Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is n
ot understood. We show that the main growth of asteroids can result from gas-drag-assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo run-away accretion of chondrules within ~3 Myr, forming planetary embryos up to Mars sizes along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size-sorting of chondrules consistent with chondrites. Accretion of mm-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disk life time outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles for the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts.
We analyze albedo data obtained using the Herschel Space Observatory that reveal the existence of two distinct types of surface among midsized transneptunian objects. A color-albedo diagram shows two large clusters of objects, one redder and higher a
lbedo and another darker and more neutrally colored. Crucially, all objects in our sample located in dynamically stable orbits within the classical Kuiper belt region and beyond are confined to the bright-red group, implying a compositional link. Those objects are believed to have formed further from the Sun than the dark-neutral bodies. This color-albedo separation is evidence for a compositional discontinuity in the young solar system.
We perform hydrodynamical simulations of the accretion of pebbles and rocks onto protoplanets of a few hundred kilometres in radius, including two-way drag force coupling between particles and the protoplanetary disc gas. Particle streams interacting
with the gas far out within the Hill sphere of the protoplanet spiral into a prograde circumplanetary disc. Material is accreted onto the protoplanet due to stirring by the turbulent surroundings. We speculate that the trend for prograde rotation among the largest asteroids is primordial and that protoplanets accreted 10%-50% of their mass from pebbles and rocks during the gaseous solar nebula phase. Our model also offers a possible explanation for the narrow range of spin periods observed among the largest bodies in the asteroid and trans-Neptunian belts, and predicts that 1000 km-scale Kuiper belt objects that have not experienced giant impacts should preferentially spin in the prograde direction.
We re-examine the correlation between the colors and the inclinations of the Classical Kuiper Belt Objects (CKBOs) with an enlarged sample of optical measurements. The correlation is strong (rho=-0.7) and highly significant (>8 sigma) in the range 0-
34 deg. Nonetheless, the optical colors are independent of inclination below ~12 deg, showing no evidence for a break at the reported boundary between the so-called dynamically hot and cold populations near ~5 deg. The commonly accepted parity between the dynamically cold CKBOs and the red CKBOs is observationally unsubstantiated, since the group of red CKBOs extends to higher inclinations. Our data suggest, however, the existence of a different color break. We find that the functional form of the color-inclination relation is most satisfactorily described by a non-linear and stepwise behavior with a color break at ~12 deg. Objects with inclinations >12 deg show bluish colors which are either weakly correlated with inclination or are simply homogeneously blue, whereas objects with inclinations <12 deg are homogeneously red.
We present high precision, time-resolved visible and near infrared photometry of the large (diameter ~ 2500 km) Kuiper belt object (136108) 2003 EL61. The new data confirm rapid rotation at period P = 3.9155+/-0.0001 hr with a peak-to-peak photometri
c range (Delta m_R) = 0.29+/-0.02 mag and further show subtle but reproducible color variations with rotation. Rotational deformation of 2003 EL61 alone would give rise to a symmetric lightcurve free of color variations. The observed photometric deviations from the best-fit equilibrium model show the existence of a large surface region with an albedo and color different from the mean surface of 2003 EL61. We explore constraints on the nature of this anomalous region set by the existing data.
We present the results of an optical lightcurve survey of 114 Jovian Trojan asteroids conducted to determine the fraction of contact binaries. Sparse-sampling was used to assess the photometric range of the asteroids and those showing the largest ran
ges were targeted for detailed follow-up observations. This survey led to the discovery of two Trojan asteroids, (17365) and (29314) displaying large lightcurve ranges (~1 magnitude) and long rotation periods (< 2 rotations per day consistent with a contact binary nature. The optical lightcurves of both asteroids are well matched by Roche binary equilibrium models. Using these binary models, we find low densities of ~600 kg/m^3 and 800 kg/m^3, suggestive of porous interiors. The fraction of contact binaries is estimated to be between 6% and 10%, comparable to the fraction in the Kuiper Belt. The total binary fraction in the Trojan clouds (including both wide and close pairs) must be higher.
