I have always been fascinated by the Kuiper Belt and the worlds that lay beyond it. The Kuiper Belt, home to Pluto and Arrokoth, is located between 30 and 50 AU from the Sun, and is estimated to contain over 100,000 small icy bodies larger than 100 km in diameter and possibly billions of icy objects between 1 and 20 km in diameter, exceeding the mass of the Asteroid Belt by about 20 to 200 times. All these bodies are called Kuiper Belt Objects or KBOs.
First theorized in the 1930s following Pluto’s discovery, the Kuiper Belt was named after the renowned Dutch-American astronomer Gerard Kuiper, misleading many into believing that he was the first astronomer to predict it. In fact, it was the Americans Frederick Leonard and Fred Whipple as well as the Irish astronomer Kenneth Edgeworth who were the first to envision the existence of a disc of objects beyond Neptune. Ironically, the name Kuiper Belt has at its source a 1951 paper written by Kuiper himself, which proposed that no objects should lie within Pluto’s orbit or beyond (apart from the Oort Cloud).
What lies after the Kuiper Belt you might ask? Until recently, we had detected another group of objects made of rocks and ice that form a large irregular disk referred to as the Scattered Disk region. Contrary to KBOs which hold steady orbits around the Sun, SDOs are home to objects whose orbits are far more irregular leading planetary scientists to conclude that SDOs are the source of short-period comets (the Oort Cloud is where long-period comets reside). One fascinating aspect of SDOs is that they usually have highly elliptical orbits, which for some can extend up to 100 AU, making them the most distant objects observed in our Solar System even if their eccentric orbit can also pass near Neptune’s orbit at their closest approach.
The biggest - and most famous - SDO is Eris at 1,163 km in radius, slightly smaller than Pluto yet 27% more massive, implying a rockier composition - we can measure Eris’ mass thanks to its moon Dysnomia. Eris is the ninth most massive planetary object directly orbiting the Sun after the planet Mercury, while Pluto is the tenth (Eris’ discovery in 2005 is one of the events that led to Pluto’s demotion to a dwarf planet). Contrary to Pluto or Triton, Eris lacks the distinctive reddish hue derived from the presence of tholins (irradiated organic compounds), pointing to a very different history. Although, due to the vast distances separating us from SDOs, what this history could be is anyone’s guess. Because of this, the Kuiper Belt and regions beyond are still poorly characterized, with surprises most likely waiting to be unveiled.
One such surprise suggested a few years back, is the possibility that a population of objects with a more regular orbit lay after the Kuiper Belt between 60 to 100 AU. Contrary to the irregular and highly elliptical SDOs, these objects might form a duplicate of the Kuiper Belt, or Kuiper 2 as some have suggested (doubly ironic given what we know about Kuiper’s views on the existence of such belts).
Just recently, NASA’s New Horizon science team gave a presentation suggesting that this second belt might indeed exist according to recent studies and that the New Horizons spacecraft might be visiting it in the 2020s and 2030s. See the diagram below.
In this diagram, the New Horizon probe is represented by the black dot at the centre of the image and its trajectory the black line crossing it. On the left are the orbits of the known KBOs and on the right, the orbits of recently discovered objects that seem to form a regular belt or Kuiper 2. Some of these objects have been discovered thanks to Earth-based observatories which have been attempting to peer through this region to find potential flyby candidates for the New Horizons probe. (It is worth noting that unfortunately, none are in range of the spacecraft).
Given this latest insight, New Horizons co-investigator Wes Fraser from the Herzberg Astrophysics Institute in Canada proposed that the spacecraft continue its exploratory quest and study this potential new belt through extended missions, as opposed to NASA’s recently announced plans to repurpose New Horizons for heliophysics instead (which would bring better science return according to NASA).
How will this turn out for the New Horizon’s mission and its spacecraft is anyone’s guess, but one thing is for sure, there are still many strange worlds out there waiting to be discovered. As always, onwards and upwards.
(Image credits: NASA & Simon Porter/New Horizons Mission)