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Planet Formation
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UC Davis researchers have dated the earliest step in the formation of the solar system -- when microscopic interstellar dust coalesced into mountain-sized chunks of rock -- to 4,568 million years ago, within a range of about 2,080,000 years.
UC Davis postdoctoral researcher Frederic Moynier, Qing-zhu Yin, assistant professor of geology, and graduate student Benjamin Jacobsen established the dates by analysing a particular type of meteorite, called a carbonaceous chondrite, which represents the oldest material left over from the formation of the solar system.
The physics and timing of this first stage of planet formation are not well understood, Yin said. So, putting time constraints on the process should help guide the physical models that could be used to explain it.
In the second stage, mountain-sized masses grew quickly into about 20 Mars-sized planets and, in the third and final stage, these small planets smashed into each other in a series of giant collisions that left the planets we know today. The dates of those stages are well established.

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Title: Observational Tests of Planet Formation Models
Authors: A. Sozzetti (1,2), G. Torres (1), D.W. Latham (1), B.W. Carney (3), J.B. Laird (4), R.P. Stefanik (1), A.P. Boss (5), D. Charbonneau (1), F.T. O'Donovan (6), M.J. Holman (1), J.N. Winn (7) ((1) CfA, (2) OATo, (3) UNC, (4) BGSU, (5) CIW, (6), NASA Goddard, (7) MIT)

We summarise the results of two experiments to address important issues related to the correlation between planet frequencies and properties and the metallicity of the hosts. Our results can usefully inform formation, structural, and evolutionary models of gas giant planets.

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Title: Three-Dimensional Simulations of Kelvin-Helmholtz Instability in Settled Dust Layers in Protoplanetary Disks
Authors: Joseph A. Barranco (San Francisco State University)

As dust settles in a protoplanetary disk, a vertical shear develops because the dust-rich gas in the midplane orbits at a rate closer to true Keplerian than the slower-moving dust-depleted gas above and below. A classical analysis (neglecting the Coriolis force and differential rotation) predicts that Kelvin-Helmholtz instability occurs when the Richardson number of the stratified shear flow is below roughly one-quarter. However, earlier numerical studies showed that the Coriolis force makes layers more unstable, whereas horizontal shear may stabilize the layers. Simulations with a 3D spectral code were used to investigate these opposing influences on the instability in order to resolve whether such layers can ever reach the dense enough conditions for the onset of gravitational instability. I confirm that the Coriolis force, in the absence of radial shear, does indeed make dust layers more unstable, however the instability sets in at high spatial wavenumber for thicker layers. When radial shear is introduced, the onset of instability depends on the amplitude of perturbations: small amplitude perturbations are sheared to high wavenumber where further growth is damped; whereas larger amplitude perturbations grow to magnitudes that disrupt the dust layer. However, this critical amplitude decreases sharply for thinner, more unstable layers. In 3D simulations of unstable layers, turbulence mixes the dust and gas, creating thicker, more stable layers. I find that layers with minimum Richardson numbers in the approximate range 0.2 -- 0.4 are stable in simulations with horizontal shear.

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Swirling eddies and chaotic vortices are crucial to the formation of new planets, suggests a counterintuitive new study.
Such turbulence is vital to helping planets go from "toddler" to "teenage" size by helping rocks and boulders stick together, the computer simulation hints.
This is a turnaround from several years ago, when scientists considered turbulence a destructive bugaboo for newly forming planets.

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Protoplanetesimals
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Title: Collisions between equal sized ice grain agglomerates
Authors: C. Schäfer, R. Speith, W. Kley (University of Tübingen)

Following the recent insight in the material structure of comets, protoplanetesimals are assumed to have low densities and to be highly porous agglomerates. It is still unclear if planetesimals can be formed from these objects by collisional growth. Therefore, it is important to study numerically the collisional outcome from low velocity impacts of equal sized porous agglomerates which are too large to be examined in a laboratory experiment. We use the Lagrangian particle method Smooth Particle Hydrodynamics to solve the equations that describe the dynamics of elastic and plastic bodies. Additionally, to account for the influence of porosity, we follow a previous developed equation of state and certain relations between the material strength and the relative density. Collisional growth seems possible for rather low collision velocities and particular material strengths. The remnants of collisions with impact parameters that are larger than 50% of the radius of the colliding objects tend to rotate. For small impact parameters, the colliding objects are effectively slowed down without a prominent compaction of the porous structure, which probably increases the possibility for growth. The protoplanetesimals, however, do not stick together for the most part of the employed material strengths. An important issue in subsequent studies has to be the influence of rotation to collisional growth. Moreover, for realistic simulations of protoplanetesimals it is crucial to know the correct material parameters in more detail.

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Planets Prefer Safe Neighbourhoods
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A star must live in a relatively tranquil cosmic neighbourhood to foster planet formation, say astronomers using NASA's Spitzer Space Telescope.

A team of scientists from the University of Arizona's Steward Observatory, Tucson, came to this conclusion after watching intense ultraviolet light and powerful winds from O-type stars rip away the potential planet-forming disks, or protoplanetary disks, around stars like our sun. At up to 100 times the mass of the sun, O stars are the most massive and energetic stars in the universe. They are at least a million times more powerful than the sun.
According to Dr. Zoltan Balog, lead author of the team's paper, the super-sensitive infrared eyes of Spitzer are ideal for capturing the "photoevaporation" of these planet-forming disks. In this process, immense output from the O star heats the disks that are surrounding nearby sun-like stars so much that gas and dust boil off (much like the evaporation of boiling water), and the disk can no longer hold together. Photon (or light) blasts from the O star then blow away the evaporated material, potentially stripping the sun-like stars of their ability to form planets.


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A Star's Close Encounter -- The potential planet forming disk (or “protoplanetary disk”) of a sun-like star is being violently ripped away by the powerful winds of a nearby hot O-type star in the upper image, from NASA's Spitzer Space Telescope. Text labels have been added to the identical image in the lower half of this picture.
Credit NASA/JPL-Caltech/Z. Balog (Univ. of Ariz./Univ. of Szeged)

The system is located about 2,450 light-years away in the star-forming cloud IC 1396.

Position(2000): RA: 21h38m57.09s Dec: 57d30m46.50s


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-- Edited by Blobrana at 18:35, 2006-10-03

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RE: Planet Formation
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Planet-Forming Disks Might Put the Brakes on Stars

Astronomers using NASA's Spitzer Space Telescope have found evidence that dusty disks of planet-forming material tug on and slow down the young, whirling stars they surround.

Young stars are full of energy, spinning around like tops in half a day or less. They would spin even faster, but something puts on the brakes. While scientists had theorized that planet-forming disks might be at least part of the answer, demonstrating this had been hard to do until now.

"We knew that something must be keeping the stars' speed in check. Disks were the most logical answer, but we had to wait for Spitzer to see the disks" - Dr. Luisa Rebull of NASA's Spitzer Science Center, Pasadena, California.

Rebull, who has been working on the problem for nearly a decade, is lead author of a new paper in the July 20 issue of the Astrophysical Journal. The findings are part of a quest to understand the complex relationship between young stars and their burgeoning planetary systems.
Stars begin life as collapsing balls of gas that spin faster and faster as they shrink, like twirling ice skaters pulling in their arms. As the stars whip around, excess gas and dust flatten into surrounding pancake-like disks. The dust and gas in the disks are believed to eventually clump together to form planets.
Developing stars spin so fast that, left unchecked, they would never fully contract and become stars. Prior to the new study, astronomers had theorised that disks might be slowing the super speedy stars by yanking on their magnetic fields. When a star's fields pass through a disk, they are thought to get bogged down like a spoon in molasses. This locks a star's rotation to the slower-turning disk, so the shrinking star can't spin faster.
To prove this principle, Rebull and her team turned to Spitzer for help. Launched in August of 2003, the infrared observatory is an expert at finding the swirling disks around stars, because dust in the disks is heated by starlight and glows at infrared wavelengths.
The team used Spitzer to observe nearly 500 young stars in the Orion nebula. They divided the stars into slow spinners and fast spinners, and determined that the slow spinners are five times more likely to have disks than the fast ones.

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Single Stars
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A new study by Charles Lada of the Harvard-Smithsonian Centre for Astrophysics (CfA) has shown that most star systems are made up of single stars. Since planets probably are easier to form around single stars, planets also may be more common than previously suspected.

Astronomers have long known that massive, bright stars, including stars like the sun, are most often found to be in multiple star systems. This fact led to the notion that most stars in the universe are multiples.
However, more recent studies targeted at low-mass stars have found that these fainter objects rarely occur in multiple systems. Astronomers have known for some time that such low-mass stars, also known as red dwarfs or M stars, are considerably more abundant in space than high-mass stars.
By combining these two facts, Lada came to the realization that most star systems in the Galaxy are composed of solitary red dwarfs.

"By assembling these pieces of the puzzle, the picture that emerged was the complete opposite of what most astronomers have believed" - Charles Lada .

Among very massive stars, known as O- and B-type stars, 80 percent of the systems are thought to be multiple, but these very bright stars are exceedingly rare. Slightly more than half of all the fainter, sun-like stars are multiples. However, only about 25 percent of red dwarf stars have companions.
Combined with the fact that about 85 percent of all stars that exist in the Milky Way are red dwarfs, the inescapable conclusion is that upwards of two-thirds of all star systems in the Galaxy consist of single, red dwarf stars.
The high frequency of lone stars suggests that most stars are single from the moment of their birth. If supported by further investigation, this finding may increase the overall applicability of theories that explain the formation of single, sun-like stars. Correspondingly, other star-formation theories that call for most or all stars to begin their lives in multiple-star systems may be less relevant than previously thought.

"It's certainly possible for binary star systems to 'dissolve' into two single stars through stellar encounters. However, suggesting that mechanism as the dominant method of single-star formation is unlikely to explain Lada's results" - Frank Shu, National Tsing Hua University in Taiwan.

Lada's finding implies that planets also may be more abundant than astronomers realized. Planet formation is difficult in binary star systems where gravitational forces disrupt protoplanetary disks. Although a few planets have been found in binaries, they must orbit far from a close binary pair, or hug one member of a wide binary system, in order to survive. Disks around single stars avoid gravitational disruption and therefore are more likely to form planets.




Stellar Multiplicity and the IMF: Most Stars Are Single Born
Authors: Charles J. Lada

In this short communication I compare recent findings suggesting a low binary star fraction for late type stars with knowledge concerning the forms of the stellar initial and present day mass functions for masses down to the hydrogen burning limit. This comparison indicates that most stellar systems formed in the galaxy are likely single and not binary as has been often asserted. Indeed, in the current epoch two-thirds of all main sequence stellar systems in the Galactic disk are composed of single stars. Some implications of this realization for understanding the star and planet formation process are briefly mentioned.

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Planet Formation
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New theoretical work shows that gas-giant planet formation can occur around binary stars in much the same way that it occurs around single stars like the Sun. The work is presented by Dr. Alan Boss of the Carnegie Institution’s Department of Terrestrial Magnetism (DTM) at the American Astronomical Society meeting in Washington, DC.
The results suggest that gas-giant planets, like Jupiter, and habitable Earth-like planets could be more prevalent than previously thought. A paper describing these results has been accepted for publication in the Astrophysical Journal.

"We tend to focus on looking for other solar systems around stars just like our Sun. But we are learning that planetary systems can be found around all sorts of stars, from pulsars to M dwarfs with only one third the mass of our Sun" - Dr. Alan Boss.

Two out of every three stars in the Milky Way is a member of a binary or multiple star system, in which the stars orbit around each other with separations that can range from being nearly in contact (close binaries) to thousands of light-years or more (wide binaries). Most binaries have separations similar to the distance from the Sun to Neptune (~30 AU, where 1 AU = 1 astronomical unit = 150 million kilometres--the distance from the Earth to the Sun).
It has not been clear whether planetary system formation could occur in typical binary star systems, where the strong gravitational forces from one star might interfere with the planet formation processes around the other star, and vice versa. Previous theoretical work had suggested, in fact, that typical binary stars would not be able to form planetary systems. However, planet hunters have recently found a number of gas-giant planets in orbit around binary stars with a range of separations.
Boss found that if the shock heating resulting from the gravitational forces from the companion star is weak, then gas-giant planets are able to form in planet-forming disks in much the same way as they do around single stars. The planet-forming disk would remain cool enough for ice grains to stay solid and thus permit the growth of the solid cores that must reach multiple-Earth-mass size for the conventional mechanism of gas-giant planet formation (core accretion) to succeed.



Boss’ models show even more directly that the alternative mechanism for gas-giant planet formation (disk instability) can proceed just as well in binary star systems as around single stars, and in fact may even be encouraged by the gravitational forces of the other star. In Boss’ new models, the planet-forming disk in orbit around one of the stars is quickly driven to form dense spiral arms, within which self-gravitating clumps of gas and dust form and begin the process of contracting down to planetary sizes.
The process is amazingly rapid, requiring less than 1,000 years for dense clumps to form in an otherwise featureless disk. There would be plenty of room for Earth-like planets to form closer to the central star after the gas-giant planets have formed, in much the same way our own planetary system is thought to have formed.

"This result may have profound implications in that it increases the likelihood of the formation of planetary systems resembling our own, because binary stars are the rule in our galaxy, not the exception" - Dr. Alan Boss.

If binary stars can shelter planetary systems composed of outer gas-giant planets and inner Earth-like planets, then the likelihood of other habitable worlds suddenly becomes roughly three times more probable--up to three times as many stars could be possible hosts for planetary systems similar to our own. NASA’s plans to search for and characterize Earth-like planets in the next decade would then be that much more likely to succeed.
One of the key remaining questions about the theoretical models is the correct amount of shock heating inside the planet-forming disk, as well as the more general question of how rapidly the disk is able to cool. Boss and other researchers are actively working to better understand these heating and cooling processes. Given the growing observational evidence for gas-giant planets in binary star systems, the new results suggest that shock heating in binary disks cannot be too large, or it would prevent gas-giant planet formation.

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