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Ex-Terre born between arms powder


What prevents the grains of dust in a protoplanetary disk plunging towards the young star at its center and thus enable it to gather and maybe form rocky planets? One possible answer is given to the study of Alan Boss, of the Carnegie Institution for Science in Washington, published in the journal The Astrophysical Journal.
Those that arise from a few years astrophysicists who study the formation of planetary systems is a simple but crucial question: how do the grains of dust surrounding young stars to avoid falling on the stars themselves and succeed well enough to aggregate into structures large that they become then real planets? Simple question, he said, but his answer. The typical scenario of a system where there are exoplanets “construction” shows a young star surrounded by a large disk of gas and dust in rotation. The grains of dust of the disk collide with one another and aggregate to form larger and larger objects: pebbles, boulders, up to planetesimals, large structures also some hundreds of kilometers which can be considered the embryonic stages of rocky planets. These aggregation processes are, however, opposed by some phenomena.

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Credit: Alan Boss

One of these is given by the spiraling motion of the gas that makes up the protoplanetary disk: its whirl triggers a real wind directed toward the star, which can drag with it the “seeds” of planets in formation, especially those sized between one and ten meters radius. If too many of these bodies are ingurgitated from the star, the disc does not remain enough material to assemble planetesimals and, therefore, planets. But there seems to be a way out: the proposed new study by Alan Boss, of the Carnegie Institution for Science in Washington (United States), based on computer simulations.
What can save pebbles and boulders from their end are the regular stages in which the stars towards which precipitate, young and not yet in a stable phase of their evolutionary cycle, showing sudden spikes in brightness, combined with equally violent explosions in the outer layers of their atmospheres. Phenomena typical in the stars size solar, lasting a hundred years and especially associated with periods of gravitational instability in their disc of gas and dust. Boss simulations show that these disturbances can push bodies the size of even a few meters to walk away from their star. Simulations that are based on recent observations of spiral arms in planetary systems in formation, which can drag due to gravitational potential bricks future rocky planets towards the periphery of the disc and accelerate their growth process. How Much? Enough for it to reach a size beyond which the dragging effect due to gas becomes negligible.
“This study shows that the celestial bodies the size of a few meters, the large boulders in short, can be dispersed in the disk of the protostar formation of spiral arms and avoid plunging toward it,” says Boss. “Once these bodies have reached the more remote areas of the disc, they can grow to successive aggregations and get to become planetesimals.” The same does not happen, however, always according to the results of the simulations, for smaller objects, the size of a few centimeters, which would tend to be surely captured by the star. “We cannot say that every protostar through these brief phases of gravitational destruction, but we believe that they play a role in the formation in the early stages of formation of terrestrial planets far more important than previously thought so far,” concludes Boss.

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