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TOPIC: Solar System formation


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Solar System nebula
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Scientists estimate solar nebula's lifetime

About 4.6 billion years ago, an enormous cloud of hydrogen gas and dust collapsed under its own weight, eventually flattening into a disk called the solar nebula. Most of this interstellar material contracted at the disk's center to form the sun, and part of the solar nebula's remaining gas and dust condensed to form the planets and the rest of our solar system.
Now scientists from MIT and their colleagues have estimated the lifetime of the solar nebula - a key stage during which much of the solar system evolution took shape.
This new estimate suggests that the gas giants Jupiter and Saturn must have formed within the first 4 million years of the solar system's formation. Furthermore, they must have completed gas-driven migration of their orbital positions by this time.
 
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Solar system could have evolved from poorly mixed elemental soup

Planetary scientists have long believed that the Earth formed from planetary objects similar to meteorites. A decade ago, perplexing new measurements challenged that assumption by showing that the Earth and its supposed "building blocks" actually contain significantly different isotopic compositions.
For the past 10 years, scientists have been trying to understand why. Recent work by the University of Chicago's Christoph Burkhardt and Nicolas Dauphas, together with their collaborators from Lawrence Livermore National Laboratory and the University of Münster, suggest a new explanation that may help illuminate both the composition of the Earth and the beginnings of the solar system.

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Title: Planetary and meteoritic Mg/Si and d30Si variations inherited from solar nebula chemistry
Author: Nicolas Dauphas, Franck Poitrasson, Christoph Burkhardt, Hiroshi Kobayashi, Kosuke Kurosawa

The bulk chemical compositions of planets are uncertain, even for major elements such as Mg and Si. This is due to the fact that the samples available for study all originate from relatively shallow depths. Comparison of the stable isotope compositions of planets and meteorites can help overcome this limitation. Specifically, the non-chondritic Si isotope composition of the Earth's mantle was interpreted to reflect the presence of Si in the core, which can also explain its low density relative to pure Fe-Ni alloy. However, we have found that angrite meteorites display a heavy Si isotope composition similar to the lunar and terrestrial mantles. Because core formation in the angrite parent-body (APB) occurred under oxidizing conditions at relatively low pressure and temperature, significant incorporation of Si in the core is ruled out as an explanation for this heavy Si isotope signature. Instead, we show that equilibrium isotopic fractionation between gaseous SiO and solid forsterite at 1370 K in the solar nebula could have produced the observed Si isotope variations. Nebular fractionation of forsterite should be accompanied by correlated variations between the Si isotopic composition and Mg/Si ratio following a slope of 1, which is observed in meteorites. Consideration of this nebular process leads to a revised Si concentration in the Earth's core of 3.6 (+6.0/-3.6) wt% and provides estimates of Mg/Si ratios of bulk planetary bodies.

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Title: Jupiter's Decisive Role in the Inner Solar System's Early Evolution
Author: Konstantin Batygin, Gregory Laughlin

The statistics of extrasolar planetary systems indicate that the default mode of planet formation generates planets with orbital periods shorter than 100 days, and masses substantially exceeding that of the Earth. When viewed in this context, the Solar System is unusual. Here, we present simulations which show that a popular formation scenario for Jupiter and Saturn, in which Jupiter migrates inward from a > 5 AU to a ~ 1.5 AU before reversing direction, can explain the low overall mass of the Solar System's terrestrial planets, as well as the absence of planets with a < 0.4 AU. Jupiter's inward migration entrained s ~ 10-100 km planetesimals into low-order mean-motion resonances, shepherding and exciting their orbits. The resulting collisional cascade generated a planetesimal disk that, evolving under gas drag, would have driven any pre-existing short-period planets into the Sun. In this scenario, the Solar System's terrestrial planets formed from gas-starved mass-depleted debris that remained after the primary period of dynamical evolution.

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Fifth giant ex-planet
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Title: Fifth giant ex-planet of the outer Solar System: characteristics and remnants
Authors: Yury I. Rogozin

In the past, the outer Solar System likely could have more planets than now. Using the new relations, we have found the orbital and physical characteristics of the icy giant explanet, which orbited the Sun about in the halfway between Saturn and Uranus. Validity of the obtained results is supported by the feasibility of these relations to other objects of the outer Solar System. Possible connection of the existing now mysterious objects of the outer Solar System such as the Saturn rings and the irregular moons Triton and Phoebe with this destroyed planet is briefly discussed.

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RE: Solar System formation
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Sun's Shock Waves May Have Staggered Solar System's Planet Formation

Our solar system's planets may have formed at differing times, determined by shock waves flowing from the young sun, one astronomer suggests.
This theory posits that Earth is one of the youngest planets in the solar system, along with Mercury, Venus and Mars.

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Jet-set young sun pushed baby planets off kilter

Something is amiss with the otherwise well-behaved planets in the solar system. They all line up with each other obediently enough, but their orbital plane is slightly offset from the sun's equator.
Now there is a clue to what caused this rebellious streak: it's possible the baby planets strayed while trying to keep up with their jet-setting parent.
Computer simulations show that asymmetrical jets on the young sun may have pushed it around in such a way that its family of planets became tilted.

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Title: Rapid Formation of Saturn after Jupiter Completion
Authors: Hiroshi Kobayashi, Chris W. Ormel, Shigeru Ida

We have investigated Saturn's core formation at a radial pressure maximum in a protoplanetary disk, which is created by gap opening by Jupiter. A core formed via planetesimal accretion induces the fragmentation of surrounding planetesimals, which generally inhibits further growth of the core by removal of the resulting fragments due to radial drift caused by gas drag. However, the emergence of the pressure maximum halts the drift of the fragments, while their orbital eccentricities and inclinations are efficiently damped by gas drag. As a result, the core of Saturn rapidly grows via accretion of the fragments near the pressure maximum. We have found that in the minimum-mass solar nebula, kilometre sized planetesimals can produce a core exceeding 10 Earth masses within two million years. Since Jupiter may not have undergone significant type II inward migration, it is likely that Jupiter's formation was completed when the local disk mass has already decayed to a value comparable to or less than Jovian mass. The expected rapid growth of Saturn's core on a timescale comparable to or shorter than observationally inferred disk lifetime enables Saturn to acquire the current amount of envelope gas before the disk gas is completely depleted. The high heat energy release rate onto the core surface due to the rapid accretion of the fragments delays onset of runaway gas accretion until the core mass becomes somewhat larger than that of Jupiter, which is consistent with the estimate based on interior modelling. Therefore, the rapid formation of Saturn induced by gap opening of Jupiter can account for the formation of multiple gas giants (Jupiter and Saturn) without significant inward migration and larger core mass of Saturn than that of Jupiter.

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Solar nebula
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Title: Cosmochemical Consequences of Particle Trajectories During FU Orionis Outbursts by the Early Sun
Authors: Alan P. Boss, Conel M. O'D. Alexander, Morris Podolak

The solar nebula is thought to have undergone a number of episodes of FU Orionis outbursts during its early evolution. We present here the first calculations of the trajectories of particles in a marginally gravitationally unstable solar nebula during an FU Orionis outburst, which show that 0.1 to 10 cm-sized particles traverse radial distances of 10 AU or more, inward and outward, in less than 200 yrs, exposing the particles to temperatures from \sim 60 K to \sim 1500 K. Such trajectories can thus account for the discovery of refractory particles in comets. Refractory particles should acquire Wark-Lovering-like rims as they leave the highest temperature regions of the disk, and these rims should have significant variations in their stable oxygen isotope ratios. Particles are likely to be heavily modified or destroyed if they pass within 1 AU of the Sun, and so are only likely to survive if they formed in the final few FU Orionis outbursts, or were transported to the outer reaches of the solar system. Calcium, aluminium-rich inclusions (CAIs) from primitive meteorites are the oldest known solar system objects and have a very narrow age range. Most CAIs may have formed at the end of the FU Orionis outbursts phase, with an age range reflecting the period between the last few outbursts.

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New isotope measurement could alter history of early solar system

The early days of our solar system might look quite different than previously thought, according to research at the U.S. Department of Energy's (DOE) Argonne National Laboratory published in Science. The study used more sensitive instruments to find a different half-life for samarium, one of the isotopes used to chart the evolution of the solar system.

"It shrinks the chronology of early events in the solar system, like the formation of planets, into a shorter time span. It also means some of the oldest rocks on Earth would have formed even earlier - as early as 120 million years after the solar system formed, in one case of Greenland rocks" - Argonne physicist Michael Paul.

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