The outer solar system is cold, distant, and (to our knowledge) lifeless—but that doesn’t mean it’s boring! Beyond Neptune is a complex landscape of small, icy bodies called the Kuiper belt. The main asteroid belt between Mars and Jupiter pales in comparison to the Kuiper belt, which is home to an estimated trillion comets and an estimated total mass of 6% the mass of the Earth (or more!), easily 100 times the mass of the asteroid belt. Most of those trillion bodies are quite small, but the Kuiper belt is also home to three of the five IAU-designated dwarf planets (Pluto, Makemake, and Haumea) and more additional dwarf planet candidates like Quaoar and Orcus.
And yet, for all this rich bounty of objects, the Kuiper belt was not observed until the early 1990s1! This is because of its distance from the sun and the size of its components. The bigger an object is, the more area it has to reflect light or emit its own, so smaller objects are much dimmer. On the other hand, the apparent brightness of an object decreases rapidly with distance due to the inverse-square law. And when we are seeing an object in reflected light, the brightness of the light it is reflecting also decreases with the inverse-square distance from the source of the light. So small objects distant both from us and the Sun are incredibly difficult to detect! Think about how bright a full moon is here on Earth. The Kuiper belt begins at the orbit of Neptune at 30 AU2 and extends out to 50 AU (or further!)—if Earth’s moon were a Kuiper belt object, the brightest it could possibly be, if everything lined up just right, is an apparent magnitude of 15. You would need about a 12-inch telescope to even be able to see it!
In fact, it took so long to observe Kuiper belt objects (KBOs), that the astronomer for whom the belt is named, Gerard Kuiper3, actually died before ever seeing the observational evidence of the belt whose existence he theorized. His 1951 paper, “On the Origin of the Solar System,” posited that a collection of bodies between 38 and 50 AU would be a natural outcome of the solar system’s formation and explain the existing observations of comets.4 Over forty years later, 15760 Albion would be discovered in 1992 and kick off the detection of more than three thousand such KBOs.
As distant as it is to us, the Kuiper belt is actually the inner outer solar system, if you will, the first of the populations of icy bodies exterior to the planets.5 Beyond it lie farther populations of icy bodies like the scattered disk, extended scattered disk, and the Oort Cloud, all of which are interesting in their own right and have dynamical and evolutionary ties with the Kuiper belt, and will be no further discussed here. All of these objects located beyond Neptune are considered trans-Neptunian objects (TNOs), of which KBOs are one subset.
Being so close to the planets, the structure and dynamics of the Kuiper belt is dominated by Neptune, whose orbit marks the interior edge of the belt. Pluto and Neptune exist in a special configuration called a mean-motion resonance, the upshot of which is that Pluto is prevented from being ejected by never getting too close to Neptune, despite the fact that the trajectories of their orbits cross. And this resonance, the 3:2 resonance, is popular! Pluto is the largest of a group of numerous bodies called plutinos, all of which exist in 3:2 resonance with Neptune. Resonances with Neptune also create groups in the belt called twotinos6 (2:1 resonance), Neptune trojans (1:1 resonance), and many other resonant groups with less fun names. The sheer abundance of KBOs in resonance with Neptune have helped us to understand the early evolution of the solar system, as Neptune’s slow outward migration scooped up these objects into their current resonances.
As important as Neptune and its resonances are to the Kuiper belt, resonant KBOs make up only about a third of known KBOs. The other type of objects in the belt, those that are not in resonance with Neptune, are known as classical Kuiper belt objects or cubewanos7. These objects tend to be a little bit farther out in the belt, between 40 and 50 AU, and have rather placid, circular, flat orbits (cold classical KBOs) or, if they had a past fling with Neptune, they can have more excited, eccentric, and inclined orbits (hot classical KBOs). Hot and cold refer not to the physical temperature of the bodies—they’re over 30 AU away from the Sun, they’re all freezing! A warm day on Pluto is 50 K (-369.7 °F)—but rather to their orbits. Just like the molecules of a hotter substance move faster and are more disordered, so too can we think of populations of objects like planets, stars, and little icy comets as being dynamically “hot” when they have higher relative velocities and more perturbed orbits.
If this feels like a lot of groups, you’re absolutely right! The Kuiper belt is a population of objects, and the classification of objects into groups isn’t just to satisfy our human urge to categorize. Taxonomy is an important part of many scientific fields. Breaking down an observed population into groups, even if the underlying causes for those categories are not known, helps us to learn more about how the population formed, what processes are actively shaping the population, and to develop a theoretical understanding to explain the observed categorizations and guide future observations for additional discoveries. The Kuiper belt still holds a lot of mysteries for us: How exactly did Neptune’s migration shape the current resonant population? How did the hot orbits get so hot? Where and how did the KBOs originally form? Why does the belt end where it ends and does it resume farther out? How well have we even characterized the population of the belt?
Our discovery of the Kuiper belt is only younger than I am! We have learned so much already about this reservoir of icy bodies, but suffice to say, there is still so much left waiting to be discovered and understood.
1. The exception being, of course, Pluto and Charon. Pluto was discovered in 1930 and its largest moon Charon in 1978. At the time of its discovery, Pluto was not known to be part of a larger body of objects and thus is not considered the discovery of the Kuiper belt. Like Ceres and the asteroid belt over a hundred years earlier, the context of the larger structure in which Pluto resides (as well as improved understanding of its size) led to its reclassification from being a planet. Given the surfeit of such objects in the Kuiper belt, a new category of solar system objects was created to better categorize them. This is not a “demotion” nor an insult to Pluto. I’m sorry that what you learned in elementary school has been updated. That’s science! I assure you, Pluto is a ball of rock and ice and has no feelings about the matter whatsoever. [back]
2. An AU is an “astronomical unit,” a distance unit based on the average distance between Earth and the Sun. Since 2012, it has been defined as exactly 149,597,870,700 meters. [back]
3. A professor at my PhD alma mater, University of Chicago, in case I needed another reason for imposter syndrome. [back]
4. This theoretical region was first referred to as the “Kuiper belt” in a 1988 paper. As we know from Stigler’s law of eponymy, naming scientific discoveries is often a haphazard business. The idea of trans-Neptunian comets is not unique to Kuiper, most notably from the work of Kenneth Edgeworth predating Kuiper’s 1951 paper, and thus the belt is sometimes called the Edgeworth-Kuiper belt (rarely, in my experience). [back]
5. This article makes no claim as to the existence, or not, of Planet 9. [back]
6. No relation to the pizza rolls. [back]
7. Yes, that’s a real term! It comes from the initial designation QB-1 of the KBO Albion, the first Kuiper belt object discovered after Pluto and Charon. [back]