Astronomical Science

The Outer Frontiers of the Solar System: Trans-Neptunian Objects and Centaurs

Maria Antonella Barucci1 nated the evolution of the early solar neb­ Pluto) being the most densely populated. Alvaro Alvarez-Candal 2 ula as well as of other planetary systems The scattering disc objects or scattered Irina Belskaya1,3 around young stars. objects are considered to be those that Catherine de Bergh1 have orbits with large eccentricities, and Francesca DeMeo1 TNO science has rapidly evolved in recent perihelion distances near the location Elisabetta Dotto4 years, linking together different popula­ of Neptune, while the detached objects Sonia Fornasier 1, 5 tions of small bodies in our planetary sys­ are those with orbits at large eccentrici­ Frédéric Merlin 1, 5 tem. The attempt to determine the physi­ ties, and with perihelion distances out of Davide Perna 4 cal properties of this population is at Neptune’s influence. The best example present one of the most active research of this last category is (90 377) Sedna, fields in planetary science. More than which has perihelion and aphelion dis­ 1 Observatoire de Paris, LESIA, France 1400 Trans-Neptunian objects, with differ­ tances of 76 and 927 AU, respectively. 2 ESO ent sizes, orbits and surface characteris­ Another group of objects, the Centaurs 3 Institute of Astronomy, Kharkiv Univer­ tics, have been discovered up to now. But (shown in orange in Figure 1), with un­- sity, Ukraine many more must be present. A few of stable orbits between those of Jupiter and 4 INAF–Osservatorio di Roma, Italy them belong to the newly defined popula­ Neptune, can also be associated with 5 Université de Paris VII–Diderot, France tion of dwarf planets. the TNO population. Planetary perturba­ tions and mutual collisions in the Kuiper The TNO population is classified into Belt are probably responsible for the ejec­ The icy bodies in orbit beyond Neptune several dynamical groups (see Figure 1), tion of objects into Centaur orbits. and known as Trans-Neptunian objects depending on their distance from the (TNOs), or Kuiper Belt objects, are the Sun and their orbital characteristics: In order to investigate the surface proper­ most distant objects of the Solar System i) classical objects; ii) resonant objects; ties of these remote and faint Solar Sys­ accessible to direct investigation from iii) scattering(ed) disc objects; and tem objects, a large programme has been the ground. The study of these objects, iv) detached objects. The first two groups carried out with the ESO Very Large containing the least processed material are also known as the Kuiper Belt, con­ Telescope (VLT) using, nearly simultane­ of the Solar System, can help in under- taining objects with an average distance ously, the Unit Telescopes UT1, UT2 and standing the still-puzzling accretion/ from the Sun of between 30 and 55 astro­ UT4. The aim of this large programme evolution processes that governed plan- nomical units (AU) and with low eccen­ was to obtain high signal-to-noise ratio etary formation in our Solar System as tricity orbits. The resonant objects are (S/N), si­­multaneous visible and near-infra­ well as in other dusty star discs. An ESO trapped in mean-motion resonances with red (NIR) spectra for almost all objects large programme has been devoted to Neptune in more than 20 resonances, with that can be observed with the VLT. Forty obtaining simultaneous high quality visi- the 3:2 mean-motion resonance (hosting objects have been studied, allowing a ble and near-infrared spectroscopy and photometry of about forty objects Figure 1. The location of with various dynamical properties. A few TNOs (on 12 February 2010) are reported in dif­ selected objects have also been ob­­ ferent colours for differ­ served with polarimetry to define their ent dynamical classes of surface characteristics better and with objects. Unusual objects detailed photometry to determine their (high-ellipticity) are shown as cyan triangles, rotational properties. The results pro- Centaurs as orange vide a unique insight into the physical ­triangles, objects in 2:3 and surface properties of these remote resonance with Neptune objects. as white circles (Pluto is shown with the crossed white symbol), scattered- disc objects as magenta Trans-Neptunian objects represent a circles, while the classi­ newly identified population of pristine cal objects are shown as red circles. Periodic material in our Solar System. Discovered comets are shown in less than twenty years ago, these icy blue squares. Objects ­bodies revo­lutionised our understanding observed at only one of the Solar System and most ideas on opposition are denoted by open symbols, while the evolution of the protoplanetary nebula. those with multiple oppo­ Located at the furthest frontiers of our sition orbits are denoted planetary system observable with ground- by filled symbols. The based telescopes, these small bodies are orbits of the planets are shown in light blue with considered to be the fossils of the proto­ their respective images. planetary disc and can provide unique (Adapted from a plot by information on the processes that domi­ the Minor Planet Center).

The Messenger 141 – September 2010 15 Astronomical Science Barucci M. A. et al., Trans-Neptunian Objects and Centaurs

Figure 2. The lower left panel shows the distribution 4 30 of the four TNO taxonomic groups (whose average 3 RR photometric colours are represented in the upper left RR 25 panel as reflectance values normalised to the Sun in 2 IR IR the V-band) within each dynamical class. The right BR 20 Reflectance 1 panel shows the distribution of the taxonomical BB BR groups, with respect to the orbital inclination relative 0 15 to the ecliptic plane, for the “classical” TNOs. 0.5 1.0 1.5 2.0 2.5 N Wavelength (µm) BB 10 broad characterisation of the brightest 5 TNOs. All the targets have also been 30 observed by V, R, I, J, H and K photome­ 0 0–5 5–10 10–15 15–20 20–25 25–30 30–35 try to determine their taxonomic classes. Orbital inclination (degrees) 20 N Surface colours, taxonomy and rotation

10 Visible and NIR photometry of forty ob­­jects have been carried out with FORS and ISAAC. Based on the computed 0 ­colour indices, we derived the taxonomic Centaurs Resonant Classical Scattered Detached classification of 38 bodies (DeMeo et al., 2009; Perna et al., 2010), by applying twelve bodies. Assuming ellipsoidal 2.4 µm. This spectral window provides the the G-mode statistical method according shapes with axes a > b > c, we derived a most sensitive technique to characterise to the Barucci et al. (2005) system. This lower limit to the axis ratio a/b from the from the ground the major mineral phases scheme identifies four classes that rea­ obtained light-curve amplitudes, under the and ices present on Trans-Neptunian sonably indicate different compositions hypothesis that the light curves are only objects. Nearly simultaneous observa- and/or evolutional history, with increas­ affected by the shape elongation and that tions of FORS visible spectroscopy, ISAAC ingly red colours from BB (blue), BR (inter­ major al­­bedo variations are not present J-band and SINFONI H- and K-band mediate blue–red), IR (moderately red) on the surface of the observed bodies. spectroscopy been performed for forty to the RR (red) class. The observations From the rotational periods and light-curve objects selected among different dynami­ performed in the framework of our ESO amplitudes we also derived a range of cal groups. The exposure time required large programme were combined with the ­variation of the density of the observed is generally long, and as the objects rotate whole data sample presently available bodies, by applying the Chandrasekar around their principal axis, the resulting in the literature. We thus analysed a total theory for rotationally stable Jacobi ellip­ spectra often contain information coming of 151 ­taxonomically classified objects soids under the simplified assumption of from different parts of the object. The and performed a statistical analysis of the cohesionless and strengthless bodies V-, R-, I-, J- and H-band photometry has rela­tionships between taxonomical and (namely fluid objects). The obtained den­ been used to tie the different spectral dy­­namical classification. The main results sity values seem to confirm the existence ranges together. we obtained are (see Figure 2): of a magnitude/density trend with larger i) the enlarged sample of analysed (brighter) TNOs being denser than smaller The visible spectra are mostly featureless, Centaurs confirms the colour bimodality (fainter) ones, as had been suggested. showing, however, very large variations suggested previously; However, this trend is strongly influenced of their spectral slope, with colours from ii) bodies belonging to the IR taxonomic by a single object (136 108 Haumea). neutral to very red. The ultra-red slopes class seem to be concentrated among The limited sample of densities currently probably indicate the presence of complex the classical and resonant populations; available in the literature, together with organic material on the surface. A few iii) within the classical objects, the most the still unresolved ambiguity between big objects, like Eris and Pluto, show sig­

spectrally red bodies (RR class) domi­ brightness and size (due to the small natures of CH4 in their spectra. We iden- nate the population at low orbital incli­ number of reliable albedo measurements), tify in a few other objects (10199 Chariklo, nation, while blue objects (BB class) are prevent us from definitively assessing 42355 Typhon, and 2003 AZ84) new faint more abundant at high orbital inclina­ any relationship between TNO density and and broad absorption features that are, in tions. These results confirm the previ­ size. general, associated with aqueous altered ously suggested relationship between silicates on the surfaces of these bodies, spectral behaviour and dynamical by analogy with features present in the evolution, the red and blue colours Composition spectra of some main belt dark asteroids. being associated with the dynamically Hydrous silicates are also known to be cold and hot populations, respectively. Detailed information on the composition of present in interplanetary dust par­ticles TNOs can only be acquired from spectro­ (IDPs). The NIR 1–2.4 µm window provides In the framework of this large programme scopic observations, especially covering powerful diagnostics for the study of we also investigated the spin rates of the wavelength range between 0.4 and astrophysical ices.

16 The Messenger 141 – September 2010 Radiative transfer models have been used water ice. It is still a matter of debate of the surface layer, and the formation to investigate TNO surface composition, whether water ice was amorphous or of a crust. and to interpret features and spectral be- crystalline in the protosolar nebula. The haviour using intimate or geographical presence of crystalline water ice implies mixtures of organics, silicate minerals, that the ice has been heated above The largest objects ­carbonaceous assemblages, ices, and/or 100–110 K. This heating could have light hydrocarbons. The red spectral resulted from impacts, or the ices might The largest TNOs, many of which are also slopes are typically well-reproduced by have formed in the warmer deep inte­ labelled as dwarf planets, have distinctly assuming the presence of organic com­ riors and were then exposed on the sur­ different surface compositions compared pounds on the surface, such as kerogens face. The quality of the observations with the rest of the population. Specifically, (complex dark organic compounds) and of fainter objects is not sufficient to dis­ they have strong signatures of methane or tholins (Titan and Triton materials — sub­ tinguish between amorphous or crys­ crystalline water ice in their spectra (see stances formed in the laboratory by irradi­ talline water ice. Nonetheless, when try­ Figure 3). During this ESO large pro­ ation of gaseous mixtures of methane and ing to model the spectra the best-fit gramme we managed to observe several nitrogen in different proportions) or ice model is usually obtained when using a of the largest TNOs: Pluto, Eris, Sedna, tholins (formed by irradiating mixtures of combination of the two water ice states. Quaoar, Charon and Orcus. essentially water and hydrocarbon ices). Other physical properties such as poros- 2) Other ices group On Pluto we detect volatile species such ity and rugosity (the numerical measure of Methane ice is present on the largest as methane, nitrogen, and carbon monox­ roughness) can, in principle, be derived TNOs such as Eris, Pluto, Sedna and ide. From our investigation, we confirm from these models, but the observation of Quaoar. The spectra of some objects, that the level of dilution of methane in unresolved sources and the small phase such as Pluto and probably Eris, show nitrogen is different on different parts of angle coverage, due to the large helio­ that some of the methane ice must the heterogeneous surface of Pluto. On centric distance of these bodies, limit con­ be dissolved in nitrogen. Methane is Eris, the largest dwarf planet, we indirectly siderably confidence in the results. These present at the surface of these large detect nitrogen on the surface based on models utilise numerical algorithms that objects because their gravity is high the wavelength position of some of the fit the object spectra by reduced chi- enough to retain such a volatile compo­ bands of methane (Merlin et al., 2009). squared minimisation. This minimisation nent, but it may also be present in The data indicate that the dilution of meth­ provides the best set of parameters lower quantities on smaller objects. A ane ice in nitrogen changes as a function among the input free parameters (such as small amount of ethane (a by-product of depth below the surface. This suggests concentration and particle size). The mod­ of methane ice irradiation) has also been the formation of a temporary atmosphere els can provide insights into the chemical detected on Quaoar. Some objects, around Eris when close to perihelion, as is composition and the dilution state of the such as Pholus and (55638) 2002 VE95, observed for Pluto. Modelling of the data various compounds, or constraints on show spectra with methanol features. also suggests a large quantity of irradiated the way they are mixed (intimate mixtures, In addition, many objects show spectra material and evolved chemistry on the sur­ areal mixtures, combinations of both, etc.). with a decreasing slope beyond 2.2 µm, face, as well as the presence of a small They also provide information on the strat­ implying the possible presence of amount of ethane that could be formed ification state of the subsurface layers ­methanol or similar molecules, even if either in the atmosphere or on the surface. providing strong evidence of volatile trans­ these faint objects have spectra with port or on limits on the irradiation level, a low S/N, especially in the K-band. The Orcus has large amounts of crystalline depending on the depth. presence of ammonia or ammonia water ice on its surface. Barucci et al. hydrate in the spectra of a few objects (2008) detected a signature attributed to The main results obtained during the ESO (Charon, Orcus) has been suggested. A hydrated ammonia, similar to the case large programme on the forty objects firm detection of ammonia would have of Charon, the satellite of Pluto, on which observed spectroscopically, and the mod­ im­­portant implications on the composi­ large amounts of water ice also exist in els of their surface by radiative transfer tion of the primitive solar nebula in low crystalline form. These observations sug­ models, can be summarised by subdivid­ density regions far from the Sun. gest processes that are able to renew ing the targets into three main groups the surface with fresh and non-irradiated according to their composition: 3) Featureless spectra group icy materials. For the largest objects Many objects have featureless spectra ­evolutionary models indicate that cryovol­ 1) Water ice group in the NIR, with a wide range of colours. canism is possible (if enough radiogenic More than 50 % of the targets show the These objects could be mantled by sources were present in their interiors). presence of water ice on their surface. a surface rich in organics or carbon. As Non-disruptive collisions could also play a The best-fit compositional models of all these objects are supposed to be role in renewing the surface, as well as these objects include water ice in the at least partly made of ices, irradiation catastrophic collisions such as that at the crystalline state as well as in the amor­ processes have to be responsible for origin of the Haumea family, which could phous state. The majority of spectra these properties. C-bearing molecules explain the presence of fresh crystalline with high S/N ratios show the presence progressively lose their hydrogen water ice even on small members of the of a feature at 1.65 µm due to crystalline atoms, which results in a polymerisation family.

The Messenger 141 – September 2010 17 Astronomical Science Barucci M. A. et al., Trans-Neptunian Objects and Centaurs

  Figure 3. The image on Haumea the left (adapted from D   Gavin Rymill, 2006) Sedna $QHR shows the circular orbits   of the eight planets ver­ DBS@MB sus the eccentric orbits Dk   of the biggest TNOs Pluto & Charon DQ (Pluto, Eris, Quaoar, Neptune   /KTSN Sedna, ...) On the right

1DK@SHU   are shown the two Uranus groups of TNO spectra:   Eris and Pluto with         methane ice dominated 6@UDKDMFSG§L spectra and Quaoar, Haumea and Charon   with water ice domi­ nated spectra. Saturn   0T@N@Q D Eris   Makemake DBS@MB   "G@QNM Quaoar   UDQDk   Gavin Rymill 2006 '@TLD@ 1DK@SH   Sedna           0 1000 2000 km 6@UDKDMFSG§L Haumea Quaoar Pluto Charon Makemake Eris

Although Quaoar’s spectrum displays object (38628) Huya, the scattered disc of large (compared to the observation clear water ice features in the crystalline object (26375) 1999 DE9, and Centaurs wavelength) inhomogeneous particles form, there is a strong red slope in the (2060) Chiron, (5145) Pholus and (10199) (Belskaya et al., 2008). Smaller size TNOs, ­visible and other weak features in the NIR Chariklo. The polarimetric characteristics characterised by a pronounced branch that suggest a small amount of methane of the scattered radiation contain much of negative polarisation, revealed a similar on the surface and small grains of irradi­ more accurate and specific infor­mation polarisation behaviour regardless of the ated material. Dalle Ore et al. (2009) model concerning the microscopic properties of fact that they have different surface a spectrum from the visible to the NIR, the surface. We found that all observed albedos and belong to different dynamical including the additional constraints of bodies revealed negative polarisation, groups. The presence of a thin frost layer Spitzer data, and find a best-fit model where the polarisation plane of linearly of submicron ice crystals on a dark sur­ consisting of crystalline and amorphous polarised light coincides with the scatter­ face is considered as one of the possible water ice, methane, nitrogen and ethane ing plane. It is a characteristic feature for ways to explain the particular polarisation ices with, in addition, Triton and Titan ­surfaces with a complex structure, as properties of these distant objects. ­tholins. Sedna, which is significantly observed for the majority of planetary sur­ ­further from the Sun (~ 90 AU) than most faces. However, the measured polarisa- known TNOs (30–50 AU), also exhibits tion phase angle behaviour of TNOs and The overall picture one of the reddest visible spectra, and ­Centaurs was found to be unique among weak features in the near-infrared that other Solar System bodies observed so These results provide unique insights into suggest a surface covered by water ice, far. Objects with a diameter smaller than the global population of these faint and methane and nitrogen ice, as well as small 1000 km exhibit a negative polarisation distant objects. Important advances in elu­ grains of irradiated material mainly formed that rapidly increases (in absolute value) cidating the surface composition of via im­­pacts of cosmic rays and interstellar with the phase angle and reaches about TNOs have been achieved. Observations medium particles (Sedna’s orbit often takes –1% at phase angles as small as 1º. The performed with SINFONI in the H and K it beyond the heliopause). largest TNOs exhibit a small fraction of regions have allowed us to detect spec- negative linear polarisation that does not tral signatures, revealing the presence

noticeably change in the observed phase of surface deposits of ices such as H2O, Surface properties angle range. It has been suggested that CH4, CH3OH, C2H6, NH3 and N2. We find the different types of polarimetric behav­ that most of the largest objects have ices To investigate the surface characteristics iour are related to different albedos and on their surface (water ice or ices of more of TNOs better we carried out polari­metric different capabilities for retaining volatiles volatile species), whatever their dynamical observations of eight objects belonging to for large and small TNOs (Bagnulo et al., class (see Figure 4) and whatever their col­ different dynamical groups. These include 2008). The modelling of the polarimetric ours, although objects with neutral colours dwarf planets Eris and ­Haumea, the clas­ behaviour of the largest objects suggests tend to be covered by water ice. The col­ sical object (20000) Varuna, the resonant that their topmost surface layer consists ours are very variable, from slightly blue

18 The Messenger 141 – September 2010 13 Figure 4. The absolute to have formed from a disc of debris 12 magnitude (in H-band) Oth – ice ejected during the collision of Pluto with a 11 of the TNOs and Cen­ LP – ice body of almost equal size. 10 taurs (with and without Oth – no ice ice detected on their 9 LP – no ice 8 surface) is plotted as a In addition, the high albedos and the function of the perihelion 7 detection of volatiles on the surfaces on distance (q, in astro­ 6 nomical units). The some TNOs indicate the possible pres­ 5 dimension of the symbol ence of an atmosphere, even if only as a Hmag 4 is related to the ­diameter transient phenomenon. The only Trans- 3 of the object (for clarity Neptunian object observed thus far with a 2 the scale used is not lin­ 1 ear). The smallest size of seasonal atmosphere is Pluto, but other 0 the Centaurs is 20 km, large objects like Eris or Sedna may have –1 while the biggest TNO one as well. A cometary-type activity –2 has a diameter close to (outburst) has also been suggested for the –3 2500 km. 5 15 25 35 45 Centaur Chariklo to explain differences in q (AU) spectra obtained at different times (Guilbert et al., 2009). to very red (Fornasier et al., 2009). The the real nature of the organics present Our understanding of the population of wide difference in surface composition and on these objects is still a matter of debate. these faint and distant objects is, however, colour within the TNO population could The red class (RR) objects are present still limited. The observations that have be connected to different original compo­ in all dynamical populations, with a higher been made so far lead to a lot of ques­ sitions and/or the different processes they concentration in the classical group. They tions. If major differences in composition have experienced, as well as to their size. are found amongst the Centaur popula- between the very large objects and the Even if TNOs are considered as the most tion as well as in the detached population, others can be attributed to a size effect, it pristine objects in the Solar System, over as for example Sedna, which is consid­ is very hard to explain differences among the 4.5 Gy of the Solar System’s life they ered as part of the inner Oort Cloud. smaller objects. The next generation of have experienced various modifying proc­ more powerful instruments on 10-metre- esses. Other processes must be at work that class telescopes will start giving us some could affect some objects more than answers, but most of the answers will It is clear that the surface of these objects others. Models of the interiors of TNOs probably come with the next generation has been affected by bombardment by indicate that cryovolcanism, which is con­ of telescopes, the ELTs, which will open cosmic ray and solar wind ions and/or sidered to be the most probable form the study of smaller objects. These smaller micrometeorites (space weathering), with of geological activity on some satellites objects are those that carry most of the the consequence that the molecular of the outer planets, may be possible on information about the dynamical/collisional ­complexes are structurally changed and the larger Trans-Neptunian objects (di- evolution of the Solar System. The defini­ the molecular compositions of ice and ameter > 800 km). This could explain, for tive evidence for atmospheres can come minerals are altered over time. Laboratory instance, the surface composition of only from occultations or direct spectro­ experiments on plausible materials for Orcus which includes both water ice in the scopic detection with spacecraft. A major TNOs show the formation of an irradiation crystalline state, which is not supposed step in the study of this population is the mantle (forming a crust), breaking bonds to exist at such low temperatures (around New Horizons–NASA mission that will fly in ice molecules, allowing the formation of 30 K), and ammonia, which is easily by Pluto in 2015 and will enable the radicals, escape of hydrogen and forma­ destroyed by irradiation. Furthermore, the detailed study of Pluto and its three satel­ tion of a carbon-rich layer of low albedo. water ice in the crystalline state should lites, Charon, Nix and Hydra. The New This can easily mask the presence of vola­ be quickly amorphised by irradiation, as Horizons spacecraft will continue on into tiles and the crust thus formed would indicated by various laboratory studies. the Trans-Neptunian population to fly by hide the real composition of these icy one or more TNOs. bodies. The statistics do not, however, Collisions must have also played an show any strong correlation between the important role in the evolution of this pop­ References surface properties and dynamical classes ulation, inducing heating and chemical or orbital properties of TNOs. Many ob- changes. The consequence of collisions Bagnulo, S. et al. 2008, A&A, 491, L33 jects from the reddest class (RR), which is not only the alteration of the surface Barucci, M. A. et al. 2005, AJ, 130, 1291 are the reddest objects in the Solar Sys­ properties, but also the modification of the Barucci, M. A. et al. 2008, A&A, 479, L13 Belskaya, I. et al. 2008, A&A, 479, 265 tem, have probably been heavily irradi­ internal structure of the targets. Collisions Dalle Ore, C. M. et al. 2009, A&A, 501, 349 ated. The slopes of the spectra of these are important both for small and large DeMeo, F. et al. 2009, A&A, 493, 283 objects, which have typically low albedos, objects. A typical example is Charon, a Guilbert, A. et al. 2009, A&A, 501, 777 have been modelled with complex organ- moon of Pluto, but completely different Fornasier, S. et al. 2009, A&A, 508, 457 Merlin, F. et al. 2009, AJ, 137, 315 ic compounds such as Titan, Triton or ice in composition (see Figure 3), which is Perna, D. et al. 2010, A&A, 508, 457 tholins, or terrestrial-type kerogens, but supposed (according to numerical models)

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