Colours and Composition of the Centaurs

Centaurs are widely believed to come from the Edgeworth-Kuiper belt, located beyond the orbit of Neptune. From here they can be injected into the inner part of the Solar System through planetary perturbations or mutual collisions. Due to their origin and dynamical evolution, Centaurs are supposed to constitute a transition population of objects from the large reservoir of Trans-Neptunian Objects (TNOs) to the active bodies of the inner Solar System. On the basis of the present knowledge of the physical properties of Centaurs and TNOs a similarity between the two populations appears evident. This is the strongest observational constraint supporting the theory of common origin.     

Spectrophotometry of Kuiper Belt Objects 20000 Varuna, 2000 Eb173 and Centaur 10199 Chariklo

Due to the distance, faintness, and very recent discovery ofKuiper Belt Objects (KBOs) and Centaurs, very little is knownabout the physical characteristics of these basic buildingblocks of the solar system. New intermediate-band photometryobservations of KBOs and Centaurs suggest that absorption bandsexist in the visible portion of their spectra, which could offerinsights into the surface composition of these objects.     

Dynamical determination of the mass of the Kuiper Belt from motions of the inner planets of the Solar system

In this paper we determine dynamically the mass of the Kuiper Belt Objects by exploiting the latest least-squares determinations of the extra rates of perihelia of the inner planets of the Solar system. By modelling classical Kuiper Belt Objects as an ecliptic ring of finite thickness, we obtain 0.033 ± 0.115 in units of terrestrial masses. For resonant Kuiper Belt Objects, a two-ring model yields 0.018 ± 0.063. These values are consistent with recent determinations obtained using ground- and space-based optical techniques. Some implications for precise tests of Einsteinian and post-Einsteinian gravity are briefly discussed.     

The Caltech Wide Area Sky Survey

The first phase of the Caltech Wide Area Sky Survey occurred from lateNovember 2001 through mid-April 2003. We present preliminary resultsfrom this survey which has detected 28 bright Kuiper Belt Objects(KBOs) and 4 Centaurs, 19 of which were discovered in our surveyincluding Quaoar, the largest KBO, as well as 6 of the 10intrinsically brightest KBOs. We have surveyed 5108 square degrees ofthe sky nearest the invariable plane to a limiting red magnitude of20.7. Correcting for the overabundance of objects near the invariableplane, this represents 27% completeness in terms of KBO numbers.Thus, approximately 100 KBOs and Centaurs brighter than m_R = 20.7exist, about 3/4 of which remain undiscovered. The bright KBOs areconsistent with the canonical q=4 size distribution, suggesting thatabout ten 1000 km diameter KBOs and about one 2000 km diameter KBOexist. Additionally, we observe only 3 KBOs with low inclination(i < 7 degrees) with 67% of the sky available to these objectssurveyed. This is in sharp contrast with the known KBOs, of whichabout 60% of the ～ 800 observed objects (as of May 2003) have i< 7 degrees. Although we observe at systematically higher invariableplane latitudes than many deeper KBO surveys, such systematic biasescannot fully explain the lack of low inclination objects, ameasurement which is significant at the > 3 σ level. Thissuggests that the bright KBOs have a fundamentally different maximumsize than the fainter KBOs. A better characterization of the surveylimiting magnitude and a more thorough modeling of observational biaseffects of different classes of KBOs will be made in a future work.     

Colour Properties and Trends in Trans-Neptunian Objects

One of the most puzzling features of the Kuiper Belt, which has been confirmedby numerous surveys, is the optical colour diversity that seems to prevail among the observed TNOs. TNOs and Centaurs have surfaces showing dramaticallydifferent colours and spectral reflectances, from neutral to very red. With therelatively few visible-NIR colour datasets available, the colour diversity seemsalso to extend to the near infrared wavelengths. Relevant statistical analyses havebeen performed and several studies have pointed out strong correlations betweenoptical colours and some orbital parameters (i, e, q) for the Classical KuiperBelt objects. On the other hand, no clear trend is obvious for Plutinos, Scatteredobjects or Centaurs. Another important result is the absence of correlation of colourswith size or heliocentric distance for any of the populations of outer Solar Systemobjects. The strong colour anisotropy found is important because it is a diagnosticof some physical effects processing the surfaces of TNOs and/or some possiblecomposition diversity. In this paper, we will review the current knowledge of thecolour properties of TNOs, describe the observed colour distribution within theEdgeworth–Kuiper belt, and address the problem of their possible origin.     

Asteroids with Satellites: Inventory, Properties, and Prospects for Future Discoveries

The population of binary asteroids numbers over 160 systems, and they can be found amongst near-Earth asteroids (NEAs), Main-Belt asteroids (MBAs), Jupiter Trojans, Centaurs and trans-Neptunian objects (TNOs). The discoveries have been made with space missions, radar observations, photometric lightcurves, and high resolution imaging from the ground and space. The properties of each population are widely different due to varying formation mechanisms and discovery techniques for each group. Future large-aperture telescopes will be capable of imaging both components for nearly all known systems and will drastically improve prospects for discovery of smaller and more tightly bound systems throughout the Solar System. The study of binary asteroids has provided valuable estimates on asteroid density and structure, a better understanding of the radiative YORP-effect, insights on catastrophic collisions, and may prove to be a key diagnostic for understanding the formation and evolution of the Kuiper Belt population.     

Physical survey of 24 Centaurs with visible photometry

We present optical observations of 24 Centaurs performed between 1998 and 2002 with the University of Hawaii 2.2-m telescope. This is the largest such Centaur survey to date. We report colors for all objects, and show that they cover a continuum with mean V–R color of 0.58±0.01 and standard deviation 0.15. The color distribution fits between those of the Kuiper Belt and the cometary nuclei, and seems consistent with the dynamical concept of the majority of Centaurs originating from the Kuiper Belt. We find no strong correlation between a Centaur's color and its orbital elements; there is at best a <3−σ correlation with semimajor axis, with redder Centaurs being farther from the Sun. We have calculated the phase-darkening slope parameters G for 5 Centaurs, 4 of which are reported for the first time. They range from −0.18 to 0.13. We have sufficient data to constrain the rotation periods of two Centaurs, 1999 UG₅ (which we reported earlier) and 1998 SG₃5. We performed a comparison of the surface brightness profiles of 10 apparently-inactive Centaurs with point sources. We found no coma around these 10 objects, including C/LINEAR (2000 B4), and generally the upper limits to the dust mass loss rates are below 0.05 kg s⁻¹.     

Which are the dwarfs in the Solar System?

The International Astronomical Union recently adopted a new definition of planets in our Solar System. A new category of objects was introduced: a “dwarf planet.” This is “a celestial body that has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape and has not cleared the neighborhood around its orbit.” In a footnote, the resolution says: “An IAU process will be established to assign borderline objects into either “dwarf planet” and other categories." In order to contribute to the establishment of this classification procedure, we analyze the problem of the minimum mass required to become a “dwarf planet,” either from the theoretical and the observational perspective. We propose classification criteria for “dwarf planets” based on the available information on the shape and size of asteroids and TNOs, principally the direct or indirect estimates of the diameter and the estimate of the shapes from the lightcurves. We compile the available observational data on large asteroids and TNOs. According to our classification scheme there is only one rocky “dwarf planet” and 12 icy “dwarf planets” among the already discovered objects.     

The Size and Shape of the Oblong Dwarf Planet Haumea

We use thermal radiometry and visible photometry to constrain the size, shape, and albedo of the large Kuiper belt object Haumea. The correlation between the visible and thermal photometry demonstrates that Haumea’s high amplitude and quickly varying optical light curve is indeed due to Haumea’s extreme shape, rather than large scale albedo variations. However, the well-sampled high precision visible data we present does require longitudinal surface heterogeneity to account for the shape of lightcurve. The thermal emission from Haumea is consistent with the expected Jacobi ellipsoid shape of a rapidly rotating body in hydrostatic equilibrium. The best Jacobi ellipsoid fit to the visible photometry implies a triaxial ellipsoid with axes of length 1,920 × 1,540 × 990 km and density $2.6$  g cm $^{-3}$ , as found by Lellouch et al. (A&A, 518:L147, 2010. doi:10.1051/0004-6361/201014648). While the thermal and visible data cannot uniquely constrain the full non-spherical shape of Haumea, the match between the predicted and measured thermal flux for a dense Jacobi ellipsoid suggests that Haumea is indeed one of the densest objects in the Kuiper belt.     

Physical studies of Centaurs and Trans-Neptunian Objects with the Atacama Large Millimeter Array

Once completed, the Atacama Large Millimeter Array (ALMA) will be the most powerful (sub)millimeter interferometer in terms of sensitivity, spatial resolution and imaging. This paper presents the capabilities of ALMA applied to the observation of Centaurs and Trans-Neptunian Objects, and their possible output in terms of physical properties. Realistic simulations were performed to explore the performances of the different frequency bands and array configurations, and several projects are detailed along with their feasibility, their limitations and their possible targets. Determination of diameters and albedos via the radiometric method appears to be possible on ∼500 objects, while sampling of the thermal lightcurve to derive the bodies’ ellipticity could be performed at least 30 bodies that display a significant optical lightcurve. On a limited number of objects, the spatial resolution allows for direct measurement of the size or even surface mapping with a resolution down to 13 milliarcsec. Finally, ALMA could separate members of multiple systems with a separation power comparable to that of the HST. The overall performance of ALMA will make it an invaluable instrument to explore the outer Solar System, complementary to space-based telescopes and spacecrafts.     

Inner Solar System dynamical analogs of plutinos

By studying orbits of asteroids potentially in 3:2 exterior mean motion resonance with Earth, Venus, and Mars, we have found plutino analogs. We identify at least 27 objects in the inner Solar System dynamically protected from encounter through this resonance. These are four objects associated with Venus, six with Earth, and seventeen with Mars. Bodies in the 3:2 exterior resonance (including those in the plutino resonance associated with Neptune) orbit the Sun twice for every three orbits of the associated planet, in such a way that with sufficiently low libration amplitude close approaches to the planet are impossible. As many as 15% of Kuiper Belt objects share the 3:2 resonance, but are poorly observed. One of several resonance sweeping mechanisms during planetary migration is likely needed to explain the origin and properties of 3:2 resonant Kuiper Belt objects. Such a mechanism likely did not operate in the inner Solar System. We suggest that scattering by the next planet out allows entry to, and exit from, 3:2 resonance for objects associated with Venus or Earth. 3:2 resonators of Mars, on the other hand, do not cross the paths of other planets, and have a long lifetime. There may exist some objects trapped in the 3:2 Mars resonance which are primordial, with our tests on the most promising objects known to date indicating lifetimes of at least tens of millions of years. Identifying 3:2 resonant systems in the inner Solar System permits this resonance to be studied on shorter timescales and with better determined orbits than has been possible to date, and introduces new mechanisms for entry into the resonant configuration.     

Micrometeorite impact annealing of ice in the outer Solar System

The spectra of water ice on the surfaces of icy satellites and Kuiper Belt Objects (KBOs) indicate that the surface ice on these bodies is in a crystalline state. This conflicts with theoretical models, which predict that radiation (galactic cosmic rays and solar ultraviolet) should damage the crystalline structure of ice on geologically short timescales. Temperatures are too low in the outer Solar System for the ice to anneal, and reflectance spectra of these bodies should match those of amorphous solid water (ASW). We assess whether the kinetic energy deposited as heat by micrometeorite impacts on outer Solar System bodies is sufficient to anneal their surface ice down to a near-infrared optical depth . We calculate the kinetic energy flux from interplanetary micrometeorite impacts, including gravitational focusing. We also calculate the thermal diffusion of impact heat in various surfaces and the rate of annealing of ice. We conclude that the rate of annealing from micrometeorite impacts is sufficient to explain the crystallinity of ice on nearly all the surfaces of the saturnian, uranian and neptunian satellites. We discuss how the model can be used in conjunction with spectra of KBOs to probe dust fluxes in the Kuiper Belt.     

Silicon vidicon imaging of jupiter: 4100- to 8300-Å absolute reflectivities and limb darkening of spatially resolved regions

Jupiter was observed in six continuum wavelength channels in the region 4100–8300 Å, using a silicon vidicon imaging photometer. Spectral reflectivities and high spatial resolution limb-darkening curves for several belts and zones have been extracted from the data. Simple model fits to the data yield information regarding spectral and spatial variations in single-scattering albedos and shape of particle single-scattering phase functions. Belts appear to be more backscattering than zones, particularly in the blue. The data are in moderate agreement with limb-darkening predicted by models derived from the center-to-limb variation in equivalent width of the H₂ 4-0 S(1) quadrupole line (Cochran, 1976) in the South Tropical Zone, but strongly disagree with the results of such models for the North Equatorial Belt.     

Albedo and photometric study of Mars with the Planetary Fourier Spectrometer on-board the Mars Express mission

The Short Wavelength Channel of the Planetary Fourier Spectrometer (PFS) covers the 8333-1750 cm⁻¹ (1.2-5.7 μm) spectral range, that is well suited to study the reflectance properties of the martian soil. These properties vary with time due to the dust dynamics in the martian environment. Wind can blow off dust exposing soil and fresh rocks and can support grain mobility inducing local dust settling. We have analyzed PFS data from January 2004 to April 2005. A detailed photometric study of the radiance acquired from the planet has been performed in order to compare correctly measurements obtained at different viewing geometries and to produce a mosaic image of the planet. The results show good agreement with data from the Thermal Emission Spectrometer (on-board NASA Mars Global Surveyor orbiter), although some variations are observed. Some albedo changes could be due to small to medium scale dust storms. A very accurate estimation of the limb-darkening parameter has been computed from the analyzed data. The obtained values are compared with a surface roughness and a thermal inertia map in order to assess the relation between the limb-darkening parameter and the physical properties of surface.     

Geometric characterization of the Arjuna orbital domain

Arjuna‐type orbits are characterized by being Earth‐like, having both low‐eccentricity and low‐inclination. Objects following these trajectories experience repeated trappings in the 1:1 commensurability with the Earth and can become temporary Trojans, horseshoe librators, quasi‐satellites, and even transient natural satellites. Here, we review what we know about this peculiar dynamical group and use a Monte Carlo simulation to characterize geometrically the Arjuna orbital domain, studying its visibility both from the ground and with the European Space Agency Gaia spacecraft. The visibility analysis from the ground together with the discovery circumstances of known objects are used as proxies to estimate the current size of this population. The impact cross‐section of the Earth for minor bodies in this resonant group is also investigated. We find that, for ground‐based observations, the solar elongation at perigee of nearly half of these objects is less than 90°. They are best observed by space‐borne telescopes, but Gaia is not going to improve significantly the current discovery rate for members of this class. Our results suggest that the size of this population may have been underestimated by current models. On the other hand, their intrinsically low encounter velocities with the Earth induce a 10–1000‐fold increase in the impact cross‐section with respect to what is typical for objects in the Apollo or Aten asteroid populations. We estimate that their probability of capture as transient natural satellites of our planet is about 8 %. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Forming Jupiter, Saturn, Uranus and Neptune in few million years by core accretion

Giant planet formation process is still not completely understood. The current most accepted paradigm, the core instability model, explains several observed properties of the Solar System’s giant planets but, to date, has faced difficulties to account for a formation time shorter than the observational estimates of protoplanetary disks’ lifetimes, especially for the cases of Uranus and Neptune. In the context of this model, and considering a recently proposed primordial Solar System orbital structure, we performed numerical calculations of giant planet formation. Our results show that if accreted planetesimals follow a size distribution in which most of the mass lies in 30-100 m sized bodies, Jupiter, Saturn, Uranus and Neptune may have formed according to the nucleated instability scenario. The formation of each planet occurs within the time constraints and they end up with core masses in good agreement with present estimations.     

The colors of cometary nuclei—Comparison with other primitive bodies of the Solar System and implications for their origin

We present new color results of cometary nuclei obtained with the Hubble Space Telescope (HST) whose superior resolution enables us to accurately isolate the nucleus signals from the surrounding comae. By combining with scrutinized available data obtained with ground-based telescopes, we accumulated a sample of 51 cometary nuclei, 44 ecliptic comets (ECs) and 7 nearly-isotropic comets (NICs) using the nomenclature of Levison [Levison, H.F., 1996. In: Rettig, T.W., Hahn, J.M. (Eds.), Completing the Inventory of the Solar System. In: ASP Conf. Ser., vol. 107, pp. 173-192]. We analyze color distributions and color-color correlations as well as correlations with other physical parameters. We present our compilation of colors of 232 outer Solar System objects—separately considering the different dynamical populations, classical KBOs in low and high-inclination orbits (respectively CKBO-LI and CKBO-HI), resonant KBOs (practically Plutinos), scattered-disk objects (SDOs) and Centaurs—of 12 candidate dead comets, and of 85 Trojans. We perform a systematic analysis of all color distributions, and conclude by synthesizing the implications of the dynamical evolution and of the colors for the origin of the minor bodies of the Solar System. We find that the color distributions are remarkably consistent with the scenarios of the formation of TNOs by Gomes [Gomes, R.S., 2003. Icarus 161, 404-418] generalized by the “Nice” model [Levison, H.F., Morbidelli, A., VanLaerhoven, Ch., Gomes, R., Tsiganis, L., 2008. Icarus 196, 258-273], and of the Trojans by Morbidelli et al. [Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R., 2005. Nature 435, 462-465]. The color distributions of the Centaurs are globally similar to those of the CKBO-HI, the Plutinos and the SDOs. However the potential bimodality of their distributions allows to possibly distinguish two groups based on their (B−R) index: Centaur I with (B−R)>1.7 and Centaurs II with (B−R)<1.4. Centaurs I could be composed of TNOs (prominently CKBO-LI) and ultra red objects from a yet unstudied family. Centaurs II could consist in a population of evolved objects which have already visited the inner Solar System, and which has been scattered back beyond Jupiter. The diversity of colors of the ECs, in particular the existence of very red objects, is consistent with an origin in the Kuiper belt. Candidate dead comets represent an ultimate state of evolution as they appear more evolved than the Trojans and Centaurs II.     

Dust Morphology Of The Inner Coma Of C/1995 O1 (Hale-Bopp)

Dust continuum imaging of comet C/1995 O1 (Hale-Bopp) was carried out with the Swedish Vacuum Solar Telescope (SVST)on La Palma in April, 1997. Images were reduced according to standard procedure, aligned, averaged, navigated and enhanced with azimuthal renormalization, rotational derivative, temporal derivative and unsharp masking processing. The rotational period of the nucleus was determined to 11.5 h and the mean projected dust outflow velocity to 0.41 km s⁻¹. Shell envelopes in the sunward side of the coma were separated by a projected distance of ∼15 000–20 000 km and spiralling inwards towards smaller radii in the direction of local evening. Small scale inhomogeneities of size 1 000–2 000 km, interpreted as correlated with variations in dust emission activity, were seen at radii ≤20 000 km. Two overlapping shell systems with a relative lag angle of ∼55° were evident at the time. The north pole of the nucleus was directed towards the Earth. The dust emission pattern is very complex and may be due to several active areas. The shape of the incomplete spiral shell pattern indicates that the angle between the line-of-sight and the rotational axis of the nucleus was not large. This revised version was published online in July 2006 with corrections to the Cover Date.     

Two dynamical classes of Centaurs

The Centaurs are a transient population of small bodies in the outer Solar System whose orbits are strongly chaotic. These objects typically suffer significant changes of orbital parameters on timescales of a few thousand years, and their orbital evolution exhibits two types of behaviors described qualitatively as random walk and resonance-sticking. We have analyzed the chaotic behavior of the known Centaurs. Our analysis has revealed that the two types of chaotic evolution are quantitatively distinguishable: (1) the random walk type behavior is well described by so-called generalized diffusion in which the rms deviation of the semimajor axis grows with time t as ∼t^H, with Hurst exponent H in the range 0.22-0.95, however (2) orbital evolution dominated by intermittent resonance sticking, with sudden jumps from one mean motion resonance to another, has poorly defined H. We further find that these two types of behavior are correlated with Centaur dynamical lifetime: most Centaurs whose dynamical lifetime is less than ∼22 Myr exhibit generalized diffusion, whereas most Centaurs of longer dynamical lifetimes exhibit intermittent resonance sticking. We also find that Centaurs in the diffusing class are likely to evolve into Jupiter-family comets during their dynamical lifetimes, while those in the resonance-hopping class do not.     

The effect on the Edgeworth–Kuiper Belt of a large distant tenth planet

We investigate the orbital evolution of both real and hypothetical Edgeworth–Kuiper Objects in order to determine whether any conclusions can be drawn regarding the existence, or otherwise, of the tenth planet postulated by Murray. We find no qualitative difference in the orbital evolution, and so conclude that the hypothetical planet has been placed on an orbit at such a large heliocentric distance that no evidence for the existence, or non-existence, can be found from a study of the known Edgeworth–Kuiper Objects.