the Sun from all different directions, spewing out an exhaust of dust as the ices sublimate due to heat from the Sun. If real, this would be a newly discovered architectural element of the solar system. It has remained invisible until very imaginative and curious astronomers, and the power of Hubble, came along.

Aside from a tapestry of glittering stars, and the glow of the waxing and waning Moon, the nighttime sky looks inky black to the casual observer. But how dark is dark?

To find out, astronomers decided to sort through 200,000 images from NASA's Hubble Space Telescope and made tens of thousands of measurements on these images to look for any residual background glow in the sky, in an ambitious project called SKYSURF. This would be any leftover light after subtracting the glow from planets, stars, galaxies, and from dust in the plane of our solar system (called zodiacal light).

When researchers completed this inventory, they found an exceedingly tiny excess of light, equivalent to the steady glow of 10 fireflies spread across the entire sky. That's like turning out all the lights in a shuttered room and still finding an eerie glow coming from the walls, ceiling, and floor.

comet dust cloud.jpeg
Skysurf illustration

The researchers say that one possible explanation for this residual glow is that our inner solar system contains a tenuous sphere of dust from comets that are falling into the solar system from all directions, and that the glow is sunlight reflecting off this dust. If real, this dust shell could be a new addition to the known architecture of the solar system.
This idea is bolstered by the fact that in 2021 another team of astronomers used data from NASA's New Horizons spacecraft to also measure the sky background. New Horizons flew by Pluto in 2015, and a small Kuiper belt object in 2018, and is now heading into interstellar space. The New Horizons measurements were done at a distance of 4 billion to 5 billion miles from the Sun. This is well outside the realm of the planets and asteroids where there is no contamination from interplanetary dust.

New Horizons detected something a bit fainter that is apparently from a more distant source than Hubble detected. The source of the background light seen by New Horizons also remains unexplained. There are numerous theories ranging from the decay of dark matter to a huge unseen population of remote galaxies.

"If our analysis is correct there's another dust component between us and the distance where New Horizons made measurements. That means this is some kind of extra light coming from inside our solar system," said Tim Carleton, of Arizona State University (ASU).

"Because our measurement of residual light is higher than New Horizons we think it is a local phenomenon that is not from far outside the solar system. It may be a new element to the contents of the solar system that has been hypothesized but not quantitatively measured until now," said Carleton.

Hubble veteran astronomer Rogier Windhorst, also of ASU, first got the idea to assemble Hubble data to go looking for any "ghost light." "More than 95% of the photons in the images from Hubble's archive come from distances less than 3 billion miles from Earth. Since Hubble's very early days, most Hubble users have discarded these sky-photons, as they are interested in the faint discrete objects in Hubble's images such as stars and galaxies," said Windhorst. "But these sky-photons contain important information which can be extracted thanks to Hubble's unique ability to measure faint brightness levels to high precision over its three decades of lifetime."

A number of graduate and undergraduate students contributed to project SKYSURF, including Rosalia O'Brien, Delondrae Carter and Darby Kramer at ASU, Scott Tompkins at the University of Western Australia, Sarah Caddy at Macquarie University in Australia, and many others.

The team's research papers are published in The Astronomical Journal and The Astrophysical Journal Letters.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

See: https://hubblesite.org/contents/news-releases/2022/news-2022-050.html

The original paper may be seen:

SKYSURF-3: Testing Crowded Object Catalogs in the Hubble eXtreme Deep Field Mosaics to Study Sample Incompleteness from an Extragalactic Background Light Perspective, by
Darby M. Kramer1, Timothy Carleton1, Seth. H. Cohen1, Rolf Jansen1, Rogier A. Windhorst1, Norman Grogin2, Anton Koekemoer2, John W. MacKenty2, and Nor Pirzkal2

Published 2022 November 18 • © 2022. The Author(s). Published by the American Astronomical Society.
The Astrophysical Journal Letters, Volume 940, Number 1

Extragalactic background light (EBL) studies have revealed a significant discrepancy between direct measurements—via instruments measuring "bare" sky from which Zodiacal and Galactic light models are subtracted—and measurements of the integrated galaxy light (IGL). This discrepancy could lie in either method, whether it be an incomplete Zodiacal model or missed faint galaxies in the IGL calculations. It has been proposed that the discrepancy is due to deep galaxy surveys, such as those with the Hubble Space Telescope, missing up to half of the faint galaxies with 24 ≲ mAB ≲ 29 mag. We address this possibility by simulating higher number densities of galaxies, and so assess incompleteness due to object overlap, with three replications of the Hubble UltraDeep Field (HUDF). SourceExtractor is used to compare the recovered counts and photometry to the original HUDF, allowing us to assess how many galaxies may have been missed due to confusion, i.e., due to blending with neighboring faint galaxies. This exercise reveals that, while up to 50% of faint galaxies with 28 ≲ mAB ≲ 29 mag were missed or blended with neighboring objects in certain filters, not enough were missed to account for the EBL discrepancy alone in any of the replications.

See: https://iopscience.iop.org/article/10.3847/2041-8213/ac9cca

Imagine looking through 200,000 Hubble ST images to find a sufficient number correlating with the belief that there is a soft glow, some very minute glow equivalent to the steady glow of 10 fireflies spread across the entire sky, possibly made by the solar reflections on minute dust particles deposited by comets within the orbit of Pluto. Wow, that's a small amount of visible photon flux. The fact that this minute flux, known as "ghost light, can be seen at all is a tribute to the, now, ancient sensitivity of the once mighty light gathering ability of the Hubble Space Telescope. It is interesting that the faint light found may have resulted in missing faint galaxies as a result of this reflective particle fog. SourceExtractor* was used to compare the recovered counts and photometry to the original Hubble Ultra Deep Field (HUDF).

* Source Extractor (Bertin and Arnouts 1996; Bertin 2016) is a widely used command-line program** for segmentation and analysis of astronomical images. It reads in FITS format files, performs a configurable series of tasks, including background estimation, source detection, deblending and a wide array of source measurements, and finally outputs a FITS format catalog file.

While Source Extractor is highly useful, the fact that it can only be used as an executable -- reading input files, producing output files and controlled by a limited set of configuration options specified in another file -- can limit its applicability or lead to awkward workflows. There is often a desire to have programmatic access to perform one or more of the above tasks on in-memory images as part of a larger custom analysis.

SEP makes available the core algorithms of Source Extractor in a library of stand-alone functions and classes. These operate directly on in-memory arrays (no FITS files or configuration files). The code is derived from the Source Extractor code base (written in C) and aims to produce results compatible with Source Extractor whenever possible. SEP consists of a C library with no dependencies outside the standard library, and a Python module that wraps the C library in a Pythonic API. The Python wrapper operates on NumPy arrays with NumPy as its only dependency. It is generated using Cython.

From Source Extractor, SEP includes background estimation, image segmentation (including on-the-fly filtering and source deblending), aperture photometry in circular and elliptical apertures, and source measurements such as Kron radius, "windowed" position fitting, and half-light radius. Additionally, several features not in Source Extractor have been added:

Optimized matched filter for variable noise in source extraction.
Circular annulus and elliptical annulus aperture photometry functions.
Local background subtraction in shape consistent with aperture in aperture photometry functions.
Exact pixel overlap mode in all aperture photometry functions.
Masking of elliptical regions on images.

Finally, note that SEP is essentially a fork of Source Extractor that has already diverged significantly from the original code base. One might ask why SEP is not part of Source Extractor itself: the command-line interface in Source Extractor could be built on top of such a library. The answer is that a vast array of changes were necessary in order to expose the functionality as stand-alone C functions. It would be a lot of work to rewrite Source Extractor itself in this way, with little gain for the executable itself.

See: https://bids.berkeley.edu/publications/sep-source-extractor-library

** Command Line is a text interface for your computer. It’s a program that takes in commands, which it passes on to the computer’s operating system to run.

From the command line, you can navigate through files and folders on your computer, just as you would with Windows Explorer on Windows or Finder on Mac OS. The difference is that the command line is fully text-based.

See: https://www.codecademy.com/article/command-line-commands
Were they able to guesstimate the spectrum? If it's dust, there will be a slight shift. If the dust in the solar system, it will be moving relative to the sun. Same thing for external dust.

If not, it's background light from the cosmos.

If the dust was detected mathematically......or filtered mathematically, ignore the study. Current math always fails us, when we try to use it as an authority.

We still can not stir plasma in a pot, neither can we simulate 3 bodies in orbit with current math.

Numbers are self related. Defining everything with math, automatically relates everything with math. Man only has related everything to math, and then, wants to worship math for it's power.

Our ignorance has never been higher.