In July of 2015, NASA’s New Horizons probe made historical past when it turned the primary mission ever to conduct an in depth flyby of Pluto.

This was adopted by the spacecraft making the first-ever encounter with a Kuiper Belt Object (KBO) – often called Arrokoth (AKA 2014 MU69) – on 31 December 2018. As well as, its distinctive place within the outer Photo voltaic System has allowed astronomers to conduct uncommon and profitable science operations.


This has included parallax measurements of Proxima Centauri and Wolf 359, the 2 closest stars to the Photo voltaic System.

As well as, a group of astronomers led by the Nationwide Optical Astronomy Observatory (NOAO) and Southwest Analysis Institute (SwRI) used archival knowledge from the probe’s Lengthy Vary Reconnaissance Imager (LORRI) to conduct measurements of the Cosmic Optical Background (COB).

The examine, which was lately accepted for publication by The Astrophysical Journal, was led by Tod R. Lauer of the NOAO. He was joined by Alan Stern (the PI of the New Horizons mission) and researchers from the SwRI, NASA, the Johns Hopkins College Utilized Physics Laboratory (JHUAPL), the House Telescope Science Institute (STSI), the Lunar and Planetary Institute (LPI), the SETI Institute, and a number of universities and establishments. 

Put merely, the COB is the sunshine from all sources exterior the Milky Manner that’s unfold diffusely all through the observable Universe.

On this sense, it’s the seen mild analogue of the Cosmic Microwave Background (CMB), and is a vital benchmark for astronomers. By measuring this mild, they can discern the places of stars, the dimensions and density of galaxies, and take a look at theories in regards to the construction and formation of the cosmos.


Precisely measuring the COB is essential for a number of causes. For starters, this background is integral to the historical past of star formation, star clusters, galaxies, black holes, galaxy clusters, and the massive scale construction of the Universe.

Subsequently, figuring out precisely how darkish the night time sky is can present perception into the formation and evolution of the Universe.

As well as, astronomers have sought to find out if there was a diffuse element to the COB (dCOB), a supply of photons not related to any presently identified objects.

The presence of such a element would enable astronomers to check how a lot of the cosmic background mild might be coming from objects within the low-density areas of the Universe, or objects that fashioned previous to the Universe organizing into its present patterns.

A dCOB may additionally mirror the manufacturing of photons by extra unique processes, such because the annihilation or decay of darkish matter particles – due to this fact aiding within the ongoing seek for this “invisible” mass.

Sadly, these kinds of research current quite a few challenges since Earth-based telescopes are topic to atmospheric distortion and space-based telescopes must take care of interference from Zodiacal Mild.

The trajectory of the New Horizons probe, which has taken it previous Pluto and into the Kuiper Belt. (NASA/JHUAPL)

In consequence, there have been critical discrepancies within the inferred brightness of the optical background over time.

However for spacecraft within the outer Photo voltaic System, these kinds of interference usually are not an issue. Therefore why astronomers have relied on all earlier missions that ventured past Neptune to conduct COB measurements – i.e. the Pioneer 10/11 and Voyager 1/2 missions.


Equally, the Hubble House Telescope additionally performed measurements of the COB, however these have been restricted in comparison with what New Horizons was capable of witness.

As Lauer, who’s a former member of the Hubble Vast Subject and Planetary Digicam group, advised Universe At the moment by way of e mail:

“NH can cleanly measure the overall mild flux emitted by the distant Universe. The Hubble is great at including up all of the distant galaxies, however does much less effectively for stuff not in galaxies that makes a diffuse background, which will get twisted up with the scattered daylight bounced round by mud within the close to Earth atmosphere.”

Curiously, this isn’t the primary time that astronomers have used LORRI knowledge to measure the COB.

In 2017, a NASA-led group examined LORRI knowledge from 4 completely different remoted sky fields that have been imaged between 2007 and 2010. This coincided with the NH’s cruise section the place it handed between the orbits of Jupiter and Uranus.

optical cosmic background lorri fieldsThe placement of the seven LORRI fields used on this work. (Lauer et al., arXiv, 2020)

For the sake of this examine, Lauer and his group examined brightness ranges noticed by LORRI of seven excessive galactic latitude fields when the New Horizons mission was 42 to 45 AU from the Solar.

At this distance, the common uncooked mild ranges have been 10 instances darker than what Hubble was capable of observe. After correcting for any remaining interference, the group ran a Monte Carlo simulation to mannequin potential sources of sunshine.


From this, they have been capable of discern the presence of a diffuse element of unknown origin, probably attributable to the presence of faint galaxies that stay undetected.

As Lauer and his colleagues concluded, this could counsel that the present census on faint galaxies falls brief and no less than half of these with an obvious magnitude degree of 30 or extra are unaccounted for.

This isn’t the primary occasion in recent times when the galactic census has needed to be revised. Till just a few years in the past, astronomers have been of the final consensus that there have been 200 billion galaxies within the observable Universe.

This was primarily based on the Hubble Extremely Deep Subject commentary marketing campaign, from which astronomers created detailed 3D maps of the Universe.

However on revised calculations in 2016, astronomers now estimate that there are as many as two trillion galaxies within the observable Universe. Based mostly on these newest outcomes, it seems as if the rely may should be up to date once more.

Regardless, the work of Lauer and his colleagues demonstrates the utility of missions like New Horizons and the sorts of analysis they’ll carry out within the outer Photo voltaic System.

This text was initially printed by Universe At the moment. Learn the authentic article.