Trans-Neptunian Dwarf Planets
Trans-Neptunian Dwarf Planets
- Bryan J. HollerBryan J. HollerSpace Telescope Science Institute
The International Astronomical Union (IAU) officially recognizes five objects as dwarf planets: Ceres in the main asteroid belt between Mars and Jupiter, and Pluto, Eris, Haumea, and Makemake in the trans-Neptunian region beyond the orbit of Neptune. However, the definition used by the IAU applies to many other trans-Neptunian objects (TNOs) and can be summarized as follows: Any non-satellite large enough to be rounded by its own gravity. Practically speaking, this means any non-satellite with a diameter larger than 400 km. In the trans-Neptunian region, there are more than 150 objects that satisfy this definition, based on published results and diameter estimates.
The dynamical structure of the trans-Neptunian region records the history of the migration of the giant planets in the early days of the solar system. The semi-major axes, eccentricities, and orbital inclinations of TNOs across various dynamical classes provide constraints on different aspects of planetary migration. For many TNOs, the orbital parameters are all that is known about them, due to their large distances, small sizes, and low albedos. The TNO dwarf planets are a different story. These objects are large enough to be studied in more detail from ground- and space-based observatories. Imaging observations can be used to detect satellites and measure surface colors, while spectroscopy can be used to constrain surface composition. In this way, TNO dwarf planets not only help provide context for the dynamical evolution of the outer solar system, but also reveal the composition of the primordial solar nebula as well as the physical and chemical processes at work at very cold temperatures.
The largest TNO dwarf planets, those officially recognized by the IAU, plus others like Sedna, Quaoar, and Gonggong, are large enough to support volatile ices on their surfaces in the present day. These ices are able to exist as solids and gases on some TNOs, due to their sizes and surface temperatures (similar to water on Earth) and include N2 (nitrogen), CH4 (methane), and CO (carbon monoxide). A global atmosphere composed of these three species has been detected around Pluto, the largest TNO dwarf planet, with the possibility of local atmospheres or global atmospheres at perihelion for Eris and Makemake. The presence of non-volatile species, such as H2O (water), NH3 (ammonia), and complex hydrocarbons, provides valuable information on objects that may be too small to retain volatile ices over the age of the solar system. In particular, large quantities of H2O mixed with NH3 point to ancient cryovolcanism caused by internal differentiation of ice from rock. Complex hydrocarbons, formed through radiation processing of surface ices, such as CH4, record the radiation histories of these objects and provide clues to their primordial surface compositions.
The dynamical, physical, and chemical diversity of the more than 150 TNO dwarf planets are key to understanding the formation of the solar system and its subsequent evolution to its current state. Most of our knowledge comes from a small handful of objects, but we are continually expanding our horizons as additional objects are studied in more detail.
- Solar System Dynamics and Orbital Structure
- Small Bodies
- Planet Formation
A Short History of the Trans-Neptunian Region
Early Discoveries and Theories
The story of the study of the trans-Neptunian region begins with the discovery of Uranus in 1781 by the British-German astronomer William Herschel. Uranus was the first planet to be “discovered,” the other five planets (not including Earth) having been known, and their positions studied, for thousands of years. By the early 1800s it had become clear that another body was perturbing the orbit of Uranus, given that tabulated positions developed larger discrepancies over time when compared to observations. In the early 1840s, the British astronomer and mathematician John Couch Adams and the French astronomer and mathematician Urbain Le Verrier independently calculated the position of the unseen planet. In 1846, the German astronomer Johann Gottfried Galle, at the behest of Le Verrier, was the first to identify the planet that would come to be known as Neptune.
The discovery of Triton, Neptune’s largest satellite, by William Lassell less than 3 weeks after the discovery of Neptune enabled a calculation of Neptune’s mass and therefore its gravitational effects on Uranus. However, within a century, additional discrepancies had been identified in the tabulated position of Uranus that were not attributable to Neptune. At the turn of the 20th century, the American businessman and astronomer Percival Lowell invoked a “Planet X,” orbiting beyond Neptune, to account for these additional, smaller perturbations. He established the Lowell Observatory in Flagstaff, Arizona, to search for Planet X, but died in 1916 without making a positive identification. In 1929, Vesto Slipher, the director of Lowell Observatory, hired Clyde Tombaugh to continue the search. Tombaugh identified Pluto in 1930, and Pluto was assumed to be the sought-after Planet X, with a mass large enough to account for the perturbations in Uranus’ orbit. The discovery of Charon, Pluto’s largest moon, in 1978 by James Christy and Robert Harrington disproved Pluto as Planet X. Studies of Charon’s orbit led to a more accurate calculation of Pluto’s mass, making it much too small to be responsible for Uranus’ orbital discrepancies (Christy & Harrington, 1978). The need for a Planet X to explain Uranus’ orbit was finally put to rest with the Voyager 2 flyby of the Neptune system in 1989 and the recalculation of Neptune’s mass (Standish, 1993).
Not all speculation about the outer solar system focused on an unidentified trans-Neptunian planet, however. After the discovery of Pluto, others theorized that Pluto may not be alone beyond Neptune. Publications by Kenneth Edgeworth and Gerard P. Kuiper (Edgeworth, 1943; Kuiper, 1951) provided firmer scientific bases for a belt of icy planetesimals beyond 30 astronomical units (au). Edgeworth correctly surmised that the density of the solar nebula was too low beyond the orbit of Neptune for planet-sized bodies to coalesce. Kuiper’s work supported this theory, but he took it a step further by saying that such a disk of objects was likely to have formed, but that Pluto would have scattered these planetesimals onto larger or even unbound orbits. This was due to the belief at the time, prior to the discovery of Charon and the determination of Pluto’s mass, that Pluto was comparable in mass to the Earth, a requirement for Pluto to be Uranus’ mysterious perturber. Pluto is, in fact, only ~ 0.2% of Earth’s mass (Stern et al., 2015), meaning it could not have cleared out the trans-Neptunian region. Following the discovery of Charon and the more accurate calculation of Pluto’s mass, Fernandez (1980) performed a dynamical study that indicated that scattering within the trans-Neptunian region could be the ultimate origin of the short-period comets. Even with mounting theoretical evidence pointing to the existence of bodies orbiting beyond Neptune, it was not until 1992 that the first Edgeworth-Kuiper belt object, typically shortened to Kuiper belt object (KBO), was identified.
Dynamical Structure of the Trans-Neptunian Region
The discovery of 1992 QB1, now known as Albion, by David Jewitt and Jane Luu opened the door to the study of the solar system’s “third zone” (Jewitt & Luu, 1993). In the nearly 30 years following the discovery of Albion, ~ 3,500 additional trans-Neptunian objects (TNOs) have been discovered (see Figure 1). These planetesimals represent the most dynamically, compositionally, and physically diverse collection of minor bodies in the solar system. These aspects provide valuable clues to the formation and early evolution of the solar system. In particular, the orbits of TNOs reveal the migration of the giant planets early in solar system history. Orbital properties also represent the most straightforward aspect of TNOs to study. Only a handful of images spread over a few months to years are required to obtain precise orbital parameters, which in turn are used to classify TNOs into different dynamical categories (see Bannister et al., 2018; Elliot et al., 2005; Gladman et al., 2008; Morbidelli & Nesvorný, 2020; Petit et al., 2011). Objects in the trans-Neptunian region are divided into four broad categories, each with a unique dynamical history: resonant TNOs, classical TNOs, scattered disk objects (SDOs), and detached TNOs.
The following orbital parameters are used in the definitions of TNO dynamical classes: semi-major axis, eccentricity, and inclination. The semi-major axis is the average distance of a body in its orbit around the Sun. It is the average of the perihelion (closest point to the Sun) and aphelion (furthest point from the Sun) of the body’s orbit. The eccentricity represents the deviation of a body’s orbit from circular, ranging from 0 to 1; a larger eccentricity means the orbit is more elliptical. TNOs on more elliptical orbits experience larger seasonal temperature differences over the course of their year. The inclination measures the tilt of the body’s orbit with respect to the plane of the ecliptic, the orbital plane of the planets. There are additional parameters used to characterize a body’s orbit, but they do not factor into the definition of TNO dynamical classifications, and so are not discussed here.
Some TNOs are in “mean-motion resonance” with Neptune, meaning that both objects complete an integer number of orbits in the same period of time. For instance, Pluto is in the 3:2 mean-motion resonance: Neptune completes 3 orbits in the time it takes Pluto to complete 2 orbits. An everyday example of a resonance is pushing a child on a swing: if pushed at the right time, with the right force, the time between pushes can be kept the same and the child will reach the same height above the ground on each swing. The gravitational attraction between Neptune and the TNO is repeated each cycle, which acts to maintain the TNO’s current orbit.
According to the Deep Ecliptic Survey classifications (see Elliot et al., 2005), the 3:2 resonance is the most heavily populated. These TNOs, sometimes referred to as plutinos, have semi-major axes of ~ 40 au and have a wide range of orbital inclinations (see Figure 2). The next most populous is the 2:1 mean-motion resonance, at ~ 47.5 au. These are both “first-order” resonances, meaning the difference between the two numbers in the resonance notation is equal to 1; the smaller the difference, the stronger the resonance. It is not entirely clear if the TNO population in the 3:2 resonance is actually larger than that in the 2:1 resonance, or if there is an observational bias due to the latter’s being fainter, as seen from Earth. Other resonances with larger numbers of known objects include 4:3, 5:2, 5:3, and 7:4.
The range of colors and sizes across the various resonant populations suggests that the TNOs currently in these orbits did not form there (see Lacerda et al., 2014; Sheppard, 2012), but instead were “picked up” at different heliocentric distances as Neptune migrated outward to its current orbit (see Gomes, 2003; Levison & Morbidelli, 2003; Malhotra, 1993, 1995; Nesvorný, 2015). Different ice species condensed at different heliocentric distances (temperatures), leading to different surface compositions and colors, which may act as tracers of where the TNOs originally formed (Brown, Schaller, & Fraser, 2011). The migration of Neptune occurred slowly enough that some primordial TNOs became captured into resonance and their orbits evolved outward with Neptune. The relative numbers of objects in each resonance may place constraints on the speed of planetary migration early in solar system history (see Chiang & Jordan, 2002; Hahn & Malhotra, 2005).
The classical Kuiper belt is typically defined as the TNOs with low eccentricities (< 0.24) and semi-major axes between the 3:2 and 2:1 resonances with Neptune (~ 40–47.5 au; Petit et al., 2011; see Figure 2). Two subpopulations exist within the larger population of classical Kuiper belt objects (KBOs), distinguished by their orbital inclinations: the cold classical KBOs (CCKBOs) and the hot classical KBOs (HCKBOs). The terms cold and hot refer to orbital inclinations, not temperatures. The CCKBOs are those with low inclinations, while the HCKBOs have larger inclinations. The boundary between the CCKBOs and the HCKBOs is a source of debate (see Fernández-Valenzuela et al., 2021) but is typically considered to be at orbital inclinations of around 4° or 5° (Elliot et al., 2005; Van Laerhoven et al., 2019).
This debate arises from the unique dynamical histories of the CCKBOs and HCKBOs. The CCKBOs, which include the New Horizons flyby target Arrokoth (see Stern et al., 2019), have uniformly small diameters, and very red surface colors, and a high fraction have satellites (see Doressoundiram et al., 2002; Noll et al., 2008, 2014, 2020; Peixinho et al., 2008). The homogeneity of the CCKBOs suggests that they represent a primordial population that formed in the same orbits that they currently occupy, making them the only TNOs not to have migrated over solar system history. On the other hand, the HCKBOs exhibit a wider range of sizes and colors and a lower fraction have satellites (see Fraser et al., 2017; Lacerda et al., 2014; Tegler et al., 2016). Combined with their broader range of inclinations, these pieces of evidence point to the HCKBOs having formed at a variety of heliocentric distances, with later emplacement onto their current orbits via the migration of Neptune, similar to resonant TNOs. As a result of this process, some of the lower-inclination HCKBOs may “contaminate” the CCKBO population; indeed some “blue CCKBOs” have been identified that may have similar origins to the HCKBOs (Fraser et al., 2017).
Scattered Disk Objects
The scattered disk objects (SDOs)—which are referred to as “scattering” disk objects by Gladman and colleagues (2008) to emphasize that the process is ongoing—are not as easily classified using their orbital parameters alone. SDOs are a category of non-resonant TNOs whose orbits are undergoing modification through gravitational interactions with Neptune. Gladman et al. (2008) defined an SDO as any TNO that has its semi-major axis altered by more than 1.5 au when its orbit is modeled over 10 million years. In other words, SDOs are minor bodies in transition. They may be the source population of the Centaurs, minor bodies with semi-major axes between Jupiter and Neptune (Volk & Malhotra, 2008). Some objects, such as Typhon, straddle the line between SDO and Centaur, revealing the transitional nature of the SDOs: Typhon’s perihelion is only ~ 17.5 au, within the orbit of Uranus, while its semi-major axis is ~ 38 au, comparable to Pluto. SDOs prove that the trans-Neptunian region is continuing to evolve in the present day, ~ 4.5 billion years after the formation of the solar system.
TNOs found outside the classical Kuiper belt (i.e., beyond the 2:1 resonance, or semimajor axes greater than ~ 47.5 au) with eccentricities larger than 0.24 are classified as detached objects (see Figure 2; Gladman et al., 2008). This combination of semi-major axis and eccentricity results in perihelia greater than ~ 36 au; the aphelia of these TNOs can be extremely large. The orbits of these TNOs cannot be explained through interactions with Neptune, hence the name “detached” for their apparent gravitational detachment from the rest of the solar system. A subclass of the detached TNOs are the extreme TNOs (ETNOs), which have perihelia greater than 40 au and semi-major axes greater than 150 au (Sheppard et al., 2019). Furthermore, a subclass of the ETNOs are the sednoids, named after the largest member, the dwarf planet Sedna, which have perihelia greater than 50 au. The presence of these subclasses within the detached population point to multiple possible origins, with no one theory applicable to all objects. The orbits of the detached objects, ETNOs, and sednoids hold the keys to understanding where they came from.
There are a handful of exotic theories that can be used to explain the orbits of the detached objects, including a more eccentric orbit for Neptune in the distant past (Gladman et al., 2002), gravitational interactions with a passing star (Morbidelli & Levison, 2004), and/or the influence of an as-yet-undetected giant planet with a semi-major axis of hundreds or thousands of au (Batygin & Brown, 2016). This last theory is the only one that can be tested observationally, whereas the previous two theories can only be modeled. An initial study by Trujillo and Sheppard (2014) revealed that the orbits of the ETNOs are almost all oriented in the same manner: their elliptical orbits all have perihelia that are roughly aligned. The probability that this alignment is due to chance or observational bias is exceedingly small, less than 0.007% (Batygin & Brown, 2016). Searches are ongoing to find this unseen giant planet, which is theorized to have a mass comparable to Uranus (Batygin & Brown, 2016), but as of the time of this writing, no announcements have been made.
Discovery of Trans-Neptunian Dwarf Planets
Dwarf planets have been identified in each of the broad dynamical categories described previously (see Figure 2). A dwarf planet is defined as any non-satellite that is large enough to be in hydrostatic equilibrium (rounded by its own gravity into a spherical or ellipsoidal shape). For practical purposes, this means any minor body larger than 400 km in diameter. The International Astronomical Union (IAU) recognizes only five dwarf planets, four of which are in the trans-Neptunian region (Pluto, Eris, Haumea, and Makemake), but based on the geophysical definition alone, there are more than 150 dwarf planets orbiting beyond Neptune (Pinilla-Alonso et al., 2020).
The discovery of the largest trans-Neptunian dwarf planet, Pluto, is already described in this article. The remainder of the largest dwarf planets were not identified until the early 2000s (see Figure 1), over 70 years later, as part of wide-area sky surveys optimized for detecting TNOs (see Brown et al., 2004, 2005; Schwamb et al., 2009; Trujillo & Brown, 2003). It may seem strange that these objects, particularly Eris, Haumea, and Makemake, were not discovered before an object as small and faint as Albion. However, the orbital positions of these TNOs led to lower apparent brightnesses, slow apparent rates across the sky, and larger distances from the ecliptic plane, which combined to reduce detectability. All three of these TNOs were (and are still) near their respective aphelia at the time of discovery, reducing apparent motions across the sky and decreasing apparent brightnesses as seen from Earth. Haumea and Makemake are currently ~ 50 au from the Sun and are ~ 15 times fainter than Pluto. Eris, at nearly 100 au, is ~ 60 times fainter than Pluto, even though it is the intrinsically brightest TNO; in other words, if all TNOs were placed at the same distance from the Sun and Earth, Eris would be the brightest. Additionally, Albion, a CCKBO, was discovered very near the ecliptic plane, where the majority of other solar system minor bodies had been detected up to that point. The fact that Albion is nearly 4,000 times fainter than Pluto proves that the area targeted by previous surveys was the primary reason that Eris, Haumea, Makemake, and other TNO dwarf planets went undiscovered for so long.
The burst of new TNO discoveries in 2013 to 2015 (see Figure 1) was largely due to the Outer Solar System Origins Survey (OSSOS; see Bannister et al., 2018) and the Panchromatic Survey Telescope and Rapid Response System survey (Pan-STARRS; see Chambers et al., 2016). Unsurprisingly, 2014 showed a peak in dwarf planet discoveries, primarily due to Pan-STARRS and smaller discovery teams. OSSOS search fields were within 10° of the ecliptic plane, so a majority of its TNO discoveries were CCKBOs, which tend to be smaller. This is important information in its own right, as it indicates that dwarf planets truly are rare in the cold classical Kuiper belt, likely due to local formation conditions in the primordial solar nebula. Pan-STARRS, on the other hand, was an all-sky survey and therefore observed the higher-inclination HCKBO and SDO populations that contain a higher proportion of dwarf planets. Since the initial data releases and discovery announcements from these surveys, the rate of TNO discoveries decreased significantly and the number of new dwarf planets identified each year dwindled to zero in the late 2010s. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) is expected to increase the number of TNOs known by an order of magnitude by the late 2020s, which would naturally increase the size of the dwarf planet population as well (Schwamb et al., 2018). As of the time of this writing, there are over 150 TNO dwarf planets currently known, which provides a sufficient sample from which to study the formation and evolution of the outer solar system. Future dwarf planet discoveries and technological advancements will help fill in the gaps.
Properties of Trans-Neptunian Dwarf Planets
The trans-Neptunian dwarf planets are laboratories for the study of outer solar system collisional history, ices and gases at cryogenic temperatures, and ocean worlds. The 150+ known dwarf planets present a rich diversity of sizes, compositions, and histories from which to understand the formation and evolution of the trans-Neptunian region as a whole. These properties are investigated using imaging and spectroscopic observations from ground- and space-based telescope facilities, with our understanding evolving side by side with advancements in technological capabilities. Our current knowledge of TNO dwarf planets, and the observational techniques used to study them, are discussed in more detail in the following sections.
The satellites of TNO dwarf planets are distinct from those around the smallest TNOs (e.g., CCKBOs), pointing to different formation mechanisms. Satellites of dwarf planets tend to be smaller with respect to their primaries and have relatively tighter orbits. On the other hand, the less massive systems have nearly equal-sized components with larger relative separations. Figure 3 shows 10 of the most massive satellite systems compared to 10 of the least massive systems, using relative sizes and separations. Separations are presented in terms of the radius of the primary, Rprimary, but could just as easily be presented in percentage of the Hill radius. An object’s Hill radius defines the surrounding sphere where its gravitational attraction is stronger than the Sun’s, and thus where satellites are potentially stable.
Three theories explain the formation of the various satellite systems in the trans-Neptunian region: capture, gravitational collapse, and giant impacts (Brunini, 2020). Capture was formerly favored for the formation of the low-mass systems (see Astakhov et al., 2005; Funato et al., 2004; Goldreich et al., 2002; Lee et al., 2007; Schlichting & Sari, 2008; Weidenschilling, 2002), but this has been overtaken by gravitational collapse due to its ability to explain the formation of equal-sized components (see Johansen et al., 2009; Li et al., 2018; Nesvorný et al., 2010; Simon et al., 2017 Youdin & Goodman, 2005). The gravitational collapse process assumes that solid particles embedded within the gas of the solar nebula started to clump together and attracted other nearby particles. After enough mass was accumulated, the clumps collapsed under their own gravity to form planetesimals, which then collided with other planetesimals to form larger objects. In some cases, collisions resulted in fragmentation of the newly formed planetesimal, with the fragments entering orbit around each other, forming the roughly equal-sized binary systems observed in the cold classical Kuiper belt today.
Capture of objects from heliocentric orbit may still be in play for some of the smaller satellites of dwarf planets, but the favored formation mechanism of these systems is by giant impact (see Brown et al., 2006; Canup, 2005, 2011; Leinhardt et al., 2010). The Pluto system is a prime example, with one large satellite (Charon) and four minor satellites (Styx, Nix, Kerberos, and Hydra). The minor satellites are uniformly small, all less than ~ 100 km in diameter (Stern et al., 2015), uniformly bright, and have surfaces composed largely of water ice (Cook et al., 2018; Weaver et al., 2016). The theory is that the proto-Charon and proto-Pluto were both partially differentiated, meaning the heavier, rocky material had begun to settle into the cores of the objects while the lighter, icy material migrated to the surface (Canup, 2005, 2011; Lithwick & Wu, 2008; Stern et al., 2006). They collided in a grazing collision, preventing both bodies from being catastrophically broken apart, while ejecting some of the icy material from their surfaces into orbit (Kenyon & Bromley, 2019a). This icy material then coalesced into the minor satellites (Kenyon & Bromley, 2019b). The mechanism that placed the minor satellites in near-resonances with Charon has yet to be satisfactorily explained (see Cheng et al., 2014; Walsh & Levison, 2015).
The Haumea system bears many signatures of an origin by giant impact (see Leinhardt et al., 2010). However, other theories cannot be ruled out, specifically formation via rotational fission, which posits that the progenitor body was originally rotating so quickly that material was ejected and later coalesced into other bodies (see Ortiz et al., 2012). Understanding the formation of Haumea’s satellites Hi’iaka and Namaka (and rings) benefits from other useful information gathered on this rich system, including: Haumea’s fast rotation period (< 4 hr; Lacerda et al., 2008), highly elongated ellipsoidal shape (see Ortiz et al., 2017), and family (Brown et al., 2007). All confirmed members of the Haumea family, including Hi’iaka and Namaka, have high albedos, neutral colors, and surfaces dominated by water ice (see Barkume et al., 2006; Brown et al., 2007; Snodgrass et al., 2010). Most importantly, the family members have similar orbital parameters (see Figure 4), implying a common origin as part of the same progenitor body; Hi’iaka and Namaka were either the fragments with velocities too low for them to escape into heliocentric orbits or they coalesced from a debris disk (see Ćuk et al., 2013; Leinhardt et al., 2010).
The fingerprints of a giant impact are not evident for all TNO dwarf planets with satellites, but similarities can be seen by comparison with the Pluto and Haumea systems. The Eris, Orcus, and Salacia systems, for instance, look similar to the Pluto/Charon binary, with a relatively large satellite (a few percent of the primary by mass) on a relatively tight orbit (Brown & Butler, 2018; Holler et al., 2021; Stansberry et al., 2012). Another characteristic that points to formation by giant impact is a satellite on a nearly circular orbit. Such satellites are thought to have formed closer to their primary than their current position and to have migrated outward through tidal interactions, similar to how the Moon is actively receding from the Earth. These tidal interactions also act to circularize the orbit over time. A capture mechanism is less likely to accomplish this because the satellite’s semi-major axis at the time of capture would be large, and tidal evolution is strongly dependent on orbital distance. Dysnomia, Vanth, and Actaea, the satellites of Eris, Orcus, and Salacia, respectively, are on nearly circular orbits.
The satellites of Makemake, Gonggong, and Quaoar are a bit harder to explain. Makemake’s satellite and Quaoar’s satellite, Weywot, are comparable in size to Namaka, Haumea’s smaller satellite (Kretlow, 2020; Parker et al., 2016). However, Weywot is on a slightly more eccentric orbit (~ 0.15; Fraser & Brown, 2010) and there have not been enough observations of Makemake’s satellite to determine its orbital parameters. Weywot’s orbital properties do not preclude it from being formed by a giant impact, but its origin is certainly less clear. Xiangliu, Gonggong’s satellite, is on an orbit twice as eccentric as Weywot’s and is perhaps half the diameter (Kiss et al., 2019). Thus, Xiangliu may be a good candidate for having been captured from a heliocentric orbit.
Regardless of whether a satellite formed via giant impact or capture, both mechanisms point to a chaotic period in the history of the trans-Neptunian region. Capture and collision both require objects on crossing orbits, something that is not common in the present-day solar system. Planetary migration models require that the original mass of the primordial disk of objects beyond the giant planets be on the order of 10s of Earth-masses to produce the observed mass of TNOs today of ~ 1/10th of an Earth-mass (see Gladman et al., 2001; Gomes et al., 2005). The difference in the original and final masses of the TNO inventory is gigantic and indicates that the majority of primordial bodies were either transferred to other minor body populations, became irregular satellites of the giant planets, or, most likely, were ejected out of the solar system entirely (see Kenyon et al., 2008). The ejection process would not have been immediate, though: some objects’ orbits would have evolved on timescales of hundreds of thousands or millions of years during the migration of the giant planets, pushing them onto crossing orbits that increased the frequency of collisions or capture events. Understanding the origins of dwarf planet satellites therefore provides insights into the early dynamical history and collisional environment of the outer solar system.
Imaging of TNO systems is used to identify the presence of a satellite as well as to characterize its orbit. The Hubble Space Telescope has been a very powerful tool for this purpose, with its greater ability to separate tighter binaries than any other facility (see Noll et al., 2020). Orbit characterization (computation of the orbital period, semi-major axis, inclination, and eccentricity of the satellite, among other parameters) is the best means of determining the mass of the system. This is done using Newton’s version of Kepler’s Third Law and assuming the mass of the satellite is negligible compared to the primary (in other words, the mass of the primary is equal to the mass of the entire system). When combined with an estimate of the primary’s diameter, the system density can be calculated, revealing more about the TNO’s interior composition and evolution.
Two different methods have been used to measure TNO diameters: thermal modeling and stellar occultations. The thermal emission from a TNO is related to its diameter and albedo; modeling the emission using measurements at different wavelengths provides constraints on both quantities. This was performed on dozens of TNOs, including a large number of dwarf planets, using Spitzer Space Telescope and Herschel Space Telescope data (see Müller et al., 2010). Both telescopes are now no longer operational; the longer-wavelength observations provide by Herschel were the key to this method and there are no other telescopes currently in development that could provide similar data. The James Webb Space Telescope will provide measurements comparable to the mid-range Spitzer wavelengths, but it will not be able to measure the thermal peak of TNOs the way Herschel did.
The alternative, and arguably more accurate, method is to measure TNO diameters using stellar occultations. A stellar occultation occurs when a TNO passes directly in front of a background star and blocks the light, as seen from an observation point on the Earth (see Figure 5). This casts a weak shadow across the Earth’s surface, similar to a solar eclipse, but that only covers a small fraction of the Earth’s surface and has a shadow that moves over a short time period. The width of the shadow and the length of the occultation provide a measurement of the target’s dimensions (projected on the sky). Time-series observations from multiple locations perpendicular to the shadow path are used to provide unparalleled constraints on the shape of the TNO. The uncertainty on the length of the “chord” obtained at each site is set by the frequency that images are obtained and can be on the order of only a few kilometers, much smaller than for the thermal modeling method. For example, an occultation of Eris resulted in a radius measurement of 1163 ± 6 km (Sicardy et al., 2011), comparable to the uncertainty on Pluto’s radius from the New Horizons flyby, 1187 ± 4 km (Stern et al., 2015). In one previous case, an occultation revealed a deep canyon on the dwarf planet 2003 AZ84, demonstrating the power of this technique (Dias-Oliveira et al., 2017).
One of the drawbacks of the thermal modeling method is that nearby satellites can inflate the diameter measurements, especially since there is no way to separate the primary from the secondary at these longer wavelengths (the diffraction limit increases with wavelength). On the other hand, the primary drawback of the stellar occultation method is that these events are rare, even for TNOs moving in front of denser star fields. They also require a lot of resources to place telescopes along the shadow path, which may sometimes be in hard-to-reach places (see Buie et al., 2020; Ortiz et al., 2020). While it is also not a drawback, per se, a stellar occultation provides a measurement of a TNO’s size and shape at a particular rotation phase. Ellipsoidal objects with particular pole orientations can give misleading results: for example, in viewing an elongated TNO equator-on, the projected area on the sky depends on when exactly the occultation is observed. For a tri-axial ellipsoid (similar to an American football or rugby ball in shape) with three dimensions a, b, and c (where c < b < a), an occultation could capture measurements of b and c when the projected area is at a minimum, a and b when the projected area is at a maximum, or anywhere in between. Multiple occultations may therefore be required to nail down the size and shape. Alternatively, a rotation light curve obtained close in time to the occultation could provide the necessary information to determine the rotation phase and thus the projected area on the sky at the time of the occultation.
Combining masses from satellite observations and radii from thermal or occultation measurements results in a density value. Densities are important for understanding the internal compositions of TNOs. The TNO dwarf planets tend to have higher densities than their smaller counterparts (see Figure 6), a general trend that is seen in icy bodies throughout the solar system (Jewitt, 2009). Some smaller TNOs are even less dense than water ice, which requires significant porosity (empty space throughout the object) to achieve. The largest dwarf planets have densities about twice as high as water ice, indicating higher rock fractions in their interiors. These TNOs thus had higher internal heat levels early in their histories, due to higher concentrations of radioactive isotopes, and they are more likely to have undergone differentiation, separating rock from ice in the interior. Higher internal heat levels and a rock–ice boundary could then have led to the formation of subsurface oceans on some TNO dwarf planets (see Hussmann et al., 2006; Neveu et al., 2015). With a sufficient concentration of contaminants, such as ammonia, which acts as an antifreeze, these oceans may persist to the present day.
The surface compositions of TNOs are a direct result of their formation environment, collisional history, and the various physical and chemical processes at work at cryogenic temperatures. TNOs are ideal laboratories for studying low-temperature processes that cannot be easily replicated on Earth, such as volatile transport, cryovolcanism, and radiation processing of ices. The surface is also the gateway to constraining interior composition and evaluating the likelihood of an atmosphere, due to its connections to both and because it is the easiest of the three zones to observe.
The primary tool for studying surface compositions of TNOs is reflectance spectroscopy. Sunlight hitting a TNO’s surface is reflected into space, with the ices on the surface absorbing at some wavelengths. These absorption bands act as a “fingerprint” for each ice species, enabling identification in spectra obtained with ground- and space-based telescope facilities. The most useful wavelength range for studying TNO surface compositions is the near-infrared, ~1–2.5 μm, which is just beyond the red end of the visible portion of the spectrum. The Earth’s atmosphere is largely transparent over these wavelengths, solar flux is still significant in this range, and many ices relevant to the study of TNOs have strong absorption features here.
The ices present on TNO surfaces can be divided into two categories: volatile and non-volatile. These terms refer to how likely the ices are to sublimate (transition from solid directly to gas) at a given temperature. At the typical surface temperature of a TNO, which is about 40 K (−233°C or −388°F), only a handful of ices are volatile and the others are effectively rocks. Nitrogen (N2), carbon monoxide (CO), and methane (CH4) are the primary volatile ices at this temperature, while water (H2O) is the most common non-volatile ice (see Fray & Schmitt, 2009). Additional non-volatile ices that have been previously detected on TNOs are the organic compounds ethane (C2H6) and ethylene (C2H4), as well as ammonia (NH3) hydrates (see Brown et al., 2015; Cook et al., 2007; Holler et al., 2014).
Volatile ices are present only on the largest, coldest TNOs (Johnson et al., 2015; Schaller & Brown, 2007a). Ices that are prone to sublimate at TNO surface temperatures are then prone to escape from the TNO entirely through a variety of processes, including thermal escape and stripping by the solar wind. In this way, smaller TNOs can lose their entire volatile inventories on timescales shorter than the age of the solar system. Only the largest TNO dwarf planets could retain a portion of their original complement of volatile ices over the age of the solar system (see Figure 7). The most easily observed volatile ice, due to its numerous strong absorption features in the near-infrared, is CH4 (see Figure 8). The volatile retention model of Schaller and Brown (2007a) correctly predicts the presence of CH4 on Pluto, Eris, Makemake, and Sedna (see Barucci et al., 2005, 2010; Cruikshank et al., 1976; Licandro et al., 2006a, 2006b). Tentative detections have been made on Gonggong, Quaoar, Orcus, and Varuna (Barucci et al., 2008; Brown, Burgasser, & Fraser, 2011; Lorenzi et al., 2014; Schaller & Brown, 2007b). The model suggests that CH4 on Gonggong and Quaoar is a reasonable expectation, but that Orcus and Varuna should have lost the entirety of their volatiles long ago. These two dwarf planets therefore provide valuable tests of volatile retention theories and could be sites of more exotic processes in the trans-Neptunian region. The TNOs that are predicted to retain CH4 should also retain the more volatile N2 and CO, but to this point they have only been directly detected on Pluto (see Owen et al., 1993). Shifts in the band centers of CH4 were detected on Eris and Makemake, an indirect indication of the presence of N2 on these dwarf planets (Licandro et al., 2006a; Tegler et al., 2008, 2010). The ETNO Sedna may also support N2 on its surface, although the evidence is still circumstantial (Barucci et al., 2005, 2010).
The presence of volatile ices opens the possibility of atmospheres around the largest dwarf planets. To date, only Pluto has a confirmed atmosphere (Elliot et al., 1989), with very low upper limits placed on atmospheric pressure for Makemake, Quaoar, and Eris (Arimatsu et al., 2019; Ortiz et al., 2012; Sicardy et al., 2011). Pluto’s atmosphere was discovered by a stellar occultation, and the upper limits for other TNO dwarf planets were determined from occultation measurements. As seen in Figure 5, the presence of a global atmosphere results in a more gradual decline and rise in the light curve as the starlight passes first through an increasingly thicker atmosphere before being blocked entirely by the surface, then back through the atmosphere before the TNO moves away from the star. Atmospheric modeling shows that Eris may have a tenuous atmosphere around local noon, even at its current distance of 96 au from the Sun, leading to volatile transport across the surface (Hofgartner et al., 2019). This could explain its uniformly bright surface (Sicardy et al., 2011) and is promising for a global atmosphere, like Pluto’s, when Eris is closer to perihelion. Makemake and possibly Gonggong, which may have CH4 on its surface, could also experience periods during their orbits when they support atmospheres. This seems less likely to apply to Quaoar, which is on a more circular orbit, so if no appreciable atmosphere is noted today, it is unlikely that it ever has one.
Pluto’s atmosphere, like its surface, is dominated by N2, with smaller amounts of CH4 and CO mixed in. The atmospheric pressure is on the order of 10 microbars, nearly 100,000 times lower than Earth’s at sea level, yet there is still significant loss of molecules into space. Some of these molecules are captured by Charon, which is too small to have retained its own initial inventory of volatiles. The CH4 molecules then find their way into cold traps on Charon’s surface and are processed by ultraviolet light (photolysis) and energetic particles (radiolysis) into more complex hydrocarbons like ethane, ethylene, acetylene (C2H2), etc. (Grundy et al., 2016). Photolysis and radiolysis of surface ices and atmospheric molecules, particularly CH4, act to redden TNO surfaces. Indeed, Charon’s north pole is redder than the rest of the surface, as seen by New Horizons, and Pluto has a large deposit of complex hydrocarbons on its equator, referred to as Cthulhu Macula (see Stern et al., 2015). Most TNO dwarf planets are at least moderately red, indicating the widespread presence of these materials on their surfaces, with some TNOs, such as Gonggong and Sedna, among the reddest objects in the solar system (Boehnhardt et al., 2014; Sheppard, 2010). In the case of Sedna, the extremely red color could be due to the fact that it spends the majority of its ~10,000-year orbit outside the heliopause, the boundary between the Sun’s radiation environment and that of interstellar space, leading to increased bombardment and surface processing by galactic cosmic rays.
The red complex hydrocarbons are present not only on the volatile-rich dwarf planets, but also on the largest volatile-poor dwarf planet as well: Haumea. The event that produced the Haumea family, its satellites and rings, elongated shape, and fast rotation period also appears to have removed its entire inventory of volatile ices. Haumea’s near-infrared spectrum and surface are dominated by H2O ice (see Pinilla-Alonso et al., 2009; Trujillo et al., 2007), but with a “Great Dark Spot” observed as Haumea rotates (Lacerda, 2009). It is possible this spot could be due to a concentration of complex hydrocarbons, but its origin and exact composition remain a mystery.
Many other, smaller TNO dwarf planets are dominated by nonvolatile H2O ice (see Barucci et al., 2011; Brown et al., 2012). As mentioned previously, large concentrations of H2O ice are suggestive of ancient cryovolcanism resulting from interior differentiation. This is further supported by the identification of ammonia (NH3) on some TNOs, specifically Charon and possibly Orcus (see Cook et al., 2007; Delsanti et al., 2010). Ammonia acts as an antifreeze when mixed with water and can help it to flow more easily at the cold temperatures experienced at TNO surfaces and subsurfaces. It is becoming clearer that NH3 ices on TNO surfaces are not indicative of ongoing cryovolcanism (see Holler et al., 2017). This is obviously difficult to determine without spacecraft flybys, but no cryovolcanic eruptions were identified during the New Horizons flyby of Charon, which has NH3 globally distributed across its surface (Hofgartner et al., 2018). The possibility of catching an active cryovolcano during a spacecraft flyby is tantalizing and would no doubt force a rethinking of the nature of H2O-dominated TNO dwarf planets.
Among the rich diversity of worlds in the trans-Neptunian region, the dwarf planets stand out the most. They are not only the biggest and brightest of the TNOs, but also the most dynamically, physically, and chemically diverse. There are over 150 currently known dwarf planets, and they are found in every dynamical population: resonant, classical Kuiper belt, scattered disk, and detached. For so many dwarf planets to be observed in the present-day trans-Neptunian region, they must have been very numerous in the primordial disk of minor bodies that was scattered and disrupted by the migration of the giant planets. As part of that scattering, the TNO dwarf planets experienced giant impact and capture events, gaining satellites in the process. It is through the study of these satellite orbits and the calculation of the system mass and density that the possibility of TNO ocean worlds arises. The dwarf planets also present unparalleled laboratories for studying atmospheric dynamics, radiation processes, and surface–interior interactions at extremely cold temperatures. Continued study of TNOs, and the dwarf planets in particular, via ground- and space-based telescopes, as well as future robotic exploration, will further reveal the origin and evolution of the solar system.
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