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Gas-giant planets like Jupiter and Saturn form soon after their stars do, according to new research.
Observations from NASA's Spitzer Space Telescope show that gas giants either form within the first 10 million years of a sun-like star's life, or not at all. The study offers new evidence that gas-giant planets must form early in a star's history. The lifespan of sun-like stars is about 10 billion years.
Ilaria Pascucci of the University of Arizona Steward Observatory in Tucson led a team of astronomers who conducted the most comprehensive search for gas around 15 different sun-like stars, most with ages ranging from 3 million to 30 million years.

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Title: Formation of Earth-like Planets During and After Giant Planet Migration
Authors: Avi M. Mandell, Sean N. Raymond, Steinn Sigurdsson

Close-in giant planets are thought to have formed in the cold outer regions of planetary systems and migrated inward, passing through the orbital parameter space occupied by the terrestrial planets in our own Solar System. We present dynamical simulations of the effects of a migrating giant planet on a disk of protoplanetary material and the subsequent evolution of the planetary system. We numerically investigate the dynamics of post-migration planetary systems over 200 million years using models with a single migrating giant planet, one migrating and one non-migrating giant planet, and excluding the effects of a gas disk. Material that is shepherded in front of the migrating giant planet by moving mean motion resonances accretes into "hot Earths", but survival of these bodies is strongly dependent on dynamical damping. Furthermore, a significant amount of material scattered outward by the giant planet survives in highly excited orbits; the orbits of these scattered bodies are then damped by gas drag and dynamical friction over the remaining accretion time. In all simulations Earth-mass planets accrete on approximately 100 Myr timescales, often with orbits in the Habitable Zone. These planets range in mass and water content, with both quantities increasing with the presence of a gas disk and decreasing with the presence of an outer giant planet. We use scaling arguments and previous results to derive a simple recipe that constrains which giant planet systems are able to form and harbour Earth-like planets in the Habitable Zone, demonstrating that roughly one third of the known planetary systems are potentially habitable.

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Title: Solar System Processes Underlying Planetary Formation, Geodynamics, and the Georeactor
Authors: J. Marvin Herndon
(revised v2)

Only three processes, operant during the formation of the Solar System, are responsible for the diversity of matter in the Solar System and are directly responsible for planetary internal-structures, including planetocentric nuclear fission reactors, and for dynamical processes, including and especially, geodynamics. These processes are: (i) Low-pressure, low-temperature condensation from solar matter in the remote reaches of the Solar System or in the interstellar medium; (ii) High-pressure, high-temperature condensation from solar matter associated with planetary-formation by raining out from the interiors of giant-gaseous protoplanets, and; (iii) Stripping of the primordial volatile components from the inner portion of the Solar System by super-intense solar wind associated with T-Tauri phase mass-ejections, presumably during the thermonuclear ignition of the Sun. As described herein, these processes lead logically, in a causally related manner, to a coherent vision of planetary formation with profound implications including, but not limited to, (a) Earth formation as a giant gaseous Jupiter-like planet with vast amounts of stored energy of protoplanetary compression in its rock-plus-alloy kernel; (b) Removal of approximately 300 Earth-masses of primordial gases from the Earth, which began Earth's decompression process, making available the stored energy of protoplanetary compression for driving geodynamic processes, which I have described by the new whole-Earth decompression dynamics and which is responsible for emplacing heat at the mantle-crust-interface at the base of the crust through the process I have described, called mantle decompression thermal-tsunami; and, (c)Uranium accumulations at the planetary centres capable of self-sustained nuclear fission chain reactions.

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It was a beautiful, sunny day on the French Riviera, and four astronomers were having lunch at the Observatoire de la CŰte d'Azur overlooking the city of Nice and the deep blue Mediterranean, as they had been doing most days for half a year. Despite the scenery and relaxed setting, the researchers were on edge. They had been struggling for months, and now, in the early spring of 2004, they were maddeningly close to proving a revolutionary concept of how our solar system had evolved.
The crux of their theory was that the four giant planets - Jupiter, Saturn, Uranus and Neptune - were born much closer together than they are today. Their orbits may have changed over time, eventually triggering a series of dramatic planetary movements and violent collisions.

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The locked migration of giant protoplanets:
Press release

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Two British astronomers, Paul Cresswell and Richard Nelson, will present new computer simulations of how planetary systems form.
They find that, in the early stages of planetary formation, giant protoplanets migrate inward, locked into mutual orbital resonances, towards the central star.



Their simulations show that, in very few cases (about 2%), a lone protoplanet is ejected far from the central star, thus lengthening its lifetime. But in most cases (98%), many of the protoplanets are locked into orbital resonances and migrate together inwards. Some may even merge with the central star.
Cresswell and Nelson thus claim that gravitational interactions within a swarm of protoplanets embedded in a disc cannot stop the inward migration of the protoplanets.
The "problem" of migration still remains.

Their research will be published in an upcoming article in Astronomy & Astrophysics.

Abstract:
Planets form from the gaseous disc that orbits a protostar. Dust grains embedded in the disc grow through collisions, accumulating first through electrostatic forces and later by gravity. The resultant objects pass through mm-, cm-, metre-sizes etc. until planetary cores of several hundred kilometres are formed. Once beyond a critical mass (around 10 Earth masses, ME) the protoplanet begins to accrete gas on neighbouring orbits, a process that takes a few million years to form a Jupiter-sized planet.
The gaseous component of the disc remains present during this process, and the imbalance of gravitational torques interior and exterior to a protoplanetary coreís near-circular orbit result in the core migrating inwards through the disk. An object the size of Jupiterís rocky core (about 10ME), starting from Jupiterís current orbital radius in a typical disc will spiral into the central star in only a few thousands of years. Hence a mechanism is needed that can halt (or even reverse) migration to prevent the destruction of protoplanets; moreover, it must be allowed to operate at a distance from the central body.

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