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The Kuiper Belt and Trans-Neptunian Objects

Author: Bella Ma

Editors: Junyu Zheng, Sumire Sumi

Artist: Sally Sun

The Kuiper Belt, named after astronomer Gerard Kuiper, who first speculated the existence of such a region along with Kenneth Edgeworth (Edgeworth, 1943) (Kuiper, 1951), is one of the largest regions within the solar system and sometimes referred to as the Trans-Neptunian region. Millions of small, icy objects named Kuiper Belt Objects (KBOs), or trans-Neptunian objects (TNOs), compose this region, occupying an enormous volume extending from the edge of Neptune, 30 AU from the sun to about 1000 AU away from the sun (Admin, 2023).

The study of the Kuiper belt helps investigate the formation and evolution of other celestial bodies (Morbidelli & Nesvorný, 2019) and, ultimately, the solar system (as demonstrated by the title of Kuiper’s original 1951 paper). TNOs also contain various volatile compounds, including water and organic molecules. Thus, studying TNOs may provide insights into the origin of life (European Research Council, 2022). 

The Kuiper Belt is a population of objects located beyond the Neptunian orbit (Morbidelli & Levison, 2014), with its existence predicted by astronomers Edgeworth and Kuiper in 1943 and 1951 respectively, sometimes also referred to as the Edgeworth-Kuiper Belt. Its inner edge begins at the Neptunian orbit, approximately 30 AU from the sun. The majority of TNOs that constitute the Kuiper Belt lie within 50 AU from the sun (the main region of the Kuiper Belt), but the “scattered disk” region stretches to 1000 AU away, with some TNOs on orbits even beyond (Kuiper Belt: Facts - NASA Science, n.d.).

The Kuiper Belt is a taurus-shaped region of the outer solar system, composed of millions of Kuiper Belt Objects or Trans-Neptunian Objects (TNOs) unevenly distributed within space (Barucci et al., 2008). TNOs are diverse in size, shape, and color, and taxonomy is typically based on the shapes and sizes of their orbits (Kuiper Belt: Facts - NASA Science, n.d.). Conventional taxonomy includes the following: (New Horizons: About the Kuiper Belt, n.d.) (Kuiper Belt: Facts - NASA Science, n.d.-b)

Classical TNOs: a large fraction of TNOs belong to this group, which orbits the sun in the main portion of the Kuiper Belt. The term “classical” refers to the fact that these TNOs occupy orbits that most befit the orbits initially predicted by the astronomers (Classical Kuiper Belt Objects, n.d.) (relatively circular orbits generally not tilted too much from the plane of the planets). They have a similar average distance from the sun of 40-50 AU (Kuiper Belt: Facts - NASA Science, n.d.-c). Classical TNOs are further classified into “cold” and “hot” based on the degree of influence exerted on the orbits by Neptune. Cold and hot TNOs mainly differ in the inclination and shape of their orbits (Delsanti & Jewitt, 2006) but also display variant physical characteristics (Doressoundiram et al., 2002).

  1. “Cold classical” TNOs: the cold population of classical TNOs has inclinations below 5˚ and nearly circular orbits (Delsanti & Jewitt, 2006). They are usually gravitationally unperturbed by Neptune, are smaller in size (not surpassing 500 miles across), and are constituted of the original materials of the Kuiper Belt (New Horizons: About the Kuiper Belt, n.d.-b). Physically, the cold population exhibits colors commonly described as red (Doressoundiram et al., 2002).

  2. “Hot classical” TNOs: orbits are commonly non-circular, with inclinations typically in the 30-40˚ range, some possibly being higher (Delsanti & Jewitt, 2006). Their colors are more heterogeneous (Doressoundiram et al., 2002) and include larger objects than those of the cold population.

Resonant TNOs: resonant TNOs orbit in resonance with Neptune (as the name suggests). Resonant TNOs are further grouped based on their resonances. Most common are the following: (Kuiper Belt: Facts - NASA Science, n.d.-d)

  1. 1:1 resonance: orbits the sun once every Neptunian orbit.

  2. 4:3 resonance: orbits the sun three times every 4 Neptunian orbits.

  3. 3:2 resonance: orbits the sun twice every three Neptunian orbits; includes Pluto, and are collectively referred to as Plutinos (New Horizons: About the Kuiper Belt, n.d.-c).

  4. 2:1 resonance: orbits the sun once every 2 Neptunian orbits.

Scattered TNOs: scattered objects usually have perihelia in the 35-40 AU range with large eccentricities and inclinations. They were possibly knocked out of stable orbits by Neptunian gravitation, making them follow extremely unstable orbits, with farthest points hundreds of AUs away and closer than Neptune at the nearest (Kuiper Belt: Facts - NASA Science, n.d.-e).

Detached Objects: sometimes also referred to as Extreme TNOs, this is a newly discovered category with very few known members, the most notable being Sedna, at least half the size of Pluto, and occupying a 12000-year orbit spanning 76-1200 AU (New Horizons: About the Kuiper Belt, n.d.-d). Theorized causes for the unusually far-away orbits of extreme TNOs include gravitational influence from an undiscovered giant planet, gravity of passing stars, and gravitational perturbations during the era of Kuiper Belt formation (Kuiper Belt: Facts - NASA Science, n.d.-f).

Centaurs: objects that occupy the space between the orbits of Jupiter and Neptune. They often interact with the gravitation of the giant planets and are either ejected into the outer solar system or pushed into the inner solar system. Theories concerning the origin of centaurs suggest that they are recent “escapees” from the Kuiper Belt, like the extreme TNOs (New Horizons: About the Kuiper Belt, n.d.-e). 

Notable TNOs and their unique features:

Pluto: Pluto is a 3:2 resonant TNO, and the first Kuiper Belt object discovered in 1930. It was thought to be the largest TNO, with a diameter of 2377 kilometers, and the second largest in mass after Eris (New Horizons: About the Kuiper Belt, n.d.-f). Pluto is a quadruple system, with a principal moon, Charon (also one of the largest TNOs), and two faint satellites, discovered in May 2005 through Hubble Space Telescope images (Weaver et al., 2006).

Arrokoth (2014 MU69): First nicknamed Ultima Thule (Talbert, 2023), it is the farthest object visited by a spacecraft (the 2019 New Horizons probe) (New Horizons: New Horizons Successfully Explores Ultima Thule, 2019). It has an average diameter of 18.26 km (composed of two planetesimals) (Keane et al., 2022). As a “cold classical” TNO, it has an aphelion of 46 AU and perihelion of 43 AU.

Eris (2003 UB313): a scattered disk object with an orbital inclination of 44˚, an aphelion approximately 97 AU from the sun, and a perihelion at 38 AU (Delsanti & Jewitt, 2006). According to estimations based on thermal emission, UB313 has a diameter of about 3000 km, making it the largest TNO (Bertoldi, n.d.).

2005 FY9: a classical TNO, with perihelion 39 AU and inclination 29˚, it’s the third brightest known TNO with a size comparable to Pluto. Spectroscopic studies by Licandro et al. (2006) indicate the existence of methane ice.

In 1943, Kenneth Edgeworth published his “The Evolution of Our Planetary System,” which, along with Kuiper’s 1951 “On the origin of the Solar System,” predicted the existence of celestial bodies outside of the orbit of Pluto (Edgeworth, 1943) (Kuiper, 1951), nowadays known as Trans-Neptunian objects.

In 1992, David Jewitt and Jane Luu discovered the first TNO, 1992QB1. In 2002, the first hundred-kilometer diameter TNO, Quaoar, was observed with the Oschin telescope at the Palomar Observatory. Two years later, Sedna, one of the largest extreme TNOs, was observed with the same telescope. A year later, the largest TNO Eris was observed (Kuiper Belt: In Depth - NASA Science, n.d.).

The most popular theory regarding the formation of the Kuiper Belt states that it’s formed from planetesimals, fragments from original protoplanetary discs that failed to coalesce into planets (Ian, 2019). The gravitational pull of Uranus and Neptune bent the orbits of icy bodies inward toward the other giant planets. Most of these were pulled inward and then “sling-shotted” by Jupiter into extremely distant orbitals, forming the Oort Cloud (Kuiper Belt: Facts - NASA Science, n.d.).

The majority of information concerning the Kuiper Belt was generated from ground-based telescope observations (such as Oschin) and the Hubble Space Telescope. Only one spacecraft, the previously mentioned New Horizons probe, visited the Kuiper Belt. Ongoing research includes the Horizon Europe Presence and Role of Organic Matter in Icy Satellites and ExtraSolar planets (PROMISE) program and the continuing New Horizons project.

So far, studying the Kuiper Belt and TNOs has led to multiple discoveries in astronomy and astrophysics. Several dwarf planets, including Pluto, Eris, and Makemake, were discovered (Gladman, 2005). Identification of volatile materials and organic molecules on icy bodies provides intriguing evidence for theories concerning the origin of life; the study of TNO orbits provides key insights into gravitational interactions within the solar system, made more valuable by the resonances and irregularities exhibited by some TNOs; Moreover, investigating the formation of the Kuiper Belt provide a glimpse into the conditions during its formation, enhancing our understanding of Solar System history (Blum & Wurm, 2008).



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