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Question 2: Accretion in the Outer Solar System

The outer solar system, stretching from Jupiter to the Oort cloud, contains the keys to understanding the formation and early evolution of the solar system.¹ Gas giant planet formation is a primary open question in theoretical astrophysics, with Jupiter and Saturn used to calibrate and test new models. As a result, a detailed characterization of these planets is required. Uranus and Neptune represent a unique class of planets that clearly differ from terrestrial and gas giant planets, and their formation challenges planet formation models because it is unclear whether they are simply failed gas giants or if they formed in a different way. That planets with similar sizes/masses appear abundant in the galaxy suggests that the formation of such intermediate-mass gas planets is common, emphasizing the need to understand the formation of Uranus and Neptune (see Questions 7 and 12; Chapters 10 and 15, respectively). The giant planets possess full planetary systems with rings and regular satellites, as well as irregular satellites thought to have been captured from heliocentric orbit early in solar system history (see Question 8, Chapter 11). The unique characteristics of these systems provide further constraints on giant planet system origin and early evolution.

The outer solar system also has diverse small body populations: comets, irregular satellites, Trojans, Centaurs, and the swarm of dwarf planets and smaller bodies in the trans-neptunian belt. The latter population presents a complex orbital structure whose full extent and character is still being explored to the limits of telescopic capabilities. However, even as they are understood today, the orbital and physical properties of the smaller objects in the outer solar system provide profound and detailed clues to its formation. The emerging view of exoplanetary systems (see Question 12), and how they differ from the solar system, provide additional strong motivations to study the formation of the outer solar system, as it is only the solar system that can be truly explored, and in time understood, in the necessary depth and detail.

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¹ A glossary of acronyms and technical terms can be found in Appendix F.

Q2.1 HOW DID THE GIANT PLANETS FORM?

The main research themes of outer planet formation have been known for decades, but developments in observations and theory continually revise our understanding of how these themes relate and how the planets formed. The leading mechanisms for giant planet formation are core accretion and disk instability (Helled et al. 2014).

In the core accretion scenario, giant planet formation begins with buildup of a heavy-element core (heavy elements referring to all elements heavier than helium), followed by accretion of hydrogen-helium, or H-He, gas. Volatile species that are heavier than helium, such as CO and N₂, can also be accreted in the form of gas (see Question 1 for details, Chapter 4). Heavy elements in the solid form are delivered by objects ranging in size from millimeter-to-decimeter-size pebbles, to planetesimals up to hundreds of kilometers in size, to finally planetary embryos/cores. H-He from the protostellar disk can be accreted during the early stages of core formation; however, as the planet grows in mass, at some point the gas accretion rate increases considerably, nearly at free fall, and a gas giant planet is formed. During the final stages of gas accretion when the planet has sufficiently contracted in size, a circumplanetary disk may form and control the transfer rate of gas to the accreting planet. Gas accretion likely ends as the circumsolar nebula dissipates (Russell et al. 2006; see Question 1), or perhaps by a gap opening in said disk. In contrast, in the disk instability scenario, giant planets form by a local gravitational collapse in the circumstellar disk (Boss 1997). This model is typically applied to giant exoplanets orbiting at tens of AU from their host star and/or of giant planets around M dwarf stars. While we cannot exclude that the gas giants in the solar system formed via disk instability, their complex internal structures are more consistent with formation by core accretion (e.g., Helled and Stevenson 2017).

Q2.1a What Is the Formation Mechanism of Gas Giant Planets? What Were the Accretion Rates of Solids (Planetesimals/Pebbles) and Gas During the Formation Process? How Long Did It Take?

A key uncertainty with the core accretion model is how cores massive enough to prompt runaway gas accretion by Jupiter and Saturn were able to accrete prior to solar nebula dispersal. The early core growth is dominated by heavy-element accretion, with many expected large objects with sizes of 100 to 1,000 km growing in separate feeding zones. At this point, the conventional wisdom has been that the protoplanets gravitationally perturb each other and merge into a few large cores. The problem is that dynamical simulations find that mergers are less common than expected, and gravitational interactions cause the protoplanets to be spread out (Levison et al. 2015), increasing the core accretion timescale to longer than the expected lifetime of the solar nebula (see Question 1 and text below), thereby preventing substantial gas accretion. In addition, it is possible that planetesimals may become isolated in rings between protoplanets, which may also lead to less efficient accretion.

A plausible solution comes from so-called pebble accretion. Small, millimeter-to-decimeter-sized protoplanetary solids (“pebbles”; see Q1.3a–b) lose orbital energy owing to aerodynamic drag by the solar nebula gas. If a pebble’s aerodynamic stopping time is less than or comparable to the time for it to encounter a growing protoplanet, then it is decelerated with respect to the protoplanet during the encounter and can become gravitationally bound, eventually spiraling into the protoplanet. This effect allows protoplanets above a threshold size to grow rapidly if they are embedded in a sea of pebbles. Numerical models suggest that pebble accretion in combination with protoplanet mergers can form the giant planet cores before the solar nebula dissipates (Levison et al. 2015; Johansen and Lambrechts 2017). Once cores reach about 10 Earth masses, rapid gas accretion can occur until the local supply of gas is depleted. A sketch of these various growth phases is shown in Figure 5-1.

Because Jupiter and Saturn are H-He dominated, and Uranus and Neptune have H-He atmospheres of a few Earth masses, the formation of the outer planets occurred within the lifetime of the gas-rich protosolar nebula. However, their exact formation timescale remains an open question. The formation timescale of the gas giant planets is thought to be on the order of 10⁶ to 10⁷ years. This timescale corresponds to the observed lifetimes of protoplanetary disks around other stars (see also Q1.4), which ensures the existence of H-He disk gas that can be accreted by the growing protoplanet. Shorter formation timescales with efficient pebble accretion are possible, but it may then be difficult to explain, for example, why Uranus and Neptune did not accrete more gas. Substantial uncertainty in formation timescale is very significant for formation models, in particular for the implied disk

conditions (which determine the accretion rates), formation efficiency, early evolution, as well as the predicted giant planet masses and compositions. Recent isotopic measurements of meteorites have been interpreted to imply a million-year formation time for Jupiter’s core and concomitant gap opening in the solar nebula (Kruijer et al. 2017), but alternative interpretations have been offered (Lichtenberg et al. 2021).

The alternative disk instability model is presently thought to be less relevant for the solar system, especially considering Juno and Cassini results for Jupiter and Saturn, respectively (Q2.1c). However, this model currently cannot be excluded, and it is therefore important to understand predicted differences that could be used to discriminate between the two models, which is an active topic of investigation. In general, core accretion requires a heavy-element core, but the presence of a core in the disk instability model cannot be ruled out. Overall, better understanding of giant planet origin requires improved information on their bulk compositions and internal structures, which can be inferred from structure models that use accurate measurements of their gravitational and magnetic fields, and atmospheric compositions (see Question 7). In combination with planet evolution models, such data can then be used to constrain formation locations, accretion rates, and formation timescales, possibly allowing us to discriminate between these two formation scenarios (see e.g., Helled et al. 2014; Helled and Morbidelli 2021).

Q2.1b How Did Uranus and Neptune Form and What Prevented Them from Becoming Gas Giants?

There are several challenges to explaining the origin of Uranus and Neptune. Planetesimal accretion is predicted to be too inefficient to form these planets at their current locations while the nebula was still present unless extreme local conditions are assumed. Uranus and Neptune could have formed faster in situ via pebble accretion, although this scenario could easily lead to their accreting too much H-He (e.g., Lambrechts and Johansen 2012). Another possibility is that Uranus and Neptune formed by collision and merging of a few low-mass planets accreted from a population of planetary embryos (e.g., Izidoro et al. 2015). As discussed below, dynamical properties of the trans-neptunian belt suggest that Neptune formed considerably closer to the Sun and later migrated outward; forming both Uranus and Neptune and smaller orbital radii would lead to faster formation potentially consistent with limited H-He accretion (see Q2.4 and Q2.6, Chapter 5).

None of the existing models, however, explain all observed properties of Uranus and Neptune, including the heavy-element to H-He ratios inferred by structure models, or even the identity of these heavy elements (i.e., are they mainly rocky, icy, or carbonaceous, or some mixture?). In addition, it is still unknown what prevented these planets from becoming H-He-dominated like Jupiter and Saturn (Helled et al. 2020). It seems unlikely that their gas accretion was truncated by the opening of gaps in the nebula, as this is generally associated with much more massive planets (≥ Jupiter mass). We infer that the growth of Uranus and Neptune was slow (i.e., accretion rates were low) and/or that it occurred late as the gaseous nebular disk had begun to dissipate, preventing substantial accretion of H-He gas. The latter idea could suggest that substantial photoevaporation in the Uranus–Neptune zone limited the size of these bodies; a signature of this could be an enrichment in noble gases, as hydrogen is expected to escape first because it is the lightest gas component (e.g., Guillot and Hueso 2006; Question 1). That Uranus and Neptune could have formed after Jupiter and Saturn is generally consistent with a heavy-element accretion rate that would be lower for larger radial distances, owing to decreasing disk solid surface densities and orbital frequencies. Further constraints on the origin of Uranus and Neptune would be greatly improved by better measurements of their gravitational and magnetic fields and atmospheric compositions.

Q2.1c What Were the Primordial Internal Structures of Giant Planets?

For decades, the giant planets’ interiors were generally assumed to be homogeneously mixed, with a compact central core. However, recent formation and internal structure models find that the heavy elements are not uniformly mixed (e.g., Helled and Stevenson 2017), and it is now understood that giant planets probably did not have distinct cores but rather an innermost region that was highly enriched with heavy elements. Once the core mass becomes sufficiently high for rapid H-He accretion, subsequent incoming solids (heavy elements) dissolve in the atmosphere, leading to deep interior compositional gradients in gaseous protoplanets. Indeed, fuzzy cores seem to exist today in both Jupiter and Saturn based on Juno and Cassini data, respectively (Wahl et al. 2017; Mankovich and Fuller 2021, see Question 7). Uranus and Neptune may have also had primordial structures with fuzzy cores and inhomogeneous interiors; however, their internal structures remain poorly constrained, hampering understanding of how they formed and evolved. More information is required to better constrain Uranus and Neptune’s formation, evolution and structure, including measurements of their gravitational and magnetic fields, determination of their atmospheric composition (preferably by an entry probe), and improved information on the behavior of planetary elements at relevant pressures and temperatures. Details on the evolution of the internal structures of giant planets are given in Chapter 10 and references therein.

Q2.1d What Were the Roles of Early Giant Impacts and Magnetic Fields in Shaping the Properties of the Outer Planets?

Collisions during the late stages of planet accretion tend to reduce the number of massive objects and increase the masses of the survivors, until a stable configuration is obtained (Izidoro et al. 2015). Simulations of giant impacts (e.g., Reinhardt et al. 2020; Rufu and Canup 2022) show that an oblique impact on Uranus can explain its 98-degree axial tilt and rotation rate (and perhaps the concomitant formation of its regular satellite system; see Q2.3b). Giant impacts are also invoked to explain Neptune’s smaller but still substantial 30-degree obliquity, key differences between Uranus and Neptune (e.g., their satellite systems, heat fluxes, and predicted moments of inertia), potential differences between Jupiter and Saturn, and the origin of fuzzy cores.

As discussed above, gas giant planets are thought to follow a core-nucleated accretion process whereby cores capture massive gaseous envelopes from the gas nebula. The concurrent accretion of angular momentum is expected to spin the protoplanet to near-breakup speeds. However, Jupiter and Saturn (and most long-period extrasolar planets; Bryan et al. 2020) rotate well below their breakup rotation rate, suggesting that some mechanism expelled angular momentum from the planet-forming region. A leading model redistributes angular momentum from the protoplanet to a circumplanetary disk (CPD) via magnetic coupling/braking (Takata and Stevenson 1996). In one of the versions of such models (Figure 5-2), vigorous convection in the protoplanet’s interior generates a strong magnetic field, which in turn couples to the ionized portion of the CPD. The terminal spin of the planet is determined by the terminal radius of the planet and the location of the magnetospheric truncation-radius of the CPD at the time of nebular gas dispersal. Testing such models requires a much better understanding of magnetic field generation within giant planets today (which constrains the internal structure implied by dynamo sustenance), as well as improved knowledge of CPDs and how their evolution controlled the formation of satellites and rings (see Q2.5). Although Uranus and Neptune accreted vastly smaller gas components than Jupiter and Saturn, and per above appear to have had their final spin states affected by late giant impacts, the current rotation periods of all four giant planets are broadly similar (about 10–17 hours), a commonality that may be coincidental.

Strategic Research for Q2.1

• Determine the atmospheric composition of Saturn, Uranus, and Neptune via in situ sampling of noble gas, elemental, and isotopic abundances, and remote sensing by spacecraft and ground/space-based telescopes.
• Determine the bulk composition and internal structure of Uranus and Neptune via gravity, magnetic field, and atmospheric profile measurements by spacecraft, as well as Doppler seismology.²
• Constrain physical properties and boundary conditions (i.e., tropospheric temperatures, shapes, and rotation rates) for structure models of Uranus and Neptune via gravity, magnetic field, and atmospheric profile measurements by spacecraft, remote sensing by spacecraft and ground/space-based telescopes.
• Better determine the formation and early evolution of the outer planets through improved numerical simulations and theoretical models.
• Improve our understanding of the behavior of planetary material at high pressures and temperatures using laboratory experiments and numerical simulations.

Q2.2 WHAT CONTROLLED THE COMPOSITIONS OF THE MATERIAL THAT FORMED THE GIANT PLANETS?

The giant planets accreted both solids and gas, regardless of the specific formation processes involved. Although these primordial components have been at least partially mixed within the planets, the current planetary composition can still be linked back to the composition of accreted primordial solids and gas (see Question 1). For the accreted gas component, current isotope ratios and elemental abundances reflect the balance between planetary growth rates and the evolution of nebular gas (including effects from photoevaporation as well as influxes of material from sources in the Sun’s stellar birth cluster). The combination of isotopic/elemental ratios from giant planets—as well as from comets and meteorites—is crucial for determining protosolar values for helium isotopes (³He/⁴He) and hydrogen isotopes (deuterium/hydrogen, or D/H), because the composition of the Sun itself has evolved after 4.5 Ga of nuclear fusion.

For accreted solids, relative abundances of the elements distinguish between classes of material such as ice, rock, and organics; between levels of physical and thermal processing such as ice crystallization; and between source region temperatures with respect to the ice lines (condensation fronts) for water and other volatiles within the protosolar nebula (described further below).

Q2.2a How Was the Overall Bulk Fraction of Heavy Elements in the Giant Planets Established?

Understanding the origin of giant planet heavy element fractions hinges on our incomplete knowledge of the bulk heavy element fractions themselves. For Jupiter, abundances of most heavy elements detected by the Galileo probe and remote sensing (Figure 5-3) are higher than their protosolar abundances. Assuming that heavy element abundances in the other giant planets scale with carbon (as measured by atmospheric methane), all the giant planets most likely accreted an excess of solid material compared to gaseous material.

In the core accretion scenario (Q2.1a), the observed supersolar enrichment of heavy elements results from both solid material that formed the giant core that was subsequently mixed into the envelope, as well as from solid material that was accreted directly into the envelope, possibly abetted by photoevaporative loss of H-He gas from the protosolar nebula itself. In situ measurements of atmosphere composition, as well as accurate gravity and magnetic field measurements, provide relevant constraints. Noble gas compositions can constrain the composition of the planetary building blocks and reveal information on the chemical and physical properties of the solar nebula, and possibly the planetary formation timescale (e.g., Guillot and Hueso 2006; see Q1.4).

The balance between these different sources is an open question, which can be explored in the coming decade with new data and continuing developments in theory. Adding data points to Figure 5-3 would require atmospheric

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² Trapped normal modes in the interiors of giant planets can create a velocity field that can be sensed remotely. Measurements of the spatial and temporal frequencies of these modes provide information on the deep interior, like seismology in the case of Earth.

probes, especially for the noble gases. Measurements of carbon-to-hydrogen (C/H) and sulfur-to-hydrogen (S/H) ratios at Uranus and Neptune have recently been advanced by submillimeter and microwave spectroscopy, which can probe atmospheric levels beneath the cloud condensation levels of at least methane (CH₄) and hydrogen sulfide (H₂S) (Tollefson et al. 2021). But key ratios such as nitrogen to hydrogen (N/H), oxygen to hydrogen (O/H), and S/H at Saturn, Uranus, and Neptune are difficult to relate to abundances in the deeper atmospheres owing to cloud condensation and chemistry (see Q7.3).

Relating atmospheric composition to bulk composition, even with improved measurements in the coming decade, will require improvements in theory. Models of radial profiles of interior density and heavy element concentrations—driven largely by new gravity field data from Juno and Cassini—achieve better fits using a “fuzzy core” in Jupiter and Saturn (see Q2.1 and Q7.2). These results suggest that heavy element enrichment in the observable envelopes is at least partially owing to mixing of core material, yet models struggle to match gravity field data with envelope enrichments higher than the protosolar abundance (e.g., Wahl et al. 2017).

Q2.2b What Were the Contributions to the Giant Planets from Different Types of Solids (Rock, Ices, and Organics)?

There is no class of solid material, whether icy or rocky, in the current solar system that reflects the generally three-times supersolar enrichment of heavy elements in Jupiter (see Figure 5-3), a fundamental challenge to understanding the types of solids accreted during planetary formation. Equilibrium condensation of a

protosolar-abundance mixture would produce more ice than rock, if the condensation temperature is low enough, but verifying a high (or any) ice-to-rock ratio in the giant planets is challenging because rock-forming species condense in deep cloud layers inaccessible to observations (although evidence for such clouds abounds in hot exo-Jupiters). Reality is even more complex because carbonaceous matter and/or graphite could also enrich the planets with carbon.

Thermochemical models constrained by disequilibrium species such as germane (GeH₄) and arsine (AsH₃) may lead to estimates of some rocky element abundances (Wang et al. 2016), and the volatile gas H₂S contains sulfur that may have been accreted in rocky material. Disequilibrium carbon monoxide (CO) abundances in giant planet tropospheres similarly provide constraints on deep O/H as a tracer of accreted icy material (Wang et al. 2016).

The D/H ratio in the giant planets is much lower than observed in comets, which may be analogs of accreted icy materials. For gas-rich Jupiter and Saturn, low D/H relative to comets indicates a high overall gas fraction. But for Uranus and Neptune, the gas fraction is lower, and D/H values, while somewhat elevated, have been interpreted as indicators of a low ice/rock fraction, or of incomplete mixing with low D/H in outer layers owing to accreted protosolar gas (Teanby et al. 2020). Higher-order gravitational moments at Uranus and Neptune, and/or internal structure constraints from Doppler seismology, are needed to constrain more advanced models of density profiles capable of distinguishing between heavy element contributions from rocky, icy, and carbonaceous material.

Advances in our understanding of the composition of Uranus and Neptune in the coming decade may lead to a reevaluation of their bulk compositions and whether the term “ice giant” should be modified to a more appropriate name that properly represents the planetary composition. The idea that solid carbonaceous material may have been as abundant as rocky and icy material has not been fully explored. Constraints on the bulk carbonaceous fraction in comets, trans-neptunian objects, and outer planet moons may test the viability of carbonaceous material as a major component of protoplanetary solid material. Studies of protoplanetary disks may provide important context in this regard (Q1.1).

Q2.2c How Were Compositional Differences Between the Gas Giants and Ice Giants Influenced by the Chemical and Physical Processing of Accreted Solids and Gas?

Composition varied spatially and temporally within the protosolar disk, with effects preserved in the present-day composition of the giant planets. The planets formed at different heliocentric distances and thus sampled spatial variation within the disk, and time variation of disk composition affected the planets differently owing to variation in individual gas accretion timescales. Disk temperature decreased as a function of radius, setting up a sequence of condensation fronts or “ice lines” for different volatile species.

Solid material spiraled inward at the disk midplane owing to gas drag, so these ice lines represent boundaries where cold ices began turning to gas. Cold amorphous water ice went through an additional phase transition to crystalline ice, releasing trapped gases. Thus, compositional differences between the gas giants and ice giants may reflect their formation at different heliocentric distances with respect to condensation temperatures of ices containing oxygen, nitrogen, carbon, noble gases, and possibly other elements such as sulfur.

The composition of gases and solids in the solar nebula was affected by many chemical processes in addition to condensation fronts (e.g., Mousis et al. 2018; Öberg and Bergin 2021). Over time, gas composition in the disk evolved as hydrogen and helium (and perhaps neon) were evaporated under the influence of protosolar ionizing ultraviolet radiation, as well as ultraviolet radiation from massive stars in the Sun’s birth cluster. These massive stars may have also affected disk composition via injection of material from stellar winds and supernovae (see Q1).

Individual simulations—including radial and/or temporal variation of gas and solid nebular composition—have matched aspects of planetary composition, but additional effects from cleared gaps, vortices, and other two- and three-dimensional structures have yet to be included in a comprehensive way. New compositional measurements, particularly in Uranus and Neptune as a contrast to Jupiter, are needed in the coming decade to constrain increasingly complex models of giant planet formation. The potential effects of volatile loss from planetesimals before they are accreted by a growing giant planet (e.g., Lichtenberg et al. 2021) should continue to be investigated.

Strategic Research for Q2.2

• Determine the atmospheric composition of Saturn, Uranus, and Neptune via in situ sampling of noble gas, elemental and isotopic abundances, and remote sensing by spacecraft and ground- or space-based telescopes.
• Understand how compositional gradients in the atmosphere and interior of Jupiter, Saturn, Uranus, and Neptune affect the determination of bulk planetary composition based on observed atmospheric composition, using gravity, magnetic field, and atmospheric profile measurements by spacecraft, Doppler seismology, and laboratory/theoretical studies of physical processes (e.g., turbulent diffusion, moist convection, precipitation, and helium rain).
• Constrain the composition of early accreted materials by determining abundance and isotopic ratios in diverse present-day objects from dust and meteorites to comets and TNOs, using sample return, in situ measurements, and remote sensing by spacecraft and ground-/space-based telescopes.
• Contextualize ice lines and elemental partitioning between gases and solids in the evolving protosolar disk using comparative observations of protoplanetary disks obtained with ground-/space-based telescopes via coronagraphy, interferometry, and spectroscopy.
• Continue to improve knowledge of protosolar elemental and isotopic abundances by study of primitive meteorites and the solar atmosphere.

Q2.3 HOW DID SATELLITES AND RINGS FORM AROUND THE GIANT PLANETS DURING THE ACCRETION ERA?

Each of the giant planets possesses satellites and rings (see Question 8). The main processes thought to have produced circumplanetary disks—gas accretion and giant impacts—would have occurred during or near the end stages of giant planet accretion. Thus, it is probable that all the giant planets had primordial satellites and rings (and perhaps multiple generations of them). A first overall question is the extent to which observed satellites and rings result from a common set of processes, or whether their individual characters derive largely from stochastic or contingent historical events. A second, related question, which has received increasing attention of late, is whether some of the current satellites and rings may have formed much later in solar system history, or even whether formation is ongoing (see Question 8).

Rings and regular satellites lie in or nearly in a given giant planet’s equator, and orbit prograde, implying an origin from a dissipative orbiting disk of gas and/or solid particles. Irregular satellites are found in dynamically stable zones that lie at great distances from their primaries, which exist for both prograde and retrograde orbits. They are judged to have been captured from heliocentric orbit (see Q2.6 and Question 8). While not giant planets, trans-neptunian dwarf planets, when telescopic observations are sufficient, are also seen to possess one or more satellites. Pluto has five moons, while Haumea has two known moons and a ring system. Indeed, so common are satellites (and to an extent, rings) about the larger bodies of the outer solar system, that the question might be not so much why they have rings and moons, but why the terrestrial planets are deficient in satellites and lack rings entirely.

Each satellite and ring system is unique in detail, and so untangling the roles of systematic and stochastic processes is challenging. This issue is highlighted by the common occurrence of satellite pairs in a given satellite system, which by rights ought to be similar but are not. The key example is the dichotomy between Ganymede and Callisto, described below. The physical processes that allowed satellites to accrete is unclear, and it has yet to be determined how similar these processes were to those, at a larger scale, that led to the formation of the planets.

Nevertheless, our understanding of giant planet satellite accretion continues to be revolutionized by new spacecraft discoveries, analysis of meteorites, theoretical modeling, and studies of exoplanetary systems. Similarly, dwarf planet satellite formation is tied to and informs us of the dynamical processes that formed the trans-neptunian belt (see Q2.5). Satellite formation in general informs us of the conditions that formed the Moon and may have led, more broadly, to the formation of habitable icy worlds throughout the solar system.

Q2.3a How Did Protosatellite Disks Form, and How Did Disk Structure and Composition Evolve During the Accretion of Primordial Satellite Systems?

Formation of the gas giants Jupiter and Saturn is thought to have been accompanied by that of a CPD surrounding each (e.g., Peale and Canup 2015; see Q2.2). As proto-Jupiter and proto-Saturn grew toward their full masses, they opened gaps in the protosolar nebula, across which gas and coupled dust continued to flow, forming the CPDs (Figure 5-4). The evolving pressure, temperature, dynamical structures, and lifetimes of these disks are actively debated. They were originally thought to be gas-dominated but may instead be gas-starved and solid-particle-rich. They may be accretion disks spreading viscously and contributing to the mass growth of the giant protoplanet or they may be decretion disks spreading outward and returning matter to the protosolar nebula, or both. Here a key uncertainty is the specific angular momentum of the inflowing CPD gaseous material, and how that relates to the supply of CPD solids. The CPDs may or may not have been truncated by inner magnetospheric cavities generated by the primordial magnetospheres of Jupiter and Saturn (Figure 5-5).

The solids (rock, ices, and organics) that accreted to form the regular satellites of Jupiter and Saturn were sourced from heliocentric solids, with a possible contribution to the more volatile components from direct condensation in the CPD, depending on the latter’s temperature and pressure history. Pebbles in the protosolar nebula may have been largely blocked from flowing into the CPD by gas pressure gradients, but the gravity of proto-Jupiter and proto-Saturn would have excited the orbits of heliocentric planetesimals, causing them to encounter the CPDs at high speeds (Raymond and Izidoro 2017; Ronnet and Johansen 2020). Ablation, disintegration, and wholesale capture of these planetesimals and recondensation of their component volatiles could have been a major source of a new generation of circum-jovian and circum-saturnian pebbles. Streaming or other instabilities within CPDs could then have formed first-generation “satellitesimals,” subsequently growing by hierarchical coagulation and/or accretion of remaining pebbles into protosatellites (Shibaike et al. 2019; Bagytin and Morbidelli 2020; Ronnet and Johansen 2020; see Figure 5-5). Different formation scenarios make different predictions for satellite structure and composition, both chemical and isotopic, and so the more we can learn about the satellites, the better we can test these scenarios. Improved numerical modeling is also key, as it has long been difficult to simultaneously model the vastly different time and spatial scales of planet and satellite formation.

Q2.3b What Were the Roles of Giant Impact and Capture in the Outer Solar System for the Origin of Primordial Satellites and Planetary Rings?

It is unclear whether Uranus and Neptune formed circumplanetary disks in a manner like that posited for Jupiter and Saturn (Peale and Canup 2015). It has been argued to be less likely because they never reached the runaway gas accretion stage (see Q2.1), although conversely their slower rate of gas accretion might have been more favorable to a contracted planet size and CPD formation (Ward and Canup 2010). A related issue is that Uranus and Neptune have substantial spin axis tilts, and it has long been thought that these were owing to late impacts. Gas accretion would generally lead to planets with a very small obliquity, like that of Jupiter. Uranus and Neptune’s current spin states (obliquity and rotation period) can be explained by an oblique impact by approximately an Earth-size body. Saturn’s tilt is also substantial, Earth-like, and may have been similarly caused by a giant impact, or alternately, by a dynamical resonance in the early solar system (Ward and Hamilton 2004; Vokrouhlický and Nesvorný 2015). Uranus’s spin is retrograde with respect to its orbital motion about the Sun, and its regular satellites orbit in the same sense as the planet’s rotation, which is in the opposite sense of a CPD formed by gas accretion. The uranian satellites may have formed from a disk produced by a Uranus-tipping giant impact (e.g., Ida et al. 2020; Woo et al. 2022), or from a combination of a CPD produced via gas accretion and a Uranus-tipping impact (Morbidelli et al. 2016; Rufu and Canup 2022; Salmon and Canup 2022). This process could have been repeated multiple times, with each major planetary reorientating impact and new impact-produced satellite-forming disk interacting with and reprocessing the previous generation of satellites. However, whereas much has been learned of the jovian and saturnian satellites from Galileo and Cassini, detailed knowledge of the principal uranian regular satellites is

lacking. Improved understanding of these satellites (including, e.g., their moments of inertia and differentiation states; compositions; geological character and signs of past/current activity; constraints on orbital evolution) is the surest way to test giant impact satellite formation models.

The role of impacts in satellite accretion is not limited to the giant impact scale or the protosolar nebula epoch. The early planetesimal (and dwarf planet) reservoir that survived beyond the orbit of Neptune, which is thought to have been the ultimate source of the trans-neptunian belt and Oort cloud (see Q2.6), was also a source for captured satellites and material for planetary rings. During the giant planet instability epoch (see Q2.4), dynamical transport of heliocentric planetesimals and dwarf planets throughout the giant planet region would have occurred but would have been especially important for Neptune as it migrated outward through the primordial Kuiper belt population. Capture into the satellite region of a giant planet could have occurred by direct collision with a preexisting regular satellite or by tidal stripping of a binary (e.g., Agnor and Hamilton 2006; Nogueira et al. 2011); such a process could have led to the capture of Neptune’s retrograde Triton, with further catastrophic consequences for any original regular midsize satellite system (McKinnon et al. 1995). Further constraints on Triton’s origin depend on a better understanding of the formation of Neptune and the Kuiper belt.

Close passes of a heliocentric body to within a giant planet’s tidal Roche limit could have led to tidal disintegration and capture of debris that could have ultimately formed rings close to the planet (Hyodo et al. 2017). Furthermore, the smallest innermost moons are susceptible to catastrophic disruption by early heavy bombardment, as both the flux (impact rate) and speed of heliocentric impactors are concentrated by a giant planet’s gravity. Such disrupted moons, however, may have subsequently reaccreted, giving these bodies complicated histories that are not easy to decipher.

Unsteady and evolving conditions at the edge of a gas giant’s gravitational sphere of influence (Hill sphere) during the giant planet instability epoch can also lead to the capture of the distant irregular satellites of each (see Q2.6). Here it is suspected that wandering objects located at the right place and time were captured during giant planet encounters. Current models predict that all the irregular satellites derive from the same trans-neptunian source population, a prediction that can be tested. Testing capture scenarios for irregular satellites and ring materials requires improved knowledge of bodies like Triton, the outer irregulars of all the giant planets, and ultimately, isotopic measurements of their components (e.g., D/H and ¹⁴N/¹⁵N).

Q2.3c What Are the Expected Properties of Satellites and Rings Formed During the Accretion Era, and Are These Consistent with the Satellite and Ring Systems Today?

It is plausible that the observed diversity of satellite and ring systems arises in large part from their accretion conditions. Primordial rings could be formed by a variety of processes, including satellite collisional or tidal disruption/stripping, or disruption during a close pass within the Roche limit per above. Accordingly, a wide range of initial ring masses and compositions could be envisioned. A ring collisionally spreads with time, causing its mass to decrease owing to accretion onto the planet or the spawning of moons as ring material spreads beyond the Roche limit. As a ring’s mass decreases, its rate of spreading slows. A massive primordial ring would after more than 4.5 billion years of evolution asymptotically evolve to a mass that is independent of its initial mass. Remarkably, this theoretically predicted asymptotic mass is essentially equal to the mass of Saturn’s rings (Salmon et al. 2010).

That said, rings may dynamically evolve sufficiently fast that none, even Saturn’s rings, can be confidently assumed to be primordial. As discussed in Question 8, that Saturn’s rings are so bright, despite their being continually bombarded by dark meteoritic material, argues against their being primordial and instead suggests they formed more recently. Whatever the origin of Saturn’s rings, satellite formation tied to outward spreading of ring material has also led to the hypothesis that the inner midsize saturnian satellites may have accreted in sequence and tidally evolved away from Saturn, one after another, over perhaps a billion years or more (e.g., Crida and Charnoz 2012; Salmon and Canup 2017). Testing such varied scenarios has proven difficult, but better understanding of giant planet interiors, and the tidal dissipation mechanisms therein, offers some promise for understanding whether satellites can tidally migrate outward by the large distances implied by the late formation hypothesis.

There appear to be two absolute size scales of regular satellites: midsize (diameter 500 to 1,500 km) and large (Moon to Mercury in size). Jupiter’s regular satellites, the Galileans, are all large, whereas those for Uranus are all midsize. Saturn’s regular satellites are a mix, with midsize moons both inside and outside Titan’s orbit. Midsize

moons also, generally, increase in mass with distance from the planet. The saturnian satellites, with their mixed character, clearly challenge formation scenarios. Detailed characterization of the midsize moons of Uranus—their internal structures, chemical and isotopic compositions, and geological histories—would greatly advance our understanding of midsize moons through comparisons with those of Saturn.

Despite the many differences seen across the giant planet regular satellite systems, they all have approximately the same total mass when compared to their host planet mass, with this ratio being about 102⁴ at Jupiter, Saturn, and Uranus. This is remarkable given the presumably varied formation histories across these systems. Satellites are expected to migrate inward within their CPDs owing to unbalanced disk torques. It has been argued that the observed ~102⁴ mass ratio reflects the balance between satellite growth versus loss owing to inward satellite migration (Canup and Ward 2006), wherein multiple generations of satellites form and are lost, with each having a common total mass compared to the planet’s mass. If so, earlier formed satellites may be lost to the parent planet, and even torn apart by tides to form a primordial ring (Canup 2010), which itself can lead to a new generation of inner satellite accretion (Crida and Charnoz 2012; Salmon and Canup 2017). Or, if there is a magnetospheric cavity close to the planet, the innermost migrating satellite may halt at the inner edge of the CPD (as suggested in analogy with exoplanet systems), with the orbits of subsequently forming and migrating satellites forming a resonant chain with the orbit of the first (e.g., Batygin and Morbidelli 2020). It is possible that such cavities could be detectable via polarimetric measurements of accreting extra-solar giant planets (sensitive to magnetic fields) by the James Webb Space Telescope. Alternatively, it has been suggested that the mass of a typical Galilean satellite is set by gap opening in the proto-jovian CPD, which slows further satellite accretion.

The compositional gradient within the Galilean satellite system, from rocky Io through icy Callisto, is an important clue to accretional conditions. This gradient could reflect local temperature conditions as well as pebble and satellitesimal composition. In contrast, the four largest and outermost uranian moons have similar compositions, while inner Miranda appears more ice rich. Significant inward satellite migration within a given circumplanetary nebula may also imply time evolution of pressure and temperature conditions at satellite birth, which can be addressed through detailed modeling of satellite formation. Note that such primordial inward migration (see Figure 5-5) is owing to gravitational interactions with nebular gas, and is distinct from the slower, outward migration of satellites owing to planetary tides over geologic time and as observed today for the Galilean satellites of Jupiter and Saturn’s inner midsize satellites (and for the Moon) (see Question 8).

The differences between Ganymede and Callisto, the two largest of the Galilean satellites, are of particular interest and significance. Both are major ice-rock worlds in adjacent orbits. Ganymede has had a complex geological history, major resurfacing, is differentiated, and exhibits an internally generated dynamo magnetic field that implies an inner metallic core. Callisto in contrast appears geologically inert, with no signs of large-scale resurfacing, and gravity data from Galileo flybys implies it may only be partially differentiated. Yet both bodies may possess internal liquid water oceans, as inferred from Galileo magnetic induction measurements.

If Callisto is only partially differentiated it places strong constraints on the timing and character of its accretion. It does not partake in the Laplace resonance between Io, Europa, and Ganymede, and thus has not been substantially tidally heated. Future measurements of its gravitational and induced magnetic field will be critical to resolve this issue and its evolutionary history. Because Callisto rotates, assessing its state of differentiation from gravity data is sensitive to nonhydrostatic effects like those known to exist in other similarly sized bodies (Gao and Stevenson 2013), so that additional approaches (e.g., utilizing shape data and/or pole position analyses) may be needed.

Strategic Research for Q2.3

• Determine fundamental properties of the midsize uranian moons through gravity, magnetic field, and geodetic measurements (by spacecraft), surface composition measurements (by remote sensing from spacecraft and ground-/space-based telescopes), and geological characterization (based on remote sensing by spacecraft, including imaging of the hemispheres unseen by Voyager 2).
• Determine fundamental properties of Neptune’s moon Triton through gravity, magnetic field, and geodetic measurements (by spacecraft), surface composition measurements (by remote sensing from spacecraft and ground-/space-based telescopes), and geological characterization (based on remote sensing by spacecraft, including imaging of the hemisphere poorly seen by Voyager 2).

• Determine Callisto’s state of differentiation to constrain the accretional conditions of large icy moons via spacecraft geodesy (shape), gravity, pole position, and magnetic field and associated charged particle measurements (the latter necessary for proper interpretation).
• Study accretion dynamics in ring systems through time-series imaging by spacecraft of rings and ring-embedded satellites (see Q7.2).
• Improve the understanding of satellite and planetary ring formation with improved numerical simulations and theoretical models.

Q2.4 HOW DID THE GIANT PLANETS GRAVITATIONALLY INTERACT WITH EACH OTHER, THE PROTOSOLAR DISK, AND SMALLER BODIES IN THE OUTER SOLAR SYSTEM?

It is now thought that the giant planets did not form where they currently reside, but instead migrated inward and/or outward because of disk torques during the protosolar nebular phase or by later gravitational interactions with remnant planetesimals. Evidence for these interactions is inferred from the orbits of the giant planets in the solar system and in extrasolar planetary systems (see Question 12), as well as how giant planet migration dynamically affected primordial small body populations (see Question 3, Chapter 6) and the bombardment history of the solar system (see Question 4, Chapter 7). The picture that has emerged so far of the solar system’s earliest history involves an instability in the orbits of all the giant planets, a hypothesis with far reaching implications, but one that requires much further elaboration, refinement, and testing.

Q2.4a Did the Giant Planets Create Gaps in the Protosolar Nebula, and What Consequences Did This Have for Their Accretion and Potential Orbital Migration?

As described in Q2.1–Q2.3, it is thought that the gas giants were massive enough to open gaps in the protostellar nebula, in analogy with protoplanetary disks around other stars (e.g., as observed by ALMA, see Question 12). Unless torques are exquisitely balanced, giant planets should migrate to some degree, that is, have their orbits move inward or outward with rates that depend on disk properties (e.g., surface density and viscosity) and the presence or absence of gaps.

The timing and extent of giant planet migration during the solar nebula phase is debated. Giant planets may have formed in or evolved into resonant chains, in analogy with some exoplanet systems. Theoretical studies suggest migration of a single body should generally be inward, but it has been shown that the proximity of gaps around proto-Jupiter and proto-Saturn may allow for the reverse, that is, outward migration. This provides a possible explanation for why the solar system does not possess close-in, “hot Jupiters” (Morbidelli and Crida 2007).

Early migration implies that Jupiter and Saturn may have accreted material over a range of heliocentric distances (.1 AU); implications of this for the asteroid belt are discussed in Question 3.

Q2.4b Did the Giant Planets of the Early Solar System Migrate, and If So, How Far, and What Was the Effect of This Migration on Other Outer Solar System Bodies?

The clearest evidence for giant planet migration in the solar system is the dynamical structure of the trans-neptunian belt (Nesvorný 2018). It is most easily explained by the outward planetesimal-driven migration of Neptune out to 30 AU through a primordial Kuiper belt population that started near ~20 AU. Some discussion of the effects of Neptune’s migration are included here, while other aspects are discussed in Q2.6.

As Neptune entered the primordial Kuiper belt, it excited the objects residing there, giving many higher eccentricities and inclinations. A small fraction was captured into resonances with Neptune—for example, Pluto’s capture into Neptune’s 2:3 mean motion resonance. Models suggest that the primordial belt population was reduced by a factor of ~1,000, with roughly this number of Pluto-size bodies and enumerable smaller bodies scattered into the giant planet zone and/or out into a scattered disk of comets associated with Neptune, or even the Oort cloud. The interactions between Neptune and ~1,000 Pluto-size bodies made Neptune’s migration grainy rather than smooth, and this in turn prevented the capture of an excess number of objects within Neptune’s resonances (Nesvorný and

Vokrouhlický 2016; Morbidelli and Nesvorný 2020). Moreover, a small fraction of the primordial Kuiper belt population ejected by Neptune was potentially retained on distant scattered orbits, where they now await discovery.

It is also possible that an earlier giant planet migration took place in the presence of nebular gas, as suggested in Q2.4a. The most viable model for this is referred to as the Grand Tack (Walsh et al. 2011). It describes how Jupiter and Saturn migrated across the asteroid belt, followed by a reversal of direction back toward their present orbits. This hypothesis derives its justification as an explanation for the low mass (inefficient accretion) of Mars and the broad mixing of compositional types in the asteroid belt, although other explanations exist for both constraints (see Q3.3).

Scenarios of giant planet migration can be tested by comparison with observations of their effects on, for example, small body populations, the formation and characteristics of the terrestrial planets, and the stability of giant planet satellites, which would strongly constrain the character (e.g., timing and physics) of giant planet migration, as well as, potentially, the formation of the Moon and the asteroids. Presently there is no fully accepted, self-consistent model for the simultaneous formation of the giant planets during the nebular era. The most important data to obtain, in the next decade, if possible, would be chronological constraints on the early bombardment of the Moon and the asteroids (Question 4), which would strongly constrain the character (timing, physics) of giant planet migration.

Q2.4c Was There a Global Instability Among the Giant Planets, and If So, When Did It Occur? Were Any Major Planets Ejected (Lost) During This Instability?

Given that migration models for Neptune can reasonably reproduce the orbital structure of the Kuiper belt and scattered disk, it is necessary to consider the broader implications of Neptune’s migration for the rest of the solar system. For example, if Neptune entered the primordial Kuiper belt at ~20 AU, it probably had to form near that location. In turn, that means the giant planets once had a different configuration and a migration mechanism had to move them to where we see them today (Nesvorný 2018) (Figures 5-6 and 5-7).

Models indicate that gas accretion of the giant planets may have left them on nearly circular coplanar orbits between ~5 and ~20 AU, with most or perhaps all locked in mutual mean motion resonances with one another. This system, however, eventually went unstable in a violent exchange of orbital energy and angular momentum that is referred to here as a giant planet instability (e.g., Tsiganis et al. 2005; Nesvorný and Morbidelli 2012). Here giant planets encountered one another while also interacting with a sea of objects liberated from the primordial Kuiper belt.

Early versions of such models postulated that the instability and migration could have occurred hundreds of millions of years after the formation of the solar system, but more recent dynamical work favors an earlier instability; as the nebula clears, the giant planets emerge in a spacing too close to remain stable without the eccentricity and inclination damping effects of nebular gas. Regardless, the dispersal of the primordial Kuiper belt led to the heavy bombardment of the solar system worlds that existed at that time by ice-rich planetesimals (see Question 4, Chapter 7). It is possible many basins and large craters on the icy satellites trace back to this epoch.

As Neptune scattered trans-neptunian objects (TNOs) into the giant planet zone, dynamical friction from gravitational interactions between these bodies and the giant planets decreased the orbital eccentricities and inclinations of the latter, but not all the way to zero (Tsiganis et al. 2005). This explains the small but nonzero eccentricities and inclinations of Jupiter, Saturn, Uranus, and Neptune, otherwise expected to be effectively zero from gas interactions or large owing to mutual perturbations.

The number of giant planets gravitationally interacting with one another at the time of the instability is unknown. Dynamical studies suggest systems with 5 or 6 giant planets (e.g., three or four Uranus/Neptune-size bodies) have greater success reproducing dynamical constraints across the solar system than those that start with 4 giant planets (Batygin et al. 2012; Nesvorný and Morbidelli 2012). This extra Uranus/Neptune-size body is useful because it can encounter Jupiter multiple times before being ejected from the solar system. The tiny “jumps” produced in Jupiter’s orbit are not only needed to explain its current orbit but also the dynamical structure of the asteroid belt and the putative capture of trans-neptunian objects in several regions (e.g., central and outer asteroid belt, Hilda asteroids, Trojan populations; see Question 3) (Nesvorný and Morbidelli 2012; Vokrouhlický et al. 2016).

Further observations to locate and characterize the most distant TNOs are the most important research activity to constrain the behavior of Neptune, along with observations and characterization of other TNOs, Centaurs, and comets to understand the installation of such extreme objects, and by implication, the instability that likely emplaced them.

The timing of the instability can be constrained by determining the chronology of impact bombardment, both in the inner and outer solar system (see Question 4). Bombardment by outer solar system objects should carry

compositional (mainly volatile) and isotopic signals as well that may be discernible on the terrestrial planets and asteroids. Additional clues to the bombardment history of the outer solar system may be found on the cratered surfaces of outer solar system moons and TNOs.

Strategic Research for Q2.4

• Determine the timing, extent, and effects of giant planet migration by measurement of impact basin ages on the terrestrial planets; compositional and isotopic constraints on early terrestrial planet evolution, including the origin of the Moon; and studies of impact crater populations on diverse outer solar system bodies.
• Further constrain the dynamical structure of the distant trans-neptunian population, including classical and resonant objects, so-called detached objects (Q2.6), and any undiscovered planet(s), through remote sensing by ground-/space-based telescopes (including surveys) and theoretical modeling.

• Characterize the basic properties of TNOs of diverse size, binarity, and dynamical subpopulations with flyby(s)/orbital/landed missions to the outer solar system and through remote sensing by ground-/ space-based telescopes (including surveys).
• Improve our understanding of giant planet formation and migration in the early solar system using improved numerical simulations and theoretical models.
• Contextualize the early configuration and evolution of the solar system using comparative observations of protoplanetary disks and exoplanets obtained with ground-/space-based telescopes.

Q2.5 HOW DID PROCESSES IN THE EARLY OUTER SOLAR SYSTEM PRODUCE THE STRUCTURE AND COMPOSITION (SURFACE AND INTERIOR) OF PLUTO AND THE TRANS-NEPTUNIAN OBJECTS?

Trans-neptunian objects, or TNOs (also Kuiper belt objects, KBOs, with the KBOs representing the stable population beyond Neptune) far outnumber any other type of bodies, but little is known about them, with the first TNO other than Pluto-Charon only discovered in 1992. TNOs retain unique information of planet formation in the outer solar system. In the past decade, thanks to an explosion in the number of TNOs being characterized with telescopic observations, combined with the New Horizons spacecraft’s flybys of the Pluto system and the small TNO Arrokoth (a bilobate, contact binary object—i.e., two bodies in physical contact but that remain distinguishable), new constraints have enabled a better understanding of how the trans-neptunian belt and, by extension, other solar system objects may have formed and evolved early on.

Q2.5a When and How Did Trans-Neptunian Objects and Cometary Bodies Form?

Crater counts suggest that Pluto, its moons, and Arrokoth developed surfaces able to retain an impact record early in solar system history, perhaps more than 4 Ga ago (Stern et al. 2018). Their bombardment history is consistent with planetesimal formation models where bodies up to ~100 km in size are formed from gravitational instability (e.g., the streaming instability) during the first few million years of solar system history when nebular gas still existed (see Q1.3b).

Until the past decade, it was thought that TNOs were assembled through successive collisions between smaller bodies. Such events can potentially form a few Pluto-sized objects in the region between ~20 and 30 AU in about 100 million years and a size distribution of bodies comparable to that seen today. Features of the trans-neptunian belt size distribution predicted from such models, however, do not produce the more massive and numerous populations of bodies implied by planet migration models and conditions needed to account for the properties of resonant TNOs (Morbidelli and Nesvorný 2020) (see Q2.5b). In contrast, models of planetesimal formation via streaming instabilities reproduce these aspects (see Q1.3b).

A major question concerns the origin of comets. These bodies, as observed, are generally small (~1 to 10 km or so in diameter) and come from the scattered disk associated with Neptune or the Oort cloud. But their ultimate origin is thought to be the primordial Kuiper belt, the same region that birthed Pluto and the other larger members of the TNO population. Debate centers on whether these “proto-comets” formed as primordial small bodies (e.g., Davidsson et al. 2016) or whether they are the fragmented and reaccreted remnants of collisions among the larger TNO planetesimals just described (e.g., Morbidelli and Nesvorný 2020). Further physical study of comets and Centaurs, defined as bodies transitioning from the scattered disk to Jupiter-family comet orbits, should be illuminating regarding the primordial versus collisional origin question. But as Rosetta observations of comet 67P/Churyumov-Gerasimenko made clear, the insolation-driven activity of comets is a complicating or obscuring factor in understanding their formation. Visiting additional primitive, cold classical Kuiper belt objects unaffected by such activity is clearly warranted.

In contrast, of all the subcategories of TNOs, the cold classical TNOs near 45 AU stand apart (see Q1.3b and Q2.6a). Their characteristics (orbital and size distributions, colors, and binarity) as well as the benign impact environment in the cold classical region imply that the cold classicals are the small bodies most closely related to primordial planetesimals. The only cold classical TNO visited to date is Arrokoth, and its singular nature shows

that these bodies can teach us a great deal about planetesimal formation (Stern et al. 2019; McKinnon et al. 2020). It is important to confirm and expand on these findings by visiting additional cold classicals as well as TNOs of diverse size, binarity, and dynamical subpopulations for comparison, and to contrast with cometary observations. Such encounters should be a part of future flyby missions to the outer solar system, supported by continuing ground and space-based observations.

Q2.5b How Many Trans-Neptunian Objects Formed, and What Were Their Initial Size Distributions?

The initial mass distribution of the trans-neptunian belt, set during the accretion era, probably exceeded the mass estimated today from observations by a factor of ~1,000 (see Q2.4b). This value is favored for several reasons: (1) it is otherwise difficult or impossible to form Pluto-sized bodies with the current mass (in any formation scenario), (2) ~1,000 Plutos are needed to keep Neptune’s migration grainy enough to capture the right number of TNOs in Neptune’s mean motion resonances, and (3) that quantity allows the depletion of the primordial Kuiper belt by Neptune’s migration to explain various small body populations captured during the giant planet instability (Nesvorný and Vokrouhlický 2016; Vokrouhlický et al. 2016, 2019; Morbidelli and Nesvorný 2020).

The largest body within this mass distribution is not known but may exceed that of Triton, likely a captured TNO and more massive than Pluto. Moving to smaller sizes, if it is inferred there are ~10⁵ bodies with diameter D .100 km in the current Kuiper belt, a factor of ~1,000 depletion would suggest there were initially ~10⁸ such bodies in the primordial Kuiper belt (Nesvorný and Vokrouhlický 2016). Moreover, if the Kuiper belt and Jupiter’s Trojan populations were captured from the same source population, it can be deduced that the cumulative power law slope (q, the exponent in a distribution versus diameter of the number of objects larger than that diameter) of the primordial Kuiper belt for D, 100 km objects once followed q 5 22.1 for 10, D, 100 km bodies (Grav et al. 2012; Nesvorný and Vokrouhlický 2016).

The smallest objects (&10 km, depending on albedo) cannot be observed directly because of their faintness, but as with most solar system populations, there are more small objects than large ones. Cratering records of Pluto/Charon and Arrokoth from New Horizons provide our best estimate of smaller objects, with craters (or pits) identified on Arrokoth as small as 200 m implying even smaller impactors. Taken together, the crater data suggests a shallow cumulative power-law slope of q ~ 21 for projectiles between a few tens of meters and ~1 km (Singer et al. 2019; Spencer et al. 2020; Morbidelli et al. 2021; Robbins and Singer 2021). Such impactors are smaller than those ostensibly created by the streaming instability, and that could suggest this shallow slope was produced by collisional fragmentation. A possible analogy would be the shallow q ~ 21 slope seen among 0.1, D, 1 km bodies in the main asteroid belt (Bottke et al. 2015; Morbidelli et al. 2021).

Comprehensive astronomical surveys would greatly improve our understanding of the population of TNOs greater than 10 km in size, while doing so for smaller TNOs requires relying on crater counts from future spacecraft encounters and/or on stellar occultation surveys, data that will place crucial constraints on planetesimal formation and evolution models.

Q2.5c How Prevalent Were Giant Impacts in the Early Trans-Neptunian Belt?

Nearly all trans-neptunian dwarf planets have satellites, and formation as a binary or multiple system is thought to be typical of TNOs in general (Noll et al. 2020). Many satellite origin scenarios have been proposed, but the dwarf planets are massive enough that the formation of moons and possibly rings around them is thought to result from relatively giant impacts between dwarf planets themselves, the Pluto system being the prime example (Canup et al. 2021).

Questions remain, however, on what led to the variety of satellite systems seen in the trans-neptunian belt. Of the ten largest known TNOs, nine have at least one satellite orbiting relatively close (in terms of gravitational binding). This contrasts with the smallest binary TNOs observed from Earth, most of which have comparably sized components and large relative separations. At the smallest end, Arrokoth is a contact binary with two similarly sized components. This suggests, subject to observational selection effects, different formation mechanisms between large and small TNO systems.

The orbit, masses, and compositions of Pluto-Charon all point to a giant impact origin (Canup et al. 2021) involving a low-velocity collision between similarly sized progenitors. Pluto-Charon could have retained volatiles through such an event, although except for CO, none of their observed volatiles are primordial (see Question 6, Chapter 9). If thousands of Pluto-class bodies formed in the ~20 to ~30 AU region prior to Neptune’s migration, giant impacts may have been common. On the other hand, if instead only a few Pluto-size objects formed (e.g., by standard pairwise accretion), giant impacts would have been rare, or possibly some key aspect of the formation problem is missing.

Differences in impact angle and/or velocity between two like-sized dwarf planets can lead to larger ice/rock fractionations, possibly explaining the properties of other dwarf planet systems, or outcomes other than a binary, such as rapidly rotating, triaxially shaped Haumea, which only has small moons, a ring, and a heliocentric family of fragments (the only one known so far among the TNOs). Alternative origins for binary systems have been proposed (Noll et al. 2020), such as three-body capture, but for (at least) dynamically cold, 100-km-class trans-neptunian binaries, these proposed capture mechanisms are inconsistent with the observed distribution of the binaries’ mutual orbit inclinations, which instead favors co-formation by streaming and gravitational instabilities (Nesvorný et al. 2019) (see Q1.3b and Q2.5a). Possible origins may also depend on formation location. Further studies of binary properties generally will help constrain these and other binary formation mechanisms.

Q2.5d What Were the Relative Proportions of Ices, Rock, and Organic Materials Accreted by Small Objects (Comets, TNOs, Moons) in the Outer Solar System?

The sizes and masses of TNOs have been refined in the past decade thanks to telescopic observations and—for the Pluto system and TNO Arrokoth—flybys with the New Horizons spacecraft. Dividing an object’s mass by its volume provides its bulk density, the first clue as to its composite materials. Mass determination generally depends on a body having a satellite whose orbit can be determined; size is especially difficult to measure for such distant bodies and is often done by thermal modeling. However, stellar occultations, if available, can be quite precise sizes. Thus, only a small minority of TNOs have well constrained bulk densities and compositions; for most others, especially objects smaller than ~500 km in diameter, densities are poorly known.

Density estimates for dwarf planets are intermediate between those of rock and ice, which is consistent with ice being the stable form of water this far from the Sun, and close to the densities of large molecular weight organic compounds; the latter are abundant in dust particles recovered on Earth and that may have originated in the outer solar system, and in comets 1P/Halley and 67P/Churyumov-Gerasimenko (Bardyn et al. 2017). A bulk density can be interpreted with specific proportions of rock and ice, but uncertainties in rock mineralogy, other ices, presence of impurities in the ice (e.g., salts), proportion of organic material (McKinnon et al. 2008, 2017), and porosity (that may exceed 70 percent in small, comet-sized bodies but is debated in larger bodies) translate into uncertain bulk proportions of these materials. Future astronomical measurements should provide a broader set of reliable TNO densities, and radio tracking of future spacecraft encounters should prove decisive in density determinations.

Q2.5e During Accretion, (How) Did the Interiors of Outer Solar System Moons and Dwarf Planets Transition from Homogeneous to Layered?

Constraining a planetary body’s internal structure requires precise measurements of shape and either its rotation or Doppler tracking of a spacecraft trajectory near the body. Such measurements have been made at the Galilean satellites, Titan, and several midsize satellites of Saturn, but not yet as satellites of ice giants or on any TNOs.

The degree of layering (differentiation) inside icy moons varies, and the timing of differentiation is unclear. Ice and rock/organics have decidedly separated inside Europa, Ganymede, and tiny Enceladus; but apparently less so inside Callisto, Titan, Rhea, Dione, and Mimas. Differentiation does not seem to depend on size or density as much as distance from the giant planet. This suggests that among accretional, radiogenic, and tidal sources of heat, it is the latter that may have driven ice to melt, letting rock and organics settle to form a core. (Further heating can differentiate metal from rock.) However, one cannot discount the other two heat sources, which may have dominated during giant planet system formation.

Early differentiation of a Titan-like satellite is a requirement of one hypothesis for the formation of Saturn’s rings, via tidal stripping of an icy mantle (Canup 2010). Loss of early surface oceans has also been suggested to explain the ice/rock compositional gradient of the Galilean satellites (Bierson and Nimmo 2020), although other explanations exist. Determining outermost Callisto’s state of differentiation is a key test for the accretional conditions of large icy moons, as Galileo flyby gravity and magnetic field measurements were not definitive on this point (see Q2.3c). Additional measurements from polar flybys and/or an orbital survey are needed.

We do not currently know whether most trans-neptunian dwarf planets are layered. The above heat sources could have allowed ice-rock/organic differentiation, especially if accretional energy includes that of giant impacts. This is consistent with if not implied by the surface geology of Pluto and Charon (i.e., undifferentiated interiors are inconsistent with observations) (Moore et al. 2016). That said, rock/organic settling may have been impeded by convection in a solid mantle, and rock and ice may not have separated in the outer regions of Charon-sized or smaller TNOs (Figure 5-8). The formation of Pluto’s rock-rich moon Charon and retinue of small but apparently very icy satellites is thought to require the giant impact of partially differentiated precursor bodies (Canup et al. 2021), that is, neither undifferentiated ice-rock bodies nor fully differentiated worlds.

Strategic Research for Q2.5

• Characterize the basic properties (size, mass, shape, cratering, rings, and binarity) of diverse TNOs and related bodies (Centaurs and comets) via remote sensing by spacecraft and ground-/space-based telescopic observations.
• Improve understanding of giant impacts between ice-rock protoplanets through state-of-the-art numerical simulations of giant impacts.
• Improve the understanding of the accretion and internal differentiation, of icy moons and TNOs, including ocean formation, through interpretation of spacecraft data in terms of consistent physicochemical models and laboratory experiments on ices and carbonaceous materials.
• Improve the understanding of binary system, TNO family, and ring formation through theory, observations, and numerical modeling.

Q2.6 HOW DID THE ORBITAL STRUCTURE OF THE TRANS-NEPTUNIAN BELT, THE OORT CLOUD, AND THE SCATTERED DISK ORIGINATE, AND HOW DID GRAVITATIONAL INTERACTIONS IN THE EARLY OUTER SOLAR SYSTEM LEAD TO SCATTERING AND EJECTION?

Small bodies across the solar system can constrain the migration of the giant planets and planet formation. The trans-neptunian belt out to the distant Oort cloud is a complex region with several interacting sub-populations, and they are the ultimate source of Jupiter family and long-period comets (also known as nearly isotropic comets). Irregular satellites of the giant planets, Trojan asteroids of Jupiter and Neptune, and even certain main belt asteroids are part of this cosmogonically linked small-body complex. Recently, unexpected small body properties, which can help us to constrain the formation and evolution of the solar system, have been discovered. They range from ring systems around some small bodies, to comets showing activity at large heliocentric distances, and to the existence of interstellar objects, with two such now known.

Q2.6a How Did the Dynamical Structure of the Trans-Neptunian Belt Originate?

During the giant planets’ migration and scattering (see Q2.4 and Q3.3), Neptune’s outward movement transformed the early trans-neptunian belt. The discovery of more than 3,000 TNOs in the past 30 years has revealed that this migration dynamically sculpted the trans-neptunian belt into four main groups of objects: (1) classical, (2) resonant, (3) scattered disk, and (4) detached (Gladman et al. 2008). Possibly the Oort cloud is a result of this scattering as well (e.g., Vokrouhlický et al. 2019; Morbidelli and Nesvorný 2020). Numerical simulations provide estimated sizes for these populations, relative to the size of the primordial Kuiper belt, and are given below (Vokrouhlický et al. 2019).

The classical TNOs are located between the 3:2 and 2:1 mean motion resonances with Neptune and reside in the semimajor-axis range of 42–47 AU. The classical group is divided into two sub-populations; the dynamically cold classicals with low eccentricities and inclinations and the hot classicals with higher eccentricities and inclinations.

The cold classicals are some of the most pristine bodies in the solar system. These bodies formed far enough away from Neptune to avoid substantial disturbances as the planet migrated outward (Nesvorný 2018). They are likely to be indigenous to this region. Their initial mass is debated, but it seems probable that the ensemble was relatively small compared to the primordial Kuiper belt. The cold classicals tend to be smaller, redder, and have a higher fraction of wide binaries compared to the hot classicals, suggesting that none of these bodies were affected by Neptune close encounters (Noll et al. 2020). The latter is consistent with dynamically brief events that may have led to ice giant ejection (e.g., Figure 5.7). Arrokoth is a resident of this population (McKinnon et al. 2020).

The hot classicals are thought to have originated in the primordial Kuiper belt and represent ~5 3 102⁴ of that population (e.g., Nesvorný and Vokrouhlický 2016). They were pushed outward by Neptune scattering events and by interactions with its mean motion resonances. Further interactions with Neptune’s resonances, secular resonances, and Pluto-sized objects allowed some to drop out of resonances while also achieving low enough eccentricities to be captured between 42 AU and 47 AU.

The resonant TNOs were trapped within Neptune’s mean motion resonances as Neptune migrated outward (e.g., Nesvorný and Vokrouhlický 2016). The size of the captured population in each resonance was regulated by the speed of Neptune’s outward migration and interactions with numerous Pluto-size bodies along its path. The resonant TNOs are physically similar to the hot classicals and contain a few 3 102⁴ of the primordial Kuiper belt. The most prominent resonant population is within Neptune’s 3:2 resonance, which contains Pluto-Charon, while smaller populations are in additional resonances (e.g., 2:1, 5:3, 7:4, and 5:2).

The major questions for the formation of the various trans-neptunian populations relate directly to Q2.4: precisely when and how did migration of the giant planets occur, and what were the properties (mass, size-frequency distribution, orbital distribution, composition, etc.) of the ancestral planetesimal or cometesimal disk that was the ultimate source of these populations? How do the properties of the various trans-neptunian populations (e.g., number, size-frequency distributions, compositions and colors, and binarity) constrain models of their formation? How have these subpopulations evolved through time? What accounts for the apparent “edge” of the classical Kuiper belt at semimajor axes of ~47 AU? Below the committee addresses the scattered disk and detached populations specifically, in Q2.6b and Q2.6c, respectively.

Q2.6b How Did the Oort Cloud and Scattered Disk Form?

It has been long inferred from the existence of long-period comets (LPCs) that the Sun is surrounded by a spherical cloud of icy small bodies, called the Oort cloud (e.g., Peixinho et al. 2020). It contains billions of objects and is located between an inner boundary of ~2,000–5,000 AU and outer boundary of ~50,000–100,000 AU. All are far beyond the reach of current telescopes.

The Oort cloud has long been thought to have been populated by planetesimals ejected from the giant planet region. Recent models suggest that much of the Oort cloud was constructed during the giant planet instability (see, e.g., Nesvorný 2018). Here small bodies scattered into the giant planet zone from the primordial Kuiper belt experienced giant planet encounters, with many placed onto orbits with very large semimajor axes. These bodies then had their perihelia increased by a combination of gravitational perturbations from galactic tides and close passing stars. Approximately ~5 percent of the primordial Kuiper belt still exists within the Oort cloud (see Vokrouhlický et al. 2019). Oort cloud objects were also potentially captured from the protoplanetary disk(s) of other star(s) when the Sun was still in its stellar birth cluster (Levison et al. 2010). A combination of the scattering and capture scenarios is plausible.

Scattered disk objects (SDOs) are bodies with semimajor axes between ~30 and ~1,000 AU on unstable crossing orbits with Neptune (Nesvorný 2018). SDOs were scattered outward from the primordial Kuiper belt during Neptune’s outward migration, leaving them on highly eccentric and inclined orbits. All will eventually be sent deeper into the giant planet zone, making them the primary source of the Centaurs (objects in chaotic orbits between Jupiter and Neptune) and Jupiter Family Comets. Their true population is thought to be larger than the classical and resonant TNOs, but because they are faint, only a few hundred SDOs have been discovered to date near their perihelion. This number should be substantially increased with telescopes coming online (e.g., Vera C. Rubin Observatory). Numerical models suggest that the SDO population includes a factor of ~300 times more objects than have been identified to date (Vokrouhlický et al. 2019). Interactions with Neptune led to the loss of ~two orders of magnitude of SDOs to the giant planet zone over billions of years. Accordingly, SDOs are the primary source of impactors on the giant planet satellites.

Note that it is possible that some objects from the giant planet zone or the inner solar system could have been passed outward through the giant planet region, had a weak interaction with Neptune and achieved SDO orbits. These objects may be rare, but they could potentially be identified by their unusual colors and spectra.

The major questions for the formation of the scattered disk relate to those in Q2.6a. For the Oort cloud, the major questions are its dynamical structure, how it relates to the more distant Detached population (Q2.6c), and whether it is truly, or mainly, sourced from the same primordial planetesimal disk as the SDOs.

Q2.6c How Did the Most Distant (e.g., Detached) Trans-Neptunian Objects Form?

Owing to the extreme distances of the detached TNOs, this population—which is transitional between the SDOs and the Oort cloud—is the least constrained. The detached population, defined as being beyond Neptune’s

gravitational influence and not on a Neptune-crossing orbit, has several sub-groups, with the extreme TNOs having perihelion distances between ~40 and ~55 AU and the Inner Oort cloud objects having perihelia beyond ~65 AU (Nesvorný 2018). So far, no TNO has been found with a perihelion between ~55 and ~65 AU (Sheppard et al. 2019).

Here the detached TNOs are divided into inner and outer parts. In the inner portion, many have semimajor axis values that are slightly on the sunward side of mean motion resonances with Neptune, and the objects are too close to the Sun to have been influenced by passing stars within the solar system’s stellar birth cluster. These constraints indicate that their origin may have been analogous to the hot classicals (see Q2.6a), in that the inner bodies were scattered outward by Neptune as it migrated through the primordial Kuiper belt; eventually, however, some interacted with distant Neptune mean-motion resonances and/or Kozai resonances, either of which lowered their eccentricities enough to remove them from Neptune-crossing orbits (Gomes 2011). These bodies then fell out of resonance while Neptune was still migrating outward, possibly when Neptune experienced tiny kicks from encounters with Pluto-size bodies (Lawler et al. 2019).

The outer portion, or the extreme KBOs, have large perihelia and semimajor axes that are too far from Neptune for Neptune’s mean motion resonances to influence their orbits. Several hypotheses have been proposed to explain how these bodies escaped a Neptune-crossing orbit: (1) a passing star from the solar system’s stellar birth cluster could scatter TNOs onto extreme TNO orbits, (2) the collective gravity of a distant massive, small body population influenced the objects and affected their orbits, or (3) a very distant planet (or planets) gravitationally shaped the outer edge of the trans-neptunian belt.

For the latter mechanism, only a handful of distant TNOs have been discovered so far, but some argue they display evidence for orbital clustering, which would be surprising if true. This was the original evidence used to suggest the hypothesis of a super-Earth planet (usually called Planet X or Planet 9) in the outer solar system (Figure 2-19 in the Small Body section of Chapter 2, Trujillo and Sheppard 2014; Batygin and Brown 2016). Alternatively, the clustering could be a by-product of observational selection effects by TNO surveys. To resolve this question, we either need to find enough extreme TNOs that the issue of observational bias can be ruled out or we need to discover the putative planet itself (Trujillo 2020).

Q2.6d How Did Scattering, Capture, and Ejection Affect the Small Body Populations?

Two critical components of the giant planet instability are that (1) giant planets are capable of having encounters with one another, which can cause them to migrate via gravitational kicks or “jumps,” and (2) these encounters are happening concurrently with Neptune’s migration through the primordial Kuiper belt, which sends approximately 20 Earth masses of TNOs into the giant planet zone. Some TNOs could have been located at the right place and time to be captured within stable reservoirs across the solar system by three-body reactions during this time (see also Question 3). Study of current populations in these reservoirs may, for example, provide constrains on the mass of the primordial trans-neptunian population (Nesvorný 2018).

The most well-known captured populations are the Trojans of Jupiter and Neptune, orbiting around the L4 and L5 Lagrange points located 60 degrees in front of and behind each planet, respectively. (Such orbits for Saturn and Uranus are unstable.) The most remarkable dynamical property of the Trojans is that their inclinations range from 0 degrees to 35 degrees. This distribution challenges most origin models that require the Trojans to be captured with low inclination orbits (Emery et al. 2015). However, objects ejected from the primordial Kuiper belt have high inclinations, and giant planet encounters can capture a small fraction of them within Jupiter and Neptune’s L4 (i.e., 60 degrees ahead) and L5 (i.e., 60 degrees behind) locations (Vokrouhlický et al. 2019), resolving this issue. Other captured populations are discussed in Question 3.

Giant planet encounters can also lead to irregular satellite capture (see Question 8, Chapter 11). Models show that several × 10^–8 of the primordial Kuiper belt population can be captured around Jupiter, Saturn, Uranus, and Neptune. The physical properties of the Trojans and irregular satellites should be the same as the resonant TNOs and the hot classicals, but the irregulars are thought to be heavily collisionally evolved (Bottke et al. 2010). Surveys have demonstrated that the Jupiter/Neptune Trojans and irregular satellites have similar surface colors and thus could share the same origin, but they lack the very red bodies found in the scattered disk, resonant TNOs, and

hot/cold classicals (Jewitt 2018). One possible explanation is that the transition to very red objects occurs beyond 30 AU in the primordial Kuiper belt, either from sublimation-driven surface depletion in some organic molecules or from collisional evolution (Nesvorný et al. 2020). The alternative is that the existing giant planet instability model is missing something.

The origin of, and dynamical processes affecting, these small body populations (e.g., Trojans, irregular satellites and TNOs) at different heliocentric locations have been, and should continue to be, studied by comparing the results of spacecraft missions and ground- and space-based telescopic observations. Combining these measurements and observations with state-of-the-art theoretical and numerical modeling offers the surest path to more complete understanding.

Q2.6e What Do Active or Unusual Phenomena Among Distant Small Body Populations Tell Us About Accretion in the Outer Solar System?

The Centaur population is mostly composed of objects from the scattered disk and a minority of TNOs from the 2:1 and 3:2 Neptune’s resonances. Through stellar occultations, ring systems (unpredicted by theory) were discovered around two Centaurs (Ortiz et al. 2020). A second interesting feature is that Centaurs can be active from a low level up to major outburst-type activity. Active Centaurs have been observed beyond 10 AU, but the physical process leading to activity is still unknown and could include (1) water-ice crystallization from an initially amorphous state providing energy to drive activity; (2) sublimation of super volatiles such as methane, carbon monoxide, or nitrogen ice; and/or (3) rotational fission. Both the distant outgassing and ring systems are unobserved among the comets—a Centaur’s final state—nor do inner solar system objects possess rings. Owing to their unusual physical characteristics and because they are in transition from the outer to the inner solar system, Centaurs are of great interest for future missions (Harris et al. 2020).

In contrast, Manx comets are on long-period orbits where other known objects are active, yet the Manx comets show no activity. They could be extinct comets that have had many volatile-depleting passages near the Sun, they could be interloper objects, perhaps formed near the main belt and then placed on extremely distant orbits through an as-yet-unknown process, or they could be planetesimals from the giant planet zone with limited initial volatile content (Meech and Castillo-Rogez 2021).

Q2.6f What Can Interstellar Objects Tell Us About the Formation and Early Evolution of Our Solar System and Others?

Although the possibility of interstellar objects (ISOs) has long been hypothesized, only recently have two been discovered passing through the solar system: 1I/‘Oumuamua and 2I/Borisov (see Question 3). These objects likely originated in an exoplanetary system and were flung out (possibly shortly after their formation, if the solar system is any guide) by an encounter with a giant planet or their host star(s).

Obvious ISOs have distinctly hyperbolic orbits. Some objects in the Oort cloud, however, may have been captured from other stars in the solar system’s stellar birth cluster. These ISOs, taking the form of near-isotropic comets, would not have hyperbolic orbits. Thus, identification of ISOs may also involve other diagnostics, such as a body’s chemical properties (e.g., deuterium to hydrogen (D/H) and other isotopic ratios, presence or outgassing of extremely volatile compounds) and/or physical properties (e.g., surface features or shapes) unlike those of the small bodies in the solar system (Meech et al. 2017). A caveat is that ISOs in the Oort cloud from the solar system’s stellar birth cluster, derived ultimately from the same natal molecular cloud, may turn out to be indistinguishable from comets native to the solar system.

The primary difficulty with observation of ISOs is that they only spend a short time in the solar system, requiring rapid-response efforts from ground- and space-based telescopes, and/or a mission designed and launched even before the ISO is discovered (such as the Comet Interceptor mission being developed by ESA and JAXA). It is expected that in the coming decade, the discovery rate of ISOs will increase dramatically. By discovering and characterizing additional ISOs, we will be able to compare them to the small body populations in the solar system, and potentially constrain their birthplace.

Strategic Research for Q2.6

• Determine the rotational, physical, chemical, geological, and interior properties of a diversity of primitive small bodies (TNOs) in the outer solar system with spacecraft and/or ground-/space-based observations.
• Determine the rotational, physical, chemical, geological, and interior properties of a diversity of irregular satellites, Trojans, Centaurs, and comets as well as investigate their ring system(s) and/or activity with spacecraft and/or ground-/space-based observations.
• Characterize the rotational, physical, chemical, geological, and interior properties of interstellar objects and comparison with small bodies in the solar system with spacecraft and/or ground-/space-based observations.
• Improve our understanding of the formation and evolution of the early solar system (and by extension, exoplanetary systems), planetary migration, including undiscovered planet(s) with improved numerical simulations, theoretical models, and/or ground-/space-based observations.

SUPPORTIVE ACTIVITIES FOR QUESTION 2

Improvement of computational capabilities for advanced numerical work (e.g., greater resolution in space and time, increased number of particles, faster computation and visualization, and machine learning).

Telescopic observations (spectroscopy, color, light curve, stellar occultation, mutual event, binarity study, etc.) of small bodies across the solar system to infer their properties, ideally utilizing both ground- and space-based assets across multiple wavelengths.

REFERENCES