* Astronomy

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Planet formation

Blobrana · L

Gas-rich planets such as Jupiter and Saturn grew from a disk of dust and gas which eventually crumpled like a piece of paper under its own gravitational instability - or so one theory goes.
Now a computer simulation suggests that this idea falls apart under the turbulent forces within early protoplanetary systems.
The old, favoured theory relies on the protoplanetary dust disk becoming denser and thinner until it reaches a tipping point, where it becomes gravitationally unstable and collapses into kilometre-sized building blocks that form the basis for gas giants. But 3-D modelling has shown for the first time that turbulence prevents the dust from settling into the dense disk necessary for gravitational instability to work.

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Baby Jupiters Must Gain Weight Fast
The planet Jupiter gained weight in a hurry during its infancy. It had to, since the material from which it formed probably disappeared in just a few million years, according to a new study of planet formation around young stars.
Smithsonian astronomers examined the 5 million-year-old star cluster NGC 2362 with NASA's Spitzer Space Telescope, which can detect the signatures of actively forming planets in infrared light. They found that all stars with the mass of the Sun or greater have lost their protoplanetary (planet-forming) disks. Only a few stars less massive than the Sun retain their protoplanetary disks. These disks provide the raw material for forming gas giants like Jupiter. Therefore, gas giants have to form in less than 5 million years or they probably won't form at all.

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New research suggests that turbulence plays a critical role in creating ripe conditions for the birth of planets. The study, to be published in The Astrophysical Journal, challenges the prevailing theory of planet formation.
Using three-dimensional simulations of the dust and gas that orbits young stars, the study demonstrates that turbulence is a significant obstacle to gravitational instability, the process that scientists have used since the 1970s to explain the early stage of planet formation.
Gravitational instability proposes that dust will settle into the middle of the protoplanetary disk around a newly-formed star. It is thought that the dust will gradually become denser and thinner until it reaches a critical point and collapses into kilometre-size clumps, which later collide to form planets. But new research by San Francisco State University professor Joseph Barranco shows that turbulent forces keep the dust and gas swirling and prevent it from forming a dense and thin enough layer for gravitational instability to occur.

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"Three-Dimensional Simulations of Kelvin-Helmholtz Instability in Settled Dust Layers in Protoplanetary Disks" will be published in the Jan. 20 issue of The Astrophysical Journal.

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Title: Ice Lines, Planetesimal Composition and Solid Surface Density in the Solar Nebula
Authors: Sarah E. Dodson-Robinson (1), Karen Willacy (2), Peter Bodenheimer (3), Neal J. Turner (2), C. A. Beichman (1,2) ((1) NASA Exoplanet Science Center, (2) Jet Propulsion Laboratory, (3) UCO/Lick Observatory)
(Version v2)

To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2-3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the solar nebula. By combining a viscously evolving protostellar disk with a kinetic model of ice formation, we calculate the solid surface density in the solar nebula as a function of heliocentric distance and time. We find three effects that strongly favor giant planet formation: (1) a decretion flow that brings mass from the inner solar nebula to the giant planet-forming region, (2) recent lab results (Collings et al. 2004) showing that the ammonia and water ice lines should coincide, and (3) the presence of a substantial amount of methane ice in the trans-Saturnian region. Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. (1996) by a factor of 3 to 4 throughout the trans-Saturnian region. We also discuss the location of ice lines and their movement through the solar nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments.

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Dusty Shock Waves Generate Planet Ingredients
Shock waves around dusty, young stars might be creating the raw materials for planets, according to new observations from NASA's Spitzer Space Telescope.
The evidence comes in the form of tiny crystals. Spitzer detected crystals similar in make-up to quartz around young stars just beginning to form planets. The crystals, called cristobalite and tridymite, are known to reside in comets, in volcanic lava flows on Earth, and in some meteorites that land on Earth.
Astronomers already knew that crystallized dust grains stick together to form larger particles, which later lump together to form planets. But they were surprised to find cristobalite and tridymite crystals. What's so special about these particular crystals? They require flash heating events, such as shock waves, to form.

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Professor Geoff Brooks, Dr Sarah Maddison and Vianney Taquet are collaborating to use mathematical models from steelmaking to determine what compounds form during the initial stages of planet development.
According to the researchers, the science behind steelmaking is comparable to the process of planet formation.

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Astronomers find grains of sand around distant stars
In a find that sheds light on how Earth-like planets may form, astronomers this week reported finding the first evidence of small, sandy particles orbiting a newborn solar system at about the same distance as the Earth orbits the sun. The report will be published online this week in the journal Nature.

"Precisely how and when planets form is an open question. We believe the disk-shaped clouds of dust around newly formed stars condense, forming microscopic grains of sand that eventually go on to become pebbles, boulders and whole planets" - study co-author Christopher Johns-Krull, assistant professor of physics and astronomy at Rice University.

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Title: Testing planet formation theories with Giant stars
Authors: Luca Pasquini, M.P. Doellinger, A. Hatzes, J. Setiawan, L. Girardi, L. da Silva, J.R. de Medeiros

Planet searches around evolved giant stars are bringing new insights to planet formation theories by virtue of the broader stellar mass range of the host stars compared to the solar-type stars that have been the subject of most current planet searches programs. These searches among giant stars are producing extremely interesting results. Contrary to main sequence stars planet-hosting giants do not show a tendency of being more metal rich. Even if limited, the statistics also suggest a higher frequency of giant planets (at least 10 %) that are more massive compared to solar-type main sequence stars. The interpretation of these results is not straightforward. We propose that the lack of a metallicity-planet connection among giant stars is due to pollution of the star while on the main sequence, followed by dilution during the giant phase. We also suggest that the higher mass and frequency of the planets are due to the higher stellar mass. Even if these results do not favour a specific formation scenario, they suggest that planetary formation might be more complex than what has been proposed so far, perhaps with two mechanisms at work and one or the other dominating according to the stellar mass. We finally stress as the detailed study of the host stars and of the parent sample is essential to derive firm conclusions.

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Researchers have dated the earliest step of planet formation in our solar system. Understanding the processes of solar system formation can give us insight into Earth's history, and indicate how Earth-like planets may form elsewhere in the universe.
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.

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