In July 2015, NASA's New Horizons spacecraft made history when it made its first mission to fly close to Pluto. This was followed by the spacecraft's first encounter with a Kuiper Belt Object (KBO) known as Arrokoth (aka. 2014 MU69) on December 31, 2018. Additionally, its unique position in the outer solar system has allowed astronomers to conduct rare and lucrative scientific operations.
This included parallax measurements from Proxima Centauri and Wolf 359, the two stars closest to the solar system. In addition, a team of astronomers led by the National Optical Astronomy Observatory (NOAO) and the Southwest Research Institute (SwRI) used archival data from the probe's Long Range Reconnaissance Imager (LORRI) to make cosmic optical background (COB) measurements.
The study, which was recently accepted for publication by The Astrophysical Journal, was led by NOAO's Tod R. Lauer. He was joined by Alan Stern (PI of the New Horizons Mission) and researchers from SwRI, NASA, the Applied Physics Laboratory (JHUAPL) at Johns Hopkins University, the Space Telescope Science Institute (STSI), the Lunar and Planetary Institute (LPI) )), the SETI Institute and several universities and institutions.
There could be quadrillion nomad planets in our galaxy alone – and they could even be ejected into intergalactic space. Photo credit: ESO / S.Brunier
Simply put, the COB is the light from all sources outside the Milky Way that is diffusely distributed in the observable universe. In this sense, it is the visible light analog of the Cosmic Microwave Background (CMB) and an important benchmark for astronomers. By measuring this light, they can identify the positions of stars, the size and density of galaxies, and test theories about the structure and formation of the cosmos.
Accurate measurement of the COB is important for several reasons. For starters, this background is an integral part of the history of star formation, star clusters, galaxies, black holes, galaxy clusters, and the large-scale structure of the universe. Knowing exactly how dark the night sky is will give you insight into how the universe was formed and evolved.
In addition, astronomers have attempted to determine whether the COB (dCOB), a source of photons not associated with currently known objects, contains a diffuse component. The presence of such a component would allow astronomers to test how much of the cosmic background light could have come from objects in the lower density regions of the universe or from objects that formed before the universe was organized in its current patterns.
A dCOB could also reflect the production of photons through more exotic processes such as the destruction or disintegration of particles of dark matter and thus support the ongoing search for this “invisible” mass. Unfortunately, these types of studies present numerous challenges as terrestrial telescopes are subject to atmospheric distortion and space-based telescopes must deal with interference from zodiacal light.
As a result, there have been significant deviations in the derived brightness of the optical background over time. However, these types of interference are not a problem for spacecraft in the outer solar system. Hence, astronomers have relied on all of the previous missions that ventured beyond Neptune to take COB measurements – i.e. H. The Pioneer 10/11 and Voyager 1/2 missions.
Similarly, the Hubble Space Telescope also took measurements of the COB, but these were limited when compared to what New Horizons could observe. As Lauer, a former member of the Hubble Wide Field and Planetary Camera team, emailed Universe Today:
“NH can accurately measure the entire flux of light emitted from the distant universe. The Hubble is great at adding up all of the distant galaxies, but is less suitable for things that are not in galaxies and creating a diffuse background that tangles with the scattered sunlight reflected from dust in the near-earth environment. "
Interestingly, this is not the first time astronomers have used LORRI data to measure COB. In 2017, a NASA-led team examined LORRI data from four different isolated sky fields imaged between 2007 and 2010. This coincided with the NH's cruise phase, when it passed between the orbits of Jupiter and Uranus.
The position of the seven LORRI fields used in this work. Photo credit: Lauer, Tod, R. (et al.)
For this study, Lauer and his team examined the brightness levels of seven fields of high galactic latitude observed by LORRI when the New Horizons mission was 42 to 45 AU from the Sun. At this distance, the average raw light levels were ten times darker than what Hubble could observe. After correcting any remaining interference, the team ran a Monte Carlo simulation to model the potential light source.
From this they were able to detect the presence of a diffuse component of unknown origin, possibly caused by the presence of faint galaxies that go undetected. As Lauer and his colleagues concluded, this would suggest that the current census of faint galaxies is inadequate, ignoring at least half of those with an apparent magnitude of 30 or more.
This is not the first time in recent years that the galactic census has had to be revised. Up until a few years ago, astronomers agreed that there are 200 billion galaxies in the observable universe. This was based on the Hubble Ultra Deep Field observation campaign, which astronomers used to create detailed 3D maps of the universe.
After revised calculations in 2016, astronomers now estimate that there are up to two trillion galaxies in the observable universe. Based on these latest results, it appears that the number may need to be updated again. Regardless, the work of Lauer and his colleagues shows how useful missions like New Horizons are and what types of research they can do in the outer solar system.
Further reading: arXiv