Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion

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 not 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 runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars's size 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 millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime 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 in 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.

From gas to stars over cosmic time

From the time the first stars formed over 13 billion years ago to the present, star formation has had an unexpectedly dynamic history. At first, the star-formation rate density increased dramatically, reaching a peak 10 billion years ago of more than 10 times the present-day value. Observations of the initial rise in star formation remain difficult, poorly constraining it. Theoretical modeling has trouble predicting this history because of the difficulty in following the feedback of energy from stellar radiation and supernova explosions into the gas from which further stars form. Observations from the ground and space with the next generation of instruments should reveal the full history of star formation in the universe, and simulations appear poised to accurately predict the observed history.

Rapid planetesimal formation in turbulent circumstellar disks

During the initial stages of planet formation in circumstellar gas disks, dust grains collide and build up larger and larger bodies. How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem: boulders are expected to stick together poorly, and to spiral into the protostar in a few hundred orbits owing to a 'headwind' from the slower rotating gas. Gravitational collapse of the solid component has been suggested to overcome this barrier. But even low levels of turbulence will inhibit sedimentation of solids to a sufficiently dense midplane layer, and turbulence must be present to explain observed gas accretion in protostellar disks. Here we report that boulders can undergo efficient gravitational collapse in locally overdense regions in the midplane of the disk. The boulders concentrate initially in transient high pressure regions in the turbulent gas, and these concentrations are augmented a further order of magnitude by a streaming instability driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar disks.