Theory of planet formation

We review the current theoretical understanding how growth from micro-meter sized dust to massive giant planets occurs in disks around young stars. After introducing a number of observational constraints from the solar system, from observed protoplanetary disks, and from the extrasolar planets, we simplify the problem by dividing it into a number of discrete stages which are assumed to occur in a sequential way. In the first stage - the growth from dust to kilometer sized planetesimals - the aerodynamics of the bodies are of central importance. We discuss both a purely coagulative growth mode, as well as a gravoturbulent mode involving a gravitational instability of the dust. In the next stage, planetesimals grow to protoplanets of roughly 1000 km in size. Gravity is now the dominant force. The mass accretion can be strongly non-linear, leading to the detachment of a few big bodies from the remaining planetesimals. In the outer planetary system (outside a few AU), some of these bodies can become so massive that they eventually accrete a large gaseous envelope. This is the stage of giant planet formation, as understood within the core accretion-gas capture paradigm. We also discuss the direct gravitational collapse model where giant planets are thought to form directly via a gravitational fragmentation of the gas disk. In the inner system, protoplanets collide in the last stage - probably after the dispersal of the gaseous disk - in giant impacts until the separations between the remaining terrestrial planets become large enough to allow long term stability. We finish the review with some selected questions.Comment: 23 pages, 8 figures, proceedings workshop "Circumstellar disks and planets: Science cases for the second generation VLTI instrumentation", to appear in Astronomy and Astrophysics Review, ed. Sebastian Wol

Application of recent results on the orbital migration of low mass planets: convergence zones

Previous models of the combined growth and migration of protoplanets needed large ad hoc reduction factors for the type I migration rate as found in the isothermal approximation. In order to eliminate these factors, a simple semi-analytical model is presented that incorporates recent results on the migration of low mass planets in non-isothermal disks. It allows for outward migration. The model is used to conduct planetary populations synthesis calculations. Two points with zero torque are found in the disks. Planets migrate both in- and outward towards these convergence zones. They could be important for accelerating planetary growth by concentrating matter in one point. We also find that the updated type I migration models allow the formation of both close-in low mass planets, but also of giant planets at large semimajor axes. The problem of too rapid migration is significantly mitigate

Rapid planetesimal formation in turbulent circumstellar discs

The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies (Safronov 1969). How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick together poorly (Benz 2000), and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas (Weidenschilling 1977). Gravitational collapse of the solid component has been suggested to overcome this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al. 2007), but turbulence must be present to explain observed gas accretion in protostellar discs (Hartmann 1998). Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are augmented a further order of magnitude by a streaming instability (Youdin & Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar discs.Comment: To appear in Nature (30 August 2007 issue). 18 pages (in referee mode), 3 figures. Supplementary Information can be found at 0708.389

Does the exotic equal pollution? Landscape methods for solving the dilemma of using native versus non‐native plant species in drylands

There is a need to resolve methods to determine the merits of native versus nonnative plant use in drylands and indeed in more temperate areas around the world. This is because whilst plant introductions may have positive objectives, they can have significant negative landscape and environmental impacts. A key discussion on this issue focuses on whether the use of non-native plant species can be considered to be pollution and pollutive based on the concept that pollution can be regarded as ‘matter out of place’. The consequences of putting the wrong plant species in the wrong place can be extremely detrimental to the landscape character, quality and value of the land, let alone the effects on ecosystem structure and functioning as well as on biodiversity. These effects can also affect human communities who may rely on the landscape, for example, for tourism. It is thus necessary that the discussion on how decisions are made in determining plant choice evolves so that the right decisions are made when planting is necessary, for the land, for nature and for the people. This discussion has been initiated through COST Action ES1104, which focused on the restoration of degraded dry and arid lands. This article discusses a number of landscape methods based on sustainability principles to determine when and where native and non-native plants could and should be used.info:eu-repo/semantics/publishedVersio

Synthetic transactivation screening reveals ETV4 as broad coactivator of hypoxia-inducible factor signaling

The human prolyl-4-hydroxylase domain (PHD) proteins 1–3 are known as cellular oxygen sensors, acting via the degradation of hypoxia-inducible factor (HIF) α-subunits. PHD2 and PHD3 genes are inducible by HIFs themselves, suggesting a negative feedback loop that involves PHD abundance. To identify novel regulators of the PHD2 gene, an expression array of 704 transcription factors was screened by a method that allows distinguishing between HIF-dependent and HIF-independent promoter regulation. Among others, the E-twenty six transcription factor ETS translocation variant 4 (ETV4) was found to contribute to PHD2 gene expression particularly under hypoxic conditions. Mechanistically, complex formation between ETV4 and HIF-1/2α was observed by mammalian two-hybrid and fluorescence resonance energy transfer analysis. HIF-1α domain mapping, CITED2 overexpression and factor inhibiting HIF depletion experiments provided evidence for cooperation between HIF-1α and p300/CBP in ETV4 binding. Chromatin immunoprecipitation confirmed ETV4 and HIF-1α corecruitment to the PHD2 promoter. Of 608 hypoxically induced transcripts found by genome-wide expression profiling, 7.7% required ETV4 for efficient hypoxic induction, suggesting a broad role of ETV4 in hypoxic gene regulation. Endogenous ETV4 highly correlated with PHD2, HIF-1/2α and several established markers of tissue hypoxia in 282 human breast cancer tissue samples, corroborating a functional interplay between the ETV4 and HIF pathways

Well-being in dairy cow farms in Baden-Wuerttemberg – an analysis particularly considering the barn’s year of construction

Das Wohlbefinden landwirtschaftlicher Nutztiere ist in den letzten Jahren verstärkt in das öffentliche Bewusstsein gerückt. Für Rinder jenseits eines Alters von sechs Monaten gibt es abgesehen von den Empfehlungen des Europarates zum Halten von Rindern (1988) und den allgemeinen Bestimmungen der Tierschutznutztierhaltungsverordnung keine rechtliche Konkretisierung der baulich-technischen Haltungsbedingungen. Allgemein besteht Konsens, dass Tierwohl gegeben ist, wenn die Tiere frei von Schmerzen und Leiden sind, ohne Überforderung der Anpassungsfähigkeit, und normales, artgemäßes Verhalten entwickeln und ausleben können. Am Tier selbst lässt sich Tierwohl anhand tierbasierter Indikatoren überprüfen. Die vorliegende Studie untersuchte in 165 Praxisbetrieben mit insgesamt 6.024 Kühen, welche Rolle das Alter eines Milchkuhstalles beim Tierwohl in baden-württembergischen Milchkuhbetrieben spielt. Dabei zeigte sich, dass weder ausgewählte Gesundheits- und Leistungsmerkmale noch die breite Streuung bei Verhaltensparametern einen Zusammenhang zum Baujahr des Stalles zeigen. Dies lässt den Schluss zu, dass Defizite beim Tierwohl nicht mit älterer Bausubstanz und gegebenenfalls unverhältnismäßig hohen Investitionen begründet oder gerechtfertigt werden können. Um das Wohlbefinden der Tiere unabhängig vom Alter der Haltungseinrichtung sicherstellen zu können, ist allerdings das Engagement der Tierhalter hinsichtlich des betrieblichen Managements erforderlich.In recent years, the well-being of agricultural farm animals has gained the awareness of the general public. For cattle beyond an age of six months, there are no legal ascertainments for structural-technical husbandry conditions besides the Council of Europe’s recommendations on cattle farming (1988) and the general regulations of animal protection and the keeping of farm animals. There is a consensus that animal well-being exists if the animals are free of pain and suffering without overextension of their adaptability and are able to develop and live out appropriate behavior to their species. The well-being can be observed on the animals themselves by examining animal-based indicators. The presented study investigated the correlation of the age of dairy cow barns and the animal well-being in 165 dairy cow farms in Baden-Wuerttemberg with a total of 6,024 cows. It was shown, that neither chosen health and performance characteristics nor the broad distribution regarding behavior parameters correlate with the barn’s construction year. This supports the conclusion that deficits in animal well-being cannot be justified with older building stock and possibly disproportionately high investment. In order to be able to guarantee the well-being of animals independently from the age of livestock buildings and installations, it is necessary that animal owners show commitment with respect to operational management

Der Einfluss elastischer Gummimatten im Melkstand auf die Klauenmaße von Milchkühen

Hoof diseases and disorders are a big animal welfare problem in dairy herds and one of the most common cause of losses on German dairy cattle farms. Any deviation from the correct claw shape may lead to unphysiological loading and as a consequence of this to hoof diseases. The present study compares the claw shape of German Simmental Cows in a stable before and after the installation of elastic rubber mats in a steep herringbone parlour. In the beginning there existed an abrasive soil and the height of the bulb was too low in 87 % of cows. Four months after the installation of the mats, 77 % of the cows had bulb heights within the reference range. No differences between claw measurements were found depending on milk yield and number of lactation. The results of this study indicated that the flooring surface in a milking parlour had a significant influence on claw shape