In an ambitious project called SKYSURF, astronomers examined 200,000 images archived with the Hubble Space Telescope and performed tens of thousands of measurements on these images to look for any residual background glow in the sky. They eliminated the light from stars, galaxies, planets, and the light of the zodiac, much like turning off the lights in a room. Surprisingly, there was still a faint, faint glow.

What causes this ghostly glow in our solar system?

One hypothesis is that sunlight is reflected from the dust layer around our solar system to Pluto. Due to the even distribution of the glow, numerous comets – free-flying, dusty snowballs of ice – are the most plausible source. As they descend toward the sun from all angles, the sun’s heat causes the ice to sublime, releasing a cloud of dust.

This would represent a brand new piece of solar system architecture if confirmed. It remained invisible until highly imaginative and curious astronomers and the power of Hubble came along.

This theory is supported by the fact that another group of astronomers measured the sky background in 2021 using information from NASA’s New Horizons probe. The New Horizons spacecraft recently entered interstellar space after passing Pluto in 2015 and a small object in the Kuiper Belt in 2018. 4 to 5 billion kilometers separated New Horizons from the sun when the observations were made. The planets and asteroids, where there is no pollution from interplanetary dust, are far from this.

Tim Carleton of Arizona State University (ASU) said: “New Horizons has detected something fainter that apparently comes from a more distant source than Hubble detected. The source of the backlight seen by New Horizons also remains unexplained. Numerous theories range from the decay of dark matter to a massive unseen population of remote galaxies.”

“If our analysis is correct, there is another dust component between us and the distance New Horizons measured. That means this kind of extra light comes from inside our solar system.”

“Because our measurement of residual light is higher than that of New Horizons, we think it’s a local phenomenon, not far outside the solar system. It could be a new element in the content of the solar system that has been hypothesized but has not yet been measured quantitatively.”

Hubble veteran astronomer Rogier Windhorst said: “More than 95% of the photons in the Hubble Archive images come from distances less than 3 billion miles from Earth. Since the very early days of Hubble, most users have discarded these aerial photons, interested in the faint discrete objects in Hubble’s images, such as stars and galaxies. But these aerial photons contain important information that can be extracted thanks to Hubble’s unique ability to measure faint brightness levels with high precision over the three decades of its life.”

Magazine references:

  1. Timothy Carleton, Rogier Windhorst, et al. SKYSURF: Limitations on Zodiacal Light and Extragalactic Backlight by Panchromatic HST All-sky Surface Brightness Measurements: II. First limits for diffused light at 1.25, 1.4 and 1.6 μm. The astronomical magazine. DOI: 10.3847/1538-3881/ac8d02
  2. Rogier Windhorst, Timothy Carleton, et al. SKYSURF: Constraints on Zodiacal Light and Extragalactic Backlight Through Panchromatic HST All-sky Surface Brightness Measurements. I. Research overview and methods. The astronomical magazine. DOI: 10.3847/1538-3881/ac82af
  3. Darby M. Kramer, Timothy Carleton, et al. SKYSURF-3: Testing crowded object catalogs in the Hubble eXtreme Deep Field mosaics to study sample incompleteness from an extragalactic backlight perspective. The Astrophysical Journal Letters. DOI: 10.3847/2041-8213/ac9cca