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 Trans­Neptunian 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 signal­to­noise ratio etary formation in our Solar System as tricity orbits. The resonant objects are (S/N), si­­multaneous visible and near­infra­ well as in other dusty star discs. An ESO trapped in mean­motion 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 mean­motion 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 (high­ellipticity) 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 Trans­Neptunian 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 revo lutionised 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., Trans­Neptunian 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 V­band) 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 G­mode 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 light­curve amplitudes, under the and ices present on Trans­Neptunian 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 J­band and SINFONI H­ and K­band 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 light­curve 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 H­band photometry has rela tionships 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 ultra­red 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.