Neptune (MidJourney)
A Trans-Neptunian Deep-Dive
- academia, science
We should all be familiar with the general arrangement of our Solar System, but let’s have a quick rundown just to put things in perspective: Our G-type main-sequence yellow dwarf star, the Sun, sits at more-or-less the center, which we know thanks in no small part to Copernicus. Then we’ve got the terrestrial planets: Mercury, Venus, Earth and its Moon, and Mars and its two moons. Then there’s the asteroid belt, with the dwarf planet Ceres hiding in there somewhere. Then we’ve got the Big Dogs of the Solar System—the gas giants—Jupiter and Saturn, and their plethora of moons; and finally, we’ve got the ice giants, Uranus and Neptune, and their respective moons. But beyond Neptune is another world altogether: a vast and frigid no-man’s-land of amorphous carbon and rock and volatile ices.
The world of trans-Neptunian objects.
The very first and certainly most famous object beyond the orbit of Neptune was discovered in February of 1930, when American astronomer Clyde William Tombaugh discovered Pluto, which, as we’ll never forget, was considered to be a full-fledged planet for over 75 years, but would face controversial reclassification in 2006 as a dwarf planet at the hands of the International Astronomical Union (Vidmachenko, 2015). But while Pluto gets all the press, since its discovery, we have actually found several more objects with similar sizes to Pluto out beyond the orbit of Neptune, one of which is even more massive than Pluto.
We classify any minor or dwarf planet we find within the Solar System that orbits the Sun at a greater average distance than Neptune as a Trans-Neptunian Object—or “TNO” if you’d prefer. Now, Neptune has a semi-major axis of 30.1 astronomical units. That’s just over 15,000 light-seconds, which means it takes light from the Sun about four hours and ten minutes just to get to Neptune, so these TNOs are the really far away, really cold leftovers from the formation of our Solar System. TNOs can essentially be divided into several subgroups based on where their orbit resides, but to simplify things, the actual space beyond Neptune gets divided into three separate volumes: the Kuiper belt, the scattered disk, and the Oort cloud.
The Kuiper Belt
The Kuiper belt was named after Dutch astronomer Gerard Kuiper, who is considered to be the father of modern planetary science. The Kuiper Belt Proper extends from the orbit of Neptune out to about 50 astronomical units, and it is a major source of short-period comets. It does have a lot in common with the asteroid belt that divides the terrestrial planets from the Jovian giants, but the Kuiper belt is much wider, way more massive, and far icier. There are estimated to be over 100,000 objects with diameters greater than 50 kilometers in the Kuiper belt, but collectively their mass makes up less than a tenth of the mass of Earth (Bhattacharya & Lichtman, 2017).
The objects found within the Kuiper Belt Proper get divided into two major subcategories: Classical KBOs, and resonant KBOs.
Classical Kuiper Belt Objects (KBOs)
Classical Kuiper Belt Objects, or KBOs, are objects that orbit the Sun beyond Neptune and aren’t in mean-motion orbital resonance with the planet. They also have a low eccentricity and semi-major axes that extend from about 40 AU all the way out to about 50 AU. More than 60% of classical KBOs have inclinations of less than five degrees, and they can be further sub-classified as either “cold” or “hot” depending on the perturbation of their orbits. “Cold” KBOs have relatively undisturbed orbits, while “hot” KBOs have noticeably perturbed orbits.
Some of the most notable classical Kuiper belt objects are: Albion, which was the first TNO to be found since Pluto and Charon; the strange-looking dual-lobed contact binary Arrokoth, which was visited by the New Horizons space probe in 2019; and the dwarf planet Makemake, which is the largest known classical Kuiper belt object, as well as the brightest Kuiper belt object right after Pluto (Brown, 2006).
Resonant Kuiper Belt Objects
Resonant Kuiper Belt Objects are objects that orbit the Sun beyond Neptune and are in mean-motion orbital resonance with the planet. This just means that Neptune and these resonant KBOs exert a gravitational influence on each other with some periodicity. The two bodies revolve around the Sun in periods that are an integer ratio of each other.
For example: a one-to-one resonance means that for every orbit the object completes around the sun, Neptune orbits once, as well. In one-to-one resonance with Neptune, we will find the Neptune trojans, which orbit the Sun close to one of Neptune’s stable Lagrangian points. Over twenty such objects have been found here (Deep Ecliptic Survey Object Classifications, 2021).
Plutinos
Plutinos are the resonant KBOs that have a two-to-three resonance with Neptune, which means that for every two orbits the object completes, Neptune completes three. Plutinos make up about 25% of the known KBOs.
Pluto
Pluto is a resonant KBO with a two-to-three resonance with Neptune, the largest member of the plutinos, and the object after which the group was named. Pluto’s orbit is fairly eccentric, and its distance from the Sun ranges from 29.7 AU, which is inside Neptune’s orbit, and 49.5 AU, near the edge of the Kuiper Belt Proper. Pluto has five known moons, and of them Charon is the largest. It’s so large that it actually creates a binary planet system with Pluto, which means that Charon and Pluto orbit around a barycenter, giving the appearance that they are orbiting each other. These binary systems are not exactly uncommon in the Kuiper belt, either: about 11% of KBOs are estimated to be part of a binary pair (Agnor & Hamilton, 2006).
The Scattered Disk
The next volume of trans-Neptunian space is called the scattered disk. While the Kuiper belt is essentially a thick doughnut-shaped torus of objects in relatively stable orbits around the Solar System, the scattered disk is representative of objects whose orbits have been, well, scattered. The scattered disk is seen by some as simply the outer region of the Kuiper belt, rather than an entirely separate and distinct population of objects (Jewitt, 2008). While the scattered disk does indeed overlap with the Kuiper Belt Proper a bit, with some objects approaching the Sun at a range of 30 to 35 AU, the objects found here have orbits that can extend out to 100 AU and beyond. The scattered disk also extends well above and below the ecliptic by up to 40 degrees (Bertoldi et al., 2006).
Scattered Disk Objects (SDOs)
Scattered disk objects, or SDOs, typically have medium- to high-eccentricity orbits with semi-major axes of 50 AU or greater. SDOs have orbits that are in constant danger of perturbation by Neptune, and sometimes they can get trapped in a temporary resonance. Since these objects are relatively unstable, this is another major source of periodic comets.
Eris
Discovered by a team of astronomers in 2003, perhaps the most significant SDO to date is the dwarf planet Eris, which is second in size only to Pluto, but it is actually more massive, making it the ninth most massive object currently orbiting the Sun (Brown & Schaller, 2007). The discovery of Eris is pretty much what led the IAU to clarify what it means to be a planet, and the eventual reclassification of Pluto as a dwarf planet (Beasley, 2005).
Resonant Scattered Disk Objects
Like objects in the Kuiper Belt Proper, SDOs can find themselves in resonance with the orbit of Neptune. Discovered by American astronomers in 2007, the dwarf planet Gonggong, named after a water god from Chinese mythology, has an eccentric and inclined orbit ranging from 34 to 101 AU, and is in three-to-ten orbital resonance with Neptune; and in terms of distance from the Sun, it is the sixth-farthest object we know of in the Solar System (International Astronomical Union, 2007).
Extreme Trans-Neptunian Objects
Extreme trans-Neptunian objects, or ETNOs, are objects in the scattered disk that have semi-major axes of at least 150 AU. These objects found their extremely eccentric orbits as a result of being scattered by the giant planets, and these are some of the most distant objects from the Sun that are still considered to be part of the Solar System.
We can further divide ETNOs into sub-categories based on how close they get to the Sun.
Detached Objects
Detached ETNOs have orbits that take them as close to the Sun as 40 astronomical units, and they appear to be relatively unaffected by the planets, which makes them look like they’re “detached” from the Solar System save for their heliocentric orbits (Blondel & Mason, 2006). Only nine of these objects have been found, the most extreme of which make up their own ETNO sub-category.
Sednoids
Sednoids have orbits that don’t even take them as close to the Sun as the Kuiper cliff, which is at about 47.8 astronomical units. Sednoids are not influenced in any appreciable way by the giant planets, and as of the present, there have been only three objects that have been found to fit into this classification, none of which get any closer to the Sun than 64 AU; though there are estimated to be many more of these objects with extreme orbits lurking out there in the frigid darkness.
Sedna
Discovered by a team of astronomers in 2003, Sedna was, at the time, the farthest object from the Sun to ever be discovered. With an average distance of about 520 astronomical units from the Sun, the farthest point of Sedna’s orbit is estimated to be around a whopping 937 astronomical units. To put that into perspective, heliopause, or the boundary of the Sun’s influence, is somewhere around 121 astronomical units, which means that for a good portion of its orbit, Sedna is hanging out in interstellar space (Vidmachenko, 2015). Since the discovery of Sedna, one even more extreme object has been found: 2015’s Leleākūhonua… or 2015 TG387., which has an extremely high eccentricity of .94, and only gets as close to the Sun as 65 AU, and as far away as 2,000 AU—a distance considered by some to be the Inner Oort Cloud.
The Oort Cloud
First theorized by Dutch astronomer Jan Oort in 1950, the Oort cloud is the theoretical sphere-shaped swarm of up to a trillion cometary bodies lying at the absolute extreme edges of the influence of the Sun in interstellar space, at a range from about 2,000 astronomical units all the way out to a little over three light-years (Morbidelli, 2005)! This is believed to be the source for all long-period comets, since these Oort cloud objects can be disturbed by triggers such as passerby stars, galactic tides, and molecular clouds. These perturbations occur as the Solar System travels through its respective arm of the Milky Way (Fernández, 2000).
We have yet to directly observe the Oort cloud, but Voyager 1 is on pace to get there in about… 300 years or so. So… [ Crosses fingers. ]
Conclusion
In 2007, a team of astronomers observed nearly 200 stars that are similar to our Sun, and they found that upwards of 20% of these stars had Kuiper belt-like debris disks around them (Trilling et al., 2008). Inevitably, mankind is going to spread out into the unknown, touching the farthest reaches of our Solar System and beyond, to other stars with their own unique systems of planets and moons and belts of asteroids and cometary debris. When we take that next step forward, I can only hope that it is as a species, free from political conflicts and arbitrary borders both physical and psychological.
Because space is already a cold, bleak, and dangerous place; and the only warmth we’re likely to find out there is with each other.
Agnor, C. B., & Hamilton, D. P. (2006). Neptune’s capture of its moon Triton in a binary–planet gravitational encounter. Nature, 441(7090), 192–194. https://doi.org/10.1038/nature04792
Beasley, D. (2005, July 29). Scientists Discover Tenth Planet. NASA. https://go.nasa.gov/38RvKEx
Bertoldi, F., Altenhoff, W., Weiss, A., Menten, K., & Thum, C. (2006). The trans-neptunian object UB313 is larger than Pluto. Nature, 439(7076), 563–564. https://doi.org/10.1038/nature04494
Bhattacharya, A. B., & Lichtman, J. M. (2017). Solar Planetary Systems: Stardust to Terrestrial and Extraterrestrial Planetary Sciences. CRC Press.
Blondel, P., & Mason, J. (2006). Solar System Update. Springer Publishing.
Brown, M. E., Barkume, K. M., Blake, G. A., Schaller, E. L., Rabinowitz, D. L., Roe, H. G., & Trujillo, C. A. (2006). Methane and Ethane on the Bright Kuiper Belt Object 2005 FY9. The Astronomical Journal, 133(1), 284–289. https://doi.org/10.1086/509734
Brown, M. E., & Schaller, E. L. (2007). The Mass of Dwarf Planet Eris. Science, 316(5831), 1585. https://doi.org/10.1126/science.1139415
Deep Ecliptic Survey Object Classifications. (2021, May 25). Southwest Research Institute. https://bit.ly/36lQdk2
Fernández, J. A. (2000). Long-Period Comets and the Oort Cloud. Earth, Moon, and Planets, 89(1/4), 325–343. https://doi.org/10.1023/a:1021571108658
International Astronomical Union. (2003, October 21). (136199) Eris = 2003 UB313. Minor Planet Center. https://bit.ly/3ElJWBo
International Astronomical Union. (2007, July 17). (225088) Gonggong = 2007 OR10. Minor Planet Center. https://bit.ly/3jNgfju
Jewitt, D. (2008, June). The 1000 km Scale KBOs. UCLA. https://bit.ly/37jV31W
Morbidelli, A. (2005, December 9). Origin and Dynamical Evolution of Comets and their Reservoirs. ArXiv.Org. https://arxiv.org/abs/astro-ph/0512256v1
Trilling, D. E., Bryden, G., Beichman, C. A., Rieke, G. H., Su, K. Y. L., Stansberry, J. A., Blaylock, M., Stapelfeldt, K. R., Beeman, J. W., & Haller, E. E. (2008). Debris Disks around Sun‐like Stars. The Astrophysical Journal, 674(2), 1086–1105. https://doi.org/10.1086/525514
Vidmachenko, A. P. (2015). Dwarf planets (to the 10th anniversary of the introduction of the new class of planets). Astronomical Almanac, 62, 228-249.