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Pluto's twin is out in the cold Four trans-Neptunian objects are currently recognized as dwarf planets: Eris, Haumea, Makemake and Pluto. Of these, the 'demoted' planet Pluto has been studied for many years and has a detected atmosphere. The others are difficult to observe because of their extreme distance from the Sun, but a stellar occultation event on 6 November 2010 provided an opportunity for a closer look at Eris. The data obtained reveal Eris as a 'twin' for Pluto in terms of size, and previous work showed the two to have similar surface compositions. Eris, however, has no detectable atmosphere and its surface is bright, possibly a result of atmospheric collapse in an extremely cold environment. The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto 1 . It resides at present at 95.7 astronomical units (1  au is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 ut . The event is consistent with a spherical shape for Eris, with radius 1,163 ± 6 kilometres, density 2.52 ± 0.05 grams per cm 3 and a high visible geometric albedo, . No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ∼1 nanobar, about 10,000 times more tenuous than Pluto's present atmosphere 2 , 3 , 4 , 5 . As Pluto's radius is estimated 3 , 4 , 5 , 6 , 7 , 8 to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun.

Observations of a stellar occultation by (10199) Chariklo, a minor body that orbits the Sun between Jupiter and Neptune, reveal that it has a ring system, a property previously observed only for the four giant planets of the Solar System. Tiny Chariklo has its own ring system Observations of a stellar occultation by (10199) Chariklo, a Centaur-class outer-system asteroid orbiting between Saturn and Uranus, reveal that it has a ring system, a feature previously observed only for the four giant planets. Chariklo, with a diameter of about 250 km, has two narrow and dense rings separated by a small gap, probably due to the presence of a (yet-to-be-found) kilometre-sized satellite. The discovery of these rings raises questions about the formation and dynamical evolution of planetary rings. For one thing, it seems likely that planetary rings are much more common than previously thought. Hitherto, rings have been found exclusively around the four giant planets in the Solar System 1 . Rings are natural laboratories in which to study dynamical processes analogous to those that take place during the formation of planetary systems and galaxies. Their presence also tells us about the origin and evolution of the body they encircle. Here we report observations of a multichord stellar occultation that revealed the presence of a ring system around (10199) Chariklo, which is a Centaur—that is, one of a class of small objects orbiting primarily between Jupiter and Neptune—with an equivalent radius of 124   9 kilometres (ref. 2 ). There are two dense rings, with respective widths of about 7 and 3 kilometres, optical depths of 0.4 and 0.06, and orbital radii of 391 and 405 kilometres. The present orientation of the ring is consistent with an edge-on geometry in 2008, which provides a simple explanation for the dimming 3 of the Chariklo system between 1997 and 2008, and for the gradual disappearance of ice and other absorption features in its spectrum over the same period 4 , 5 . This implies that the rings are partly composed of water ice. They may be the remnants of a debris disk, possibly confined by embedded, kilometre-sized satellites.

Aims. The aim of this work is to study the dynamical effects of the Galaxy on binary star systems with physical and orbital charac- teristics similar to those of the population of known wide binary stars with exoplanets. As secondary goal we analyse the possible consequences on the stability of a hypothetical planetary system orbiting one of the stellar components. Methods. We numerically reproduced the temporal evolution of a sample of 3 * 10 5 binary star systems disturbed by the Galactic potential and passing stars in an environment similar to the solar neighbourhood. Results. Our results show that the dynamical evolution of the population of wide binary stars with exoplanets in the solar neigh- bourhood is modelled by the process of disruption of binary star systems induced by the Galaxy. We found that this process depends mainly on the separation between both stars, whereas it is almost independent of the initial orbital configuration. Moreover, our calcu- lations are in agreement with the results of previous works regarding the indirect influence of the Galaxy on the stability of planetary systems in wide binary stars. However, the effects on the planetary region show a dependence on the initial configuration of binary stars. Finally, we obtain an indirect test of the impulse approximation model for dynamical studies of binary star systems.

The icy dwarf planet Makemake has projected axes of 1,430 ± 9 and 1,502 ± 45 km and a V-band geometric albedo larger than Pluto’s but smaller than Eris’s, with no global Pluto-like atmosphere. Makemake shapes up against Pluto and Eris Makemake is thought to be the third-largest dwarf planet in our Solar System, a little smaller than Pluto and Eris, but until now knowledge of its size and albedo were only approximate. This paper reports the results of observations of the occultation of a faint star known as NOMAD 1181-0235723 by Makemake on 23 April 2011. The data confirm that Makemake is smaller than Pluto and Eris, with axes of 1,430±9 km and 1,502±45 km. Makemake's mean geometric albedo — the ratio of light reflected to light received — is intermediate between that of Pluto and that of Eris. All three are icy, making them among the most reflective objects in the Solar System. And the occultation light curves rule out the presence of a global Pluto-like atmosphere on Makemake, although the presence of dark terrain might imply the presence of a localized atmosphere. Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies 1 , 2 . Pluto possesses an atmosphere whereas Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos 3 . Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto 4 , and is currently at a distance to the Sun intermediate between the two. Although Makemake’s size (1,420 ± 60 km) and albedo are roughly known 5 , 6 , there has been no constraint on its density and there were expectations that it could have a Pluto-like atmosphere 4 , 7 , 8 . Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1 σ ) and 1,502 ± 45 km, implying a V-band geometric albedo p V = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4–12 nanobar (1 σ ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm −3 is inferred from the data.

Context. Among the main effects that the Milky Way exerts in binary systems, the Galactic tide is the only one that is not probabilistic and can be deduced from a potential. Therefore, it is possible to perform an analysis of the global structure of the phase space of binary systems in the solar neighbourhood using the Galactic potential. Aims. The aim of this work is to obtain a simple model to study the collisionless dynamical evolution of generic wide binaries systems in the solar neighbourhood. Methods. Through an averaging process, we reduced the three-dimensional potential of the Galaxy to a secular one-degree of freedom model. The accuracy of this model was tested by comparing its predictions with numerical simulations of the exact equations of motion of a two-body problem disturbed by the Galaxy. Results. Using the one-degree of freedom model, we developed a detailed dynamical study, finding that the secular Galactic tide period changes as a function of the separation of the pair, which also gives a dynamical explanation for the arbitrary classification between \"wide\" and \"tight\" binaries. Moreover, the secular phase space for a generic gravitationally bound pair is similar to the dynamical structure of a Lidov-Kozai resonance, but surprisingly this structure is independent of the masses and semimajor axis of the binary system. Thus, the Galactic potential is able to excite the initially circular orbit of binary systems to high values of eccentricity, which has important implications for studies of binary star systems (with and without exoplanets), comets, and Oort cloud objects.

The Hilda asteroids are located in the outer main belt in a stable 3:2 mean-motion resonance with Jupiter, while the quasi-Hildas (qH) have similar orbits but are not directly under the effect of the MMR. Moreover, cometary activity has been detected in qH. In this study, we investigate the collisional evolution of Hilda asteroids and apply it to study the cratering on asteroid 334 Chicago, as well as to determine whether impacts between Hildas and qH can serve as a viable mechanism for inducing cometary activity. We simulated the collisional evolution of Hilda asteroids over a period of 4 Gyr. We considered three initial size-frequency distributions (SFD) and two scaling laws for the collisional outcomes and performed a large set of simulations for each scenario which we used to construct median SFDs of the Hilda population. We also derived impactor SFD on asteroid 334 Chicago and used it to calculate the crater SFD on 334 Chicago. Additionally, we evaluated the subcatastrophic impact timescale between Hilda and qH objects. The observed SFD of Hilda asteroids larger than 3 km is best matched by scenarios assuming that such SFD is mostly primordial, implying minimal collisional activity over time. For smaller sizes, although unconstrained, the SFD steepens significantly due to the catastrophic fragmentation of a small number of multikilometer-sized bodies. We determined that the largest impactor on 334 Chicago measures a few kilometers in size, resulting in a maximum crater size of approximately 30 km. Furthermore, the slope of the crater SFD mirrors that of the initial SFD for subkilometric bodies. While impact events between Hildas and qH can induce observable activity and although stochastic in nature, the timescale of such events exceeds the dynamical lifetime of qH, making them an unlikely primary mechanism for inducing observable activity.

We study the dynamical and collisional evolution of Near-Earth asteroids (NEAs) in Main Belt-crossing orbits (NEACs). We select NEACs with H < 18 and integrate their orbits for 1e7 yr with N-body simulations. Objects are grouped by initial semi-major axis (G1: a < 2.06 au; G2: 2.06 < a < 2.5 au; G3: a > 2.5 au). We compute the fraction of each orbit spent within the main belt (MB), dynamical occupancy maps in the (a,e) plane, and median lifetimes. Using collisional evolution, we obtain size-dependent timescales, the change in the NEA size-frequency distribution (SFD) over 1 Myr, and impactor and crater SFDs on 150 m to 1 km targets, representative of NEAs visited by space missions. Median dynamical lifetimes decrease with increasing a: ~1.3e7 yr (G1), ~2.1e6 yr (G2), and ~0.9e6 yr (G3). NEACs in G2-G3 maintain nearly constant MB residence fractions with short intervals of full containment, while G1 exhibits stronger 0-0.8 oscillations (median ~0.55 for ~1e6 yr). DART-analog impacts occur on ~1e5 yr timescales for targets smaller than about 300 m (rising to ~1e6 yr for larger bodies), whereas catastrophic collisions are negligible within NEAC lifetimes. Over 1 Myr, collisional erosion reduces the meter-size NEA population by only 0.1-1.4% depending on Q_D*. Comparison with the observed crater SFDs on Bennu, Didymos, and Ryugu indicates target strengths of Y ~ 100 Pa for Bennu, young effective surface ages for Didymos, and short crater-retention times of order 1e4-1e5 yr for craters with diameters smaller than 100 m on Ryugu, consistent with rapid resurfacing. NEACs spend a substantial fraction of their lifetimes inside the MB and undergo frequent small-scale impacts, yet collisions weakly modify the global NEA SFD on Myr timescales. Our combined dynamical-collisional framework constrains NEAC lifetimes, orbital pathways, collisional timescales, and surface processing.

In this paper, we perform a dynamical study of the population of objects in the unstable quasi-Hilda region. The aim of this work is to make an update of the population of quasi-Hilda comets (QHCs) that have recently arrived from the Centaurs region. To achieve our goal, we have applied a dynamical criteria to constrain the unstable quasi-Hilda region that allowed us to select 828 potential candidates. The orbital data of the potential candidates was take from the ASTORB database and we apply backward integration to search by those that have recently arrived from the outer regions of the Solar System. Then we studied the dynamical evolution of the candidates from a statistical point of view by calculating the time-averaged distribution of a number of clones of each candidate as a function of aphelion and perihelion distances. We found that 47 objects could have been recently injected into the inner Solar System from the Centaur or transneptunian regions. These objects may have preserved volatile material and are candidates to exhibit cometary activity.

Aims. We aim to investigate how polarimetric observations can improve our understanding of the nature and diversity of M/X-type asteroids. Methods. Polarimetric observations of the selected M/X-type asteroids were carried out at the Tohoku 0.6-m telescope at Haleakala Observatory, Hawaii (simultaneously in BVR filters), the 2-m telescope of the Bulgarian National Astronomical Observatory in Rozhen (in R filter), and the 2.15-m telescope of the Complejo Astronómico El Leoncito (CASLEO), Argentina (in V filter). We analysed the polarimetric characteristics of M/X-type asteroids along with the available data obtained by other techniques. Results. New polarimetric observations of 22 M/X-type asteroids combined with published observations provide a data set of 41 asteroids for which the depth of a negative polarisation branch and/or inversion angle were determined. We found that the depth of the negative polarisation branch tends to increase with decreasing steepness of the near-infrared spectra. Asteroids with a deeper negative polarisation branch tend to have a higher radar circular polarisation ratio. We show that, based on the relationship of the depth of the negative polarisation branch and inversion angle, two main sub-types can be distinguished among M-type asteroids. We suggest that these groups may be related to different surface compositions similar to (1) irons and stony-irons and (2) enstatite and iron-rich carbonaceous chondrites.