The Trans-Neptunian Solar System

By Dina Prialnik and Leslie Young

The Trans-Neptunian Solar System is a timely reference highlighting the state-of-the-art in current knowledge on the outer solar system. It not only explores the individual objects being discovered there, but also their relationships with other Solar System objects and their roles in the formation and evolution of the Solar System and other planets. Integrating important findings from recent missions, such as New Horizons and Rosetta, the book covers the physical properties of the bodies in the Trans-Neptunian Region, including Pluto and other large members of the Kuiper Belt, as well as dynamical indicators for Planet 9 and related objects and future prospects.

Offering a complete look at exploration and findings in the Kuiper Belt and the rest of the outer solar system beyond Neptune, this book is an important resource to bring planetary scientists, space scientists and astrophysicists up-to-date on the latest research and current understandings.

• Provides the most up-to-date information on the exploration of the Trans-Neptunian Solar System and what it means for the future of outer solar system research
• Contains clear sections that provide comprehensive coverage on the most important facets of the outer Solar System
• Includes four-color images and data from important missions, including New Horizons and Rosetta
• Concludes with suggestions and insights on the future of research on Trans-Neptunian objects

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 28, 2019
• ISBN: 9780128175255

Book preview

600–609.)

Chapter 1

Introduction: The Trans-Neptunian belt—Past, present, and future

Julio A. Fernández    Department of Astronomy, Faculty of Sciences, Universidad de la República, Montevideo, Uruguay

Abstract

We present a historical account of advances in the study of the Trans-Neptunian (TN) population since Pluto’s discovery until present. Early cosmogonic ideas about a TN belt or ring assumed that it was the leftover at the edge of a protoplanetary disk where densities were too low for the planetesimals to accumulate into a single planet. Early attempts to constrain its mass were based on the perturbations that such a belt could cause on the motions of Uranus, Neptune, or comets that penetrate it like 1P/Halley. It was shown later that Jupiter family comets (JFCs) are likely to come from a flat source at the edge of the planetary region. JFCs then became the smoking gun that uncovered the existence of a large population of TN bodies arranged in a flat near-ecliptic distribution. We will next describe the discovery of TN objects, some rivaling Pluto in size, and how they started to challenge Pluto’s status as the ninth planet of the solar system, that finally led to the revision of the definition of planet. Finally, we will briefly describe our current understanding of the population and nature of TN objects and some of the hot issues under discussion.

Keywords

Trans-Neptunian belt; Accretion processes; Planet definition; Trans-Neptunian objects: dynamics; Trans-Neptunian objects: physics

Acknowledgments

The authors gratefully acknowledge financial support from Project CSIC Grupo I+D 831725 Planetary Sciences.

1.1 The solar system beyond Neptune: The search for planet X

The discovery of Neptune in 1846 stimulated the search for other more distant planets, motivated by the allegedly irregularities in the motion of Uranus that required another more distant planet besides Neptune. Percival Lowell devoted several years to search for this hypothetical planet that he called planet X until his death in 1916. The search was restarted at the Lowell Observatory in Flagstaff, Arizona, in 1929 by a young observer Clyde Tombaugh who succeeded in discovering Pluto on February 18, 1930. He continued his search for other Trans-Neptunian (TN) planets for another 13 years but none appeared. It seemed that the inventory of planets was complete, so Pluto marked the outer edge of the solar system. A detailed chronicle of Pluto’s discovery can be found in Davies et al. (2008).

Right after its discovery, Pluto was hailed as the ninth planet with an uncertain mass but that, in principle, could be as large as the Earth’s, depending on its albedo and bulk density. Yet, it showed significant differences with the other planets that moved on regular, near-circular, and near-coplanar orbits. Pluto’s orbit was indeed so eccentric that it crossed that of Neptune, and its inclination was of about 17 degrees, far higher than those of the other planets. With such an orbit the puzzling question was why Pluto had not been ejected by Neptune’s perturbations throughout the solar system lifetime. Lyttleton (1936) suggested that Pluto was a former Neptune’s satellite that moved with Tritton in direct regular orbits around the planet. Tidal friction brought their orbits close together allowing close encounters between these satellites. One of such encounters led to the ejection of Pluto and the reversal of Tritton’s orbit. The ejected Pluto remained in an orbit around the Sun crossing that of Neptune. Where Lyttleton’s hypothesis proved to be true, it would have exposed a curious situation: the changing role of Pluto during its lifetime, being at the beginning a satellite to become later a planet.

The puzzle about the stability of Pluto’s orbit was finally solved by Cohen and Hubbard (1965), who carried out numerical computations for 120,000 year showing that the orbit of Pluto librates relative to that of Neptune around the 2:3 commensurability, in such a way that Pluto can never approach Neptune to less than 18 AU.

Nicholson and Mayall (1931) estimated Pluto’s mass at about two-thirds that of the Earth from the presumed perturbations that this planet caused on Neptune and Uranus. Later Kuiper (1950) measured Pluto’s diameter with the 200-in. Palomar telescope, finding an apparent diameter of 0″.20 ± 0″.01 or 0.46 of the Earth’s diameter which would imply a rather low visual albedo of 0.17. Yet Cruikshank et al. (1976) found evidence for methane frost on Pluto’s surface suggesting a much higher albedo than previously thought, probably above 0.4. Therefore, Pluto should be much smaller, perhaps even smaller than the Moon. In 1977, a large moon was discovered around Pluto by Christy and Harrington (1978) that was later named Charon. The motion of the Pluto-Charon system around their center of mass allowed to estimate its mass. The result of the combined mass was about 0.0017 M⊕ (Earth’s mass), thus confirming that Pluto was really very small, much smaller than the Moon. New astrometric measurements of Pluto, Charon, and the four other satellites discovered later (Styx, Nix, Kerberos, and Hydra) allowed to refine Pluto’s mass to 0.0022 M⊕ (Buie et al., 2006; Stern et al., 2018).

1.2 Early cosmogonic ideas

The discovery of Pluto prompted Leonard (1930) to speculate about the existence of a population of ultra-Neptunian and ultra-Plutonic planets on the basis that the Sun’s gravitational field extends far beyond Pluto’s orbit. But the first thorough cosmogonic model was developed by Edgeworth (1938, 1943, 1949) who assumed that the early Sun was surrounded by a vast disk of meteorites that extended beyond the orbits of the planets, retaking the idea of a Laplacian disk where planets formed. While in the inner regions of the disk densities were high enough to allow the formation of big planets by gravitational collapse, in the outer edge of the disk densities were too low for this to happen. He went on to argue that the particulate fluid became turbulent favoring the agglomeration of dust particles within eddies into comet-size bodies, described as 10-km size heap of gravel loosely held together by its own gravity. The total number of comets would have been of about 200–2000 million, amounting to a total mass of a few tenths M⊕ scattered over a region between ∼10¹⁰ km and 3−4 × 10¹⁰ km (∼70 to ∼200−300 AU). Edgeworth conjectured that this vast reservoir was the source of the observed comets. It is interesting to mention that Edgeworth’s ideas were recently taken by Johansen et al. (2007) who invoked turbulence to explain two opposite effects. On one side, the inhibition of particle sedimentation in the midplane of the protoplanetary disk prevented densities from growing to the point of triggering gravitational collapse into kilometer-size planetesimals. On the other side, turbulence can create transient high-pressure regions that can grow by an order of magnitude by a streaming instability driven by the relative flow of gas and solids. These dense regions can collapse into planetesimals from comet to dwarf-planet size bodies.

In an independent work, Kuiper (1951) also discussed a model of solar system formation in which he envisaged that beyond Neptune densities were too low for the solid material to collect into a single massive body, leaving instead a swarm of planetesimals spread in a ring between ∼30 and 50 AU. Some years later, and seemingly independent from the previous authors, Cameron (1962) argued that in a massive solar nebula a large mass of nebular gas was shed beyond Neptune, so a large amount of solid material must remain there as comets. Inspired by the work of Cameron (1962), Whipple (1964) developed further the idea of a stable belt of comets beyond the orbit of Neptune. In order to account for observed disturbances in the motion of Neptune’s latitude, he estimated that the belt should have a mass of 10–20 M⊕ if it were located at 40–50 AU from the Sun. Later Standish (1993) showed that such disturbances in Neptune’s motion were spurious so Whipple’s result was invalidated. Whipple and colleagues tried another approach by using 1P/Halley as a natural probe since its aphelion lies right in the putative belt. No perturbations were detected on 1P/Halley’s motion which allowed the authors to set an upper limit of 0.5 M⊕ if the belt was located near the invariable plane at 40 AU, or 1.3 M⊕ at 50 AU (Hamid et al., 1968). In this early stage, Whipple was probably the one who did the most to disseminate the idea of a TN belt in a quite explicit way as it is shown by his sketch of the putative belt (Fig. 1.1). It is interesting to mention that Whipple did not believe that this belt could contribute appreciably to the observable comets, rather he thought on a belt dynamically frozen in the outer edge of the planetary system beyond the reach of planetary perturbations. In the same sense, Delsemme (1973) argued that because belt comets had circular orbits, they could not be the source of the visible comets which must come from the Oort cloud.

Fig. 1.1 Whipple’s concept of a Trans-Neptunian ring or belt beyond the reach of planetary perturbations ( Whipple, 1972).

1.3 The Jupiter family comet connection

Until the 1970s, the discussion about the existence of a TN belt was entirely theoretical since there was no observational evidence supporting it. For instance, the concept of the Oort cometary cloud was developed to explain the anomalous excess of near-parabolic orbits (semimajor axes a > 10⁴ AU) among the observed long-period comets (LPCs). From this observation, Oort (1950) inferred that a vast spherically distributed comet reservoir should exist at several 10⁴ AU subject to stellar perturbations. But there seemed to be no observed comet population in the Earth’s vicinity coming from a flat source beyond Neptune. If this were the case, how could we learn about a putative belt safe from planetary or stellar perturbations, and thus dynamically frozen?

At the end of the 1970s, I became interested in the problem of the evolution of comets and their transfer from long-period orbits to short-period orbits. The standard theory at that time was that short-period comets (SPCs) were the end product of the dynamical evolution of Oort cloud comets captured by Jupiter (the classical Laplace’s hypothesis), with the difference that the single-encounter capture was substituted by the multiple-step capture by Jupiter and the other Jovian planets (Kazimirchak-Polonskaya, 1967; Everhart, 1972). I found rather puzzling to have an evolutionary path connecting both populations since LPCs have a more or less isotropic distribution of orbital inclinations whereas SPCs show a flat distribution strongly concentrated to the ecliptic plane. How was it possible to pass from an isotropic distribution to a flat distribution of inclinations? Everhart (1972) tried to solve the problem by assuming that SPCs were the end product of the evolution of Oort cloud comets through multiple encounters with Jupiter. By setting an upper limit of 2000 revolutions to the evolution, he found that captures to short-period orbits occurred for comets within the region of perihelion distances 4 < q < 6 AU and inclinations i < 9degrees. The cut at 2000 revolutions seemed to be tailored to allow only low-i Oort cloud comets to reach short-period orbits. But what would happen if we extended the evolution to a greater number of revolutions? In that case, many high-inclination and retrograde comets would leak into short-period orbits in conflict with the observations. One could argue that the limit of 2000 revolutions had a physical explanation in terms of the finite physical lifetimes, but this argument had no quantitative basis.

Vaghi (1973) pointed out to another inconsistency in Everhart’s hypothesis: the Tisserand parameter with respect to Jupiter, TJ, so they could not have come, in principle, from original parabolic orbits. Vaghi’s objection is illustrated in Fig. 1.2, where we plot the Tisserand parameters of different comet populations: SPCs, LPCs, and intermediate-period or Halley-type comets (HTCs), versus their orbital energies (represented by the reciprocal of the original semimajor axis, 1/a, as a proxy). Note that the figure plots JFCs with orbital periods P < 20 years, while SPCs in Everhart’s nomenclature were those with P  AU−1). This is a minor difference that does not change the arguments given here. We can see that SPCs (open circles at the upper right corners of the panels) are clearly detached from the remaining populations represented by filled circles. Another problem came from the capture efficiency from Everhart’s scheme, Joss (1973) found that the rate of captures into short-period orbits would be 40,000 times too low to keep the current observed population of SPCs in steady state. To overcome this shortcoming, Delsemme (1973) argued about the existence of a population of 30,000–100,000 intermediate-period, low-inclination comets with perihelia come between 4 < q < 6 AU as the immediate source of SPCs. Yet Delsemme did not explain how this population was formed and maintained. The ultimate question was: what was the origin of the flat, intermediate-period comet source?

Fig. 1.2 Tisserand parameter T J versus reciprocal semimajor axis for comets with q < 2 AU ( upper panel ) and 2 < q < 5 AU ( bottom panel ). Jupiter family comets with periods P < 20 years ( open circles ) have in general T J , while intermediate-period comets and long-period comets ( filled circles ) have in most cases T J < 2 ( Fernández, 2005).

We rechecked the efficiency of the transfer process from near-parabolic to short-period orbits by considering Everhart’s capture window and also the contribution from captures by the other Jovian planets (Fernández, 1980). In agreement with Joss’s conclusion that the capture efficiency would be extremely low, we found that for every captured SPC, 300 near-parabolic comets would be ejected per year, which would give ∼1.3 × 10¹² throughout the solar system lifetime at a steady loss rate. This is an enormous wastage of matter, one order of magnitude greater than the Oort cloud population estimated by Oort. By applying Okham’s razor principle of choosing a theory that requires the smaller number of hypothesis, we thought: Why not to assume that SPCs come from a flat source? After reading the works by Kuiper and Whipple, we came to the conclusion that the TN belt should be a more suitable source of SPCs. This idea was developed in a paper (Fernández, 1980) where from a numerical model we showed that weak perturbations by Pluto-size or Moon-size bodies could drive some TN objects (TNOs) near Neptune’s orbit. Thus, contrary to what Whipple (1972) and Delsemme (1973) thought of a dynamically frozen TN belt, we found that a steady leakage of bodies to Neptune’s gravitational influence zone could take place. Once under the control of Neptune, the bodies would be either ejected or handed down to the next planet inside (Uranus) and so on until ending up under the gravitational control of Jupiter. The reasoning was that if for each one of the Jovian planets half of the bodies were handed down to the next planet inside (or to the inner planetary region in the case of Jupiter) and the other half were ejected (see, e.g., Everhart, 1977), the efficiency of the transfer process from the TN belt to short-period orbits would be: (1/2)⁴ = 0.0625. This back-of-the-envelope estimate turned out to be too low, later numerical simulations by Duncan et al. (1988) rose the transfer efficiency to 0.17, and the Duncan et al.’s (1995) estimate rose again the ratio to 0.34. The transfer efficiency turns out to be two orders of magnitude greater than that from an isotropic flux of near-parabolic comets.

1.4 The naming controversy

The TN belt did not have a proper name until Duncan et al. (1988) coined the term Kuiper belt that was rapidly adopted by the scientific community at large. The name seems to be arbitrary and unfair with other researchers. Regrettably, Edgeworth’s pioneer work on a TN belt of comets was overlooked until Bailey and Stagg (1990) mentioned Edgeworth’s (1949) paper. McFarland (1996) advocated strongly in favor of recognizing Edgeworth’s important contribution to the subject, which led Davis and Farinella (1997) to coin the name Edgeworth-Kuiper belt. Even though for a time both names coexisted in the literature, the latter has faded with time, perhaps because it is just simpler to use a single name. But the story is more complex since there were other important players. As we saw before, Whipple—inspired by Cameron’s work—was possibly the one who best tried to convey the idea of a TN belt in a more explicit way and to constrain its mass (at least the article by Whipple (1972) was the first to call the writer’s attention about this subject).

But, after all, why does the belt have to have a proper name? For instance, does the asteroid belt have a proper name? No, it does not. It could have been named, for example, the Piazzi belt (after Ceres’s discoverer) or Olbers belt (after the first one that proposed a theory about the origin of asteroids) but, with good criterion, it was left unnamed thus preventing potential controversies. In summary, if we choose a certain name, surely we will be unfair with others, so why not to just simply adopt the neutral term Trans-Neptunian belt (TN) and Trans-Neptunian object (TNO)? These will be the terms that we will be using in the rest of the chapter.

1.5 The discovery

In the late 1980s, the situation was ripe to search for TNOs inspired in the previous theoretical and computational studies. Kowal (1989) searched 6400 square degrees photographically to approximately mV = 20 which led to the discovery of the Centaur 2060 Chiron in 1977 but no TNOs. Other deep searches were carried out by Luu and Jewitt (1988) and Levison and Duncan (1990), again with negative results. It was a matter of perseverance, access to the medium- and large-size telescopes, and better CCD cameras (that at the beginning of the 1990s were replacing the old photographic plates) to detect these elusive bodies. Observing with the 2.2 m-telescope of Mauna Kea, Hawaii, Jewitt and Luu (1993) finally succeeded in finding a couple of TNOs: 15,670 Albion and a few months later 1993 FW. Both objects were found to have low-eccentricity orbits and semimajor axes of a = 43.82 AU and a = 44.07 AU, right at the distances where the TN belt was expected to be.

Once an observing campaign was successful, it was just a matter of time to find new members. This was what actually happened, so the population of discovered TNOs has been steadily growing until reaching a number of 2183 at the end of 2017 (see Fig. 1.3). The discovery rate of TNOs has been particularly high during the period 1999–2005 thanks to the Deep Ecliptic Survey a program carried out from Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory (Elliot et al., 2005) that yielded more than 300 TNOs and Centaurs with well-determined orbits, and again during 2013–17 thanks to the Outer Solar System Origins Survey (OSSOS) program that used the Canada-France-Hawaii Telescope, surveying 155.3 square degrees of sky near the invariable plane to a limiting (red) magnitude m(R) = 25.2 providing more than 800 discoveries (e.g., Bannister et al., 2018).

Fig. 1.3 The discovery rate of TNOs with perihelion distances q > 30 AU during 1992–2017. Data taken from JPL Solar System Dynamics.

1.6 Dynamical structure and transfer mechanisms

The increasing computer power prompted several authors to carry out massive numerical integrations to build dynamical maps of stable and unstable regions in the TN region ( AU (Fig. 1.4). Early orbit computations of the first discovered TNOs showed that about 40% moved in Pluto-like orbits, that is, in the 2:3 MMR with Neptune. They have been called Plutinos  AU because the overlapping of MMRs causes large-scale chaos, and Neptune’s proximity increases the instability of bodies located there (Morbidelli et al., 1995). The slow diffusion of TNOs located at the borders of chaotic zones causes a slow leakage of TNOs to Neptune-crossing orbits which feeds a transient population of bodies in the outer planetary region that were called Centaurs (Stern and Campins, 1996). A fraction of them will end up in the inner planetary region, the majority as JFCs and a small fraction as HTCs (Fernández et al., 2016).

Fig. 1.4 Dynamical erosion in the planetary zone and the TN belt depicted through the survival times of test particles with semimajor axes covering the range 5 < a < 45 AU. It is seen that for a > 43 AU the particles survive for the whole integration period of 200 Myr ( Holman and Wisdom, 1993).

Jewitt et al. (1998) characterized three different dynamical classes: (1) Classical TNOs, which occupy low-eccentricity (e  AU, they are estimated to constitute ∼70% of the observed population. They are subdivided according to their inclinations into the cold (i < 5degrees) and hot (i > 5degrees) populations. We will see below that both populations show distinct features. (2) Resonant TNOs, which occupy MMRs with Neptune, in particular the 2:3 (a ≈ 39.4 AU) and comprise ∼20% of the known objects. (3) Scattered disk objects (SDOs), which possess the most extreme orbits, with perihelion distances 30 ≤ q ≤ 40 AU, median semimajor axis a ∼ 90 AU and eccentricities e ≈ 0.6, and comprise about 10% of the known TNOs. Yet, there is a strong bias against the detection of SDOs since they are on average more distant and therefore fainter. The actual population of SDOs should therefore be substantially larger than the observed one, probably of similar size or slightly larger than the classical (cold + hot) population (Trujillo et al., 2001; Adams et al., 2014). Because SDOs approach Neptune near perihelion, they are subject to weak perturbations by this planet that excite their eccentricities to higher and higher values. It is then likely that an important fraction of them will end up in the Oort cloud (Fernández et al., 2004).

Besides the dynamical classes described earlier, we have also found detached SDOs with q > 40 AU which are too far away for planetary perturbations to have excited their eccentricities by the classical diffusion mechanism. Yet the Kozai mechanism (coupling between e and i when ω librates around a certain value) can force high q up to values around 70 AU (Gomes et al., 2005). Fig. 1.5 shows the distribution of all the discovered TNOs in the parametric plane (a, e). Besides the TNOs heavily concentrated in the 2:3 MMR (a = 39.4 AU), there is a quite notorious concentration around the 1:2 MMR at a = 47.7 AU. The OSSOS survey has enriched the observed sample of TNOs, in particular highlighting some other clusterings of TNOs around other MMRs, such as 3:4 (a = 36.4 AU), 4:7 (a = 43.7 AU), and 2:5 (a = 55.4 AU) (Bannister et al., 2018).

Fig. 1.5 Distribution of the discovered TNOs with q > 30 AU in the parametric plane eccentricity versus semimajor axis. We can see several groupings: classical TNOs (cold and hot), TNOs in the 2:3 and 1:2 MMRs (Plutinos and Twotinos), and SDOs. We can also see some detached TNOs. There are some distant TNOs with a > 120 AU that are not plotted in the figure. Data taken from JPL Solar System Dynamics.

1.7 Size distribution and massive TNOs

Early results showed that the cumulative luminosity function (CLF) could fit to a linear relation

   (1.1)

where Σ(mR) is the density of TNOs per square degree near the ecliptic brighter than the apparent (red) magnitude mR, and C and α are constants. Trujillo et al. (2001) derived for the CLF a slope: α = 0.63 ± 0.06.

By making appropriate corrections for heliocentric and geocentric distances, we can convert the CLF into a cumulative size distribution (CSD) which will follow a power law of the kind

   (1.2)

where NR(> R) is the number of objects with radii greater than R. Bearing in mind that α is related to the exponent s through s = 5α we obtain s = 3.15 ± 0.3 if we take the value of α derived by Trujillo et al. The conversion from luminosity to radius implies to adopt an average albedo for the TNOs. Typical geometric albedos of all TNOs and Centaurs are about 0.07–0.08 (Stansberry et al., 2008), though there is a large dispersion among individual objects and albedos seem to be correlated with color. Lacerda et al. (2014) distinguished the two groups: one with dark neutral surfaces and albedos ∼0.05, and other with bright red surfaces and higher albedos ∼0.15.

More recently it was found that the CLF is better represented by a broken power law for both the hot population (hot classicals + SDOs) and cold classicals (Fuentes et al., 2010; Adams et al., 2014; Fraser et al., 2014). Yet the slopes and the break magnitude, Hb(radius Rbc (radius Rbh ≃ 72 km) for the hot population. The radii Rbc and Rbh are derived by adopting an average albedo p = 0.07. As regards the faint-end slopes both populations show a similar value α2 ∼ 0.2. The masses of the hot and cold populations are estimated to be ∼0.01 and 3 × 10−4 M⊕, respectively. How much mass is enclosed in the scattered disk and the detached scattered disk is still very uncertain given the high degree of incompleteness of the observed sample and the difficulty to estimate biases.

The size distribution we observe today must arise from the combination of two processes: (1) The primordial distribution of planetesimals in the protoplanetary disk and their growth to massive bodies via runaway and oligarchic accretion, and (2) their later collisional evolution. From theoretical models  km while for greater sizes it was keptprimordial. It is then possible to associate the bimodal CLF found before with two different evolutionary stages: one primordial and one collisionally evolved.

Most of the largest TNOs seem to have been detected by now, some of them rivaling in size with Pluto, as are the cases of Haumea with an estimated diameter D ≃ 1600 km, Makemake with D ≃ 1430 km, and Eris with a size very similar to Pluto’s and slightly more massive. All the largest TNOs belong to the hot population (either the hot classical or the SD). Schwamb et al. (2014) estimated that the inventory of large classical TNOs brighter than apparent (red) magnitude m(R) ∼ 19.5 is nearly complete. This roughly corresponds to an object of D ∼ 750 km of albedo ∼0.1 at r = 42 AU. This of course does not rule out the possibility that massive TNOs of Pluto size or larger are lurking at distances greater than 100 AU.

At the other end of the size distribution, we have small comet-size TNOs beyond our current capabilities of detection. The most meaningful constraint on the small TNOs population (diameters D ≃ 1−10 km) comes from the Taiwanese-American Occultation Survey (TAOS) based on serendipitous occultations of stars by small TNOs. From the lack of observed occultations, the TAOS team (Zhang et al., 2013) found an upper limit of s = 2.34−2.82 for the exponent of the CSD, which is consistent with a decrease in the slope at the faint end of the size distribution found by Fraser et al. (2014). The 1–10 km size range is relevant since it roughly corresponds to the size range of the observed JFCs which show a power-law CSD with an estimated s ≃ 1.9−2.7 (Lamy et al., 2004; Tancredi et al., 2006; Fernández et al., 2013). It is also possible that bodies coming from the TN belt will experience sublimation, outbursts and splittings, during their journey to the inner planetary region, leaving two or more daughter comets that will change the CSD of the incoming small TNOs. Therefore, the CSD of JFCs does not have to match that observed in small TNOs. One of the striking features of the population of near-Earth JFCs is the scarcity of comets at the faintest end (roughly in the size range ∼0.1−0.5 km) (Fernández and Morbidelli, 2006), Is such a scarcity an original property of the population in the TN belt? or Is it due to their depletion as they approach to the Sun? This is no doubt an interesting topic whose solution will depend on expanding capabilities to investigate the subkilometer size population at larger heliocentric distances.

1.8 Is Pluto a planet? Discussion of its status and redefinition of planet

In ancient times, the concept of planet was very clear: It applied to those celestial bodies moving among the stars. The word itself comes from the Greek and means wandering star. In the Copernican heliocentric universe, planets were those bodies moving around the Sun known at that time, namely Mercury, Venus, Earth, Mars, Jupiter, and Saturn. The discovery of Uranus in 1781 and Neptune in 1846 did not need any reformulation of the classical definition of planet. The search for a planet between the orbits of Mars and Jupiter, as predicted by the Titius-Bode law, led to the discovery of Ceres in 1801. It was immediately hailed as the eighth planet, but its tiny size and the quick discovery in a row of three other bodies in the same interplanetary region, Pallas, Juno, and Vesta between 1801 and 1807, led astronomers to conclude that rather than a single planet, the region between Mars and Jupiter was populated by a swarm of small bodies that were called asteroids alluding to their star-like appearance as seen through the telescope.

The fact that Pluto, discovered in 1930, stood unchallenged as the sole known object of the TN region for more than six decades (with the satellite Charon since 1977) helped to cement the idea that it was the ninth planet, despite successive estimates of its size showing that it was extremely small, even smaller than the Moon. One of the unexpected consequences of the discovery of a TN population was to challenge the status of Pluto as a planet. The first discovered TNOs were too small in comparison to Pluto to promote any revision, but the situation changed when massive TNOs were discovered, and the crisis finally erupted with the discovery of Eris rivaling in size with Pluto itself. So, what to do? Should Eris, Makemake, Haumea, and others be added to the list of planets? or Should Pluto and the other Pluto-sized TNOs be classified as a new class of objects, following the historical path of Ceres?

To be useful, any classification in science must be supported by some theoretical background. In the case of the definition of planet, once that the old descriptions as a wandering star or a body that circles the Sun proved to be insufficient, a new theoretical framework was required. From a cosmogonic perspective, the concept of planet should be associated to the end products of the accretion process in the protoplanetary disk. The process is not fully efficient: A fraction of the mass in a certain accretion zone will remain unaccreted and be either ejected or remain in dynamically stable niches as comets, asteroids, TNOs, or trojans. In the sketch of Fig. 1.6, we depict in a simple way the passage from an initial population of kilometer-size planetesimals to a planet through intermediate stages. Planetesimals accrete until the most massive members reach the stage of runaway accretion in which their gravitational fields become strong enough to significantly increase their collision cross-sections, so their masses start to detach from those of the rest of the planetesimals. When runaway bodies become sufficiently massive, it is the gravitational scattering of background planetesimals which dominates the random velocity evolution of the population in the accretion zone rather than the interactions among planetesimals. The accretion cross-section of massive runaway bodies will decrease with an increase of the random velocity, and then the accretion rate starts to slow down. The more massive the runaway bodies are, the greater the random velocities, and thus the smaller the cross-section, so the massive runaway bodies will end up with similar masses orderly distributed in the disk. The subsequent accretion mode receives the name of oligarchic accretion (Kokubo and Ida, 1998; Thommes et al., 2004). The oligarchs dominate the field of relative velocities. There is accretion via collision and also dispersal of the residual matter, leaving as a final product a set of planets in regular orbits, perhaps with the ejection of one or more massive protoplanets.

Fig. 1.6 Sketch showing the different stages of accretion in the solar nebula: From kilometer-size planetesimal to a planet as the final product. The process left a residual population that survived until now with the names indicated at the bottom of the figure.

The crisis triggered by the discovery of Eris and other large TNOs generated an interesting debate that reached its climax during the IAU General Assembly in Prague in 2006. From a cosmogonical point of view, it made sense to reserve the category of planet to the final product of accretion in a given accretion zone, leaving the products of the initial and intermediate stages of the accretion process (planetesimals, runaway bodies, oligarchs) for other categories of objects. Contrary to what cosmogonical reasons would have advised, an ad hoc committee appointed by the IAU tried to push for a lax definition that kept Pluto in the planet category. In essence such definition said that a planet is a body in hydrostatic equilibrium circling the Sun, and that it is not a satellite. Talking in cosmogonic terms, it was like putting in the same bag planets, oligarchs, and even runaway bodies. Facing this proposal, a group of astronomers attending the Prague General Assembly presented an alternative definition that took into account cosmogonic principles as described earlier. In the alternative definition, a new cosmogonic-dynamic requirement was added to qualify as a planet: The body must have been able to clean its influence zone of planetesimals and competing embryo planets during its accretion. While the lax definition would open the door to a myriad of planet candidates in the solar system that fulfill the criterion of hydrostatic equilibrium (e.g., Tancredi and Favre, 2008), the more restricted definition would constrain the number of planets to eight, from Mercury to Neptune, corresponding to the end products of the accretion process in their respective influence zones.

At the closing assembly both definitions were subject to vote, gathering the second one a clear majority, thus leaving Pluto and the other planet candidates of the first option in an unspecified category. They were certainly larger than most comets, asteroids, and TNOs. Finally, bodies fulfilling the first criterion, but not the second one were categorized as dwarf planets. They were the oligarchs and even runaway bodies that remained unaccreted in dynamically stable niches. In retrospect, it is amazing the hot debate generated by the issue of planet definition that spilled over the academy reaching the public at large, few times seen in the history of astronomy or of science in general, and in which the issue was finally settled through a show of hands more typical of a political debate than of a scientific one. This is particularly astonishing for laymen that consider science as objective, cool, describing the reality, and far away from human passions and interests. For the general public, the episode has come to be known as the demotion of Pluto. Certainly, Pluto was the victim of the astronomers’ success at discovering TNOs.

1.9 TNOs today: Current picture and new challenges

Our current view depicts TNOs as icy bodies characteristic of the outer solar system, a mixture of water ice, other volatiles, and minerals. Water ice has been detected on the surface of some TNOs in the near-infrared region (e.g.,  km) show indeed clear signatures of volatile retention and they seem to be coated by a frost layer of methane, molecular nitrogen and in some cases carbon monoxide (Schaller and Brown, 2007). Their albedos are quite high going from about 9% for Quaoar to about 86% for Eris. Another process that might contribute to raise the albedo is impact gardening and cryovolcanism that leave exposed fresh unirradiated ice (Jewitt and Luu, 2004).

The icy surfaces of TNOs are subject to chemical alteration by cosmic-ray bombardment that can dissociate water molecules leading to a hydrogen depletion. Cosmic rays can also trigger the formation of ultrared material, constituted by carbon-rich, complex organic molecules like tholins that are synthetic macromolecular compounds produced by the irradiation of gaseous or solid mixtures of hydrocarbons and water. The ultrared material can be removed by collisions that leave exposed fresh ice, thus leading to changes in the surface color (Luu and Jewitt, 1996). It is then to be expected that TNOs will show different colors and this is what is actually observed. What is interesting and rather surprising is that colors are related to the dynamical class: cold classical TNOs (inclinations i < 5degrees) show red surface colors, while hot classical TNOs (i > 5degrees), high-i  AU are usually gray, while objects formed at greater distances tend to be red. They explain the color difference in terms of the sublimation of CH4 or its release from clathrates closer to the Sun, leaving a colorless water ice crust and depriving the surface of a substance like the CH4 able to form red organic compounds.

The bulk density of TNOs is a key parameter to understand their chemical composition and physical structure. A pure-ice body will have a density of 1 g cm−3 (or slightly higher due to the gravitational compression of the material). An ice/rock mixture would give densities 2–2.5 g cm−3, as are the cases of Pluto and Eris. Bodies growing to sizes of several hundred kilometers will produce at an early stage enough internal heat from colliding planetesimals and radioactivity to melt water ice. This solid-liquid phase transition will reduce porosity to negligible values, thus increasing the bulk density of the body to values of 2−2.5 g cm−3 typical of porosity-free, rock/ice mixtures (Brown, 2013). In order to determine bulk densities, it is necessary to measure both size and mass. Dynamical masses can be determined in binary pairs, while sizes can be estimated from occultations of stars or from observations in the IR from space-based observatories like Spitzer and Herschel.

As regards some open problems of current active research we can mention:

1.9.1 In situ formation versus implantation

One key question to learn about formation, evolution, and transport of matter in the early solar system is to estimate what are the fractions of matter in the TN belt that are primordial and implanted from inner regions. Current theories of planet formation suggest that the Jovian planets underwent migration during their formation. In particular, Uranus and Neptune would have experienced a significant outward migration (Fernández and Ip, 1984; Tsiganis et al., 2005). In the course of their migration, these outer planets would have also pushed outward via resonant coupling swarms of planetesimals that ended up in the TN region, most of them in MMRs with Neptune (Malhotra, 1995). This push-out mechanism of planetesimals would have originated a high-inclination TNO population with moderate eccentricities, the source of the current hot population (Gomes, 2003). A slow migration of Neptune from ∼26 to 27 AU to its current location would have produced a hot population with the observed inclination distribution (Nesvorný, 2015). From numerical simulations Nesvorný (2015) derived a capture efficiency in the classical disk of ∼2−4 × 10−4 for each initial particle, which would require a massive transplanetary disk of planetesimals at <30 AU of about 20 M⊕.

The scattering of planetesimals by Neptune would have destroyed wide binary TNOs, thus explaining their scarcity in the hot population (Parker and Kavelaars, 2010). In a more extreme view, Levison and Morbidelli (2003) and Levison et al. (2008) argued that the cold classicals were also implanted from a region <30 AU. However, the existence of a large fraction of wide binary pairs in the cold disk argues against their implantation from an inner region which would have implied close interactions with Neptune. The cold classical belt could be the remnant of the primordial population that survived the high eccentric phase of Neptune’s migration before settling in its current low-eccentricity orbit (Batygin et al., 2011). All the implantation models require a massive disk outside the early Neptune of ∼20 M⊕ that abruptly ends at ∼30 AU, condition that it is required in order to stop the outward migration of Neptune beyond its current distance. The question is: What caused the disk to end so abruptly? Could it have been possible that the mass in the cold disk was much larger at the beginning and suffered collisional attrition that left only a small fraction of its primordial content? Stern and Campins (1996) found that the primordial disk must have been two orders of magnitude more massive than the current one in order to form 100-km size bodies at distances ∼45 AU. The question is how was it possible to lose most of the mass. Morbidelli and Valsecchi (1997) conjectured that Neptune scattered Earth-sized bodies that passed through the TN belt exciting higher eccentricities and inclinations of TNOs, thus favoring mutual collisions that ground down most of them to dust and meteoroids.

1.9.2 The outer edge of the belt

Is there a sharp cutoff in the number density of classical TNOs at r = 48 AU? The fact that the edge is very close to the 1:2 MMR with Neptune (a = 47.7 AU) might suggest that it is related to the push-out of bodies drove by Neptune’s migration ( km)? The rationale for setting an upper size limit to distant TNOs is that the low densities of the protoplanetary disk at distances >50 AU would have slowed down or impeded altogether the growth of planetesimals to larger sizes.

1.9.3 The detached population

We can divide this population into two groups: (1) That formed by planetesimals scattered by Neptune from <30 to >40 AU where they were trapped in high-amplitude MMRs, as discussed by, for example, Nesvorný (2015), then the Kozai mechanism and other secular resonances worked for decreasing their eccentricities, that is, raising their q’s above 40 AU. This mechanism could have been efficient to raise perihelia up to ∼70 AU (Gomes et al., 2005), and able to create a numerous population in MMRs of type 1: k with q ≃ 50−70 AU and a  AU, like Sedna, for which the previous mechanism did not work. The source was the same: These bodies were scattered by the migrating Neptune from <30 AU but with the difference that they were not trapped in MMRs, so they could continue evolving with ever larger semimajor axes until they fell under the gravitational influence of Galactic tides and passing stars that raised their perihelia (Fernández et al., 2004). If the planetesimal disk encountered by the migrating Neptune had a mass of 20 M⊕, as estimated by Nesvorný (2015), we can conjecture that a significant mass worth of the order of one M⊕ could have been trapped in the detached disk/inner Oort cloud, including planet-size bodies. In third regard, the discovery of the detached TNO 2013 SY99 with a perihelion distance q = 50.0 AU and a semimajor axis a ≃ 733 AU led Bannister et al. (2017) to suggest the existence of a numerous detached population whose perihelia have been lifted to a ∼ 40−50 AU and their semimajor axes to 1000–2000 AU by the interplay between the weak Galactic tides and planetary