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Post Info TOPIC: Giant Planets


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Title: A desert of gas giant planets beyond tens of au
Author: Sergei Nayakshin (University of Leicester, UK)

Direct imaging observations constrain the fraction of stars orbited by gas giant planets with separations greater than 10 au to about 0.01 only. This is widely believed to indicate that massive protoplanetary discs rarely fragment on planetary mass objects. I use numerical simulations of gas clumps embedded in massive gas discs to show that these observations are consistent with ~0.2-10 planetary mass clumps per star being born in young gravitationally unstable discs. A trio of processes -- rapid clump migration, tidal disruption and runaway gas accretion -- destroys or transforms all of the simulated clumps into other objects, resulting in a desert of gas giants beyond separation of approximately 10 au. The cooling rate of the disc controls which of the three processes is dominant. For cooling rates faster than a few local dynamical times, clumps always grow rapidly and become massive brown dwarfs or low mass stars. For longer cooling times, post-collapse (high density) planets migrate inward to ~10-20 au where they open a gap in the disc and then continue to migrate inward much less rapidly. Pre-collapse (low density) planets are tidally disrupted and may leave massive solid cores behind. Gas giant planets observed inside the desert, such as those in HR 8799, must have followed an unusual evolutionary path, e.g., their host disc being dispersed in a catastrophic fashion.

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Title: A Definition for Giant Planets Based on the Mass-Density Relationship
Author: Artie P. Hatzes Heike Rauer

We present the mass-density relationship (log M - log rho) for objects with masses ranging from planets (M ~ 0.01 M_Jup) through stars (M > 0.08 M_Sun). This relationship shows three distinct regions separated by a change in slope in log M -- log rho plane. In particular, objects with masses in the range 0.3 M_Jup to 60 M_Jup follow a tight linear relationship with no distinguishing feature to separate the low mass end (giant planets) from the high mass end (brown dwarfs). The distinction between giant planets and brown dwarfs thus seems arbitrary. We propose a new definition of giant planets based simply on changes in the slope of the log M versus log rho relationship. By this criterion, objects with masses less than ~ 0.3 M_Jup are low mass planets, either icy or rocky. Giant planets cover the mass range 0.3 M_Jup to 60 M_Jup. Analogous to the stellar main sequence, objects on the upper end of the giant planet sequence (brown dwarfs) can simply be referred to as "high mass giant planets", while planets with masses near that of Jupiter can be considered to be "low mass giant planets".

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Massive Planets Might Escape Stellar Engulfment Largely Undiminished

Having your planet swallowed by a star is no fun. But some planets might be able to run the astrophysical gauntlet and make it through more or less intact.
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Title: The frequency of giant planets around metal-poor stars
Authors: A. Mortier, N. C. Santos, A. Sozzetti, M. Mayor, D. Latham, X. Bonfils, S. Udry

Context. The discovery of about 700 extrasolar planets, so far, has lead to the first statistics concerning extrasolar planets. The presence of giant planets seems to depend on stellar metallicity and mass. For example, they are more frequent around metal-rich stars, with an exponential increase in planet occurrence rates with metallicity.
Aims. We analysed two samples of metal-poor stars (-2.0 \leq [Fe/H] \leq 0.0) to see if giant planets are indeed rare around these objects. Radial velocity datasets were obtained with two different spectrographs (HARPS and HIRES). Detection limits for these data, expressed in minimum planetary mass and period, are calculated. These produce trustworthy numbers for the planet frequency.
Methods. A general Lomb Scargle (GLS) periodogram analysis was used together with a bootstrapping method to produce the detection limits. Planet frequencies were calculated based on a binomial distribution function within metallicity bins.
Results. Almost all hot Jupiters and most giant planets should have been found in these data. Hot Jupiters around metal-poor stars have a frequency lower than 1.0% at one sigma. Giant planets with periods up to 1800 days, however, have a higher frequency of f_p = 2.63^{+2.5}_{-0.8}%. Taking into account the different metallicities of the stars, we show that giant planets appear to be very frequent (f_p = 4.48^{+4.04}_{-1.38}%) around stars with [Fe/H] > -0.7, while they are rare around stars with [Fe/H] \leq -0.7 (\leq 2.36% at one sigma).
Conclusions. Giant planet frequency is indeed a strong function of metallicity, even in the low-metallicity tail. However, the frequencies are most likely higher than previously thought.

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Title: Collisional destruction of the giant planets and rare types of meteorites
Authors: V. I. Dokuchaev, Yu. N. Eroshenko

The probability of the collision and destruction of the giant planets at various stages of planetary systems evolution is calculated. The flow of the fragments of various sizes and the probability of their observations near the Earth are estimated. Of the particular interest is the case of the fragments of metallic hydrogen under the condition of its metastability at low pressure. The radio bursts, which can be generated at the collapses of the planets-giant's magnetospheres during their collisions, are also discussed.

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Giant planet interiors
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Title: A new vision on giant planet interiors: the impact of double diffusive convection
Authors: Jérémy Leconte, Gilles Chabrier

While conventional interior models for Jupiter and Saturn are based on the simplistic assumption of a solid core surrounded by a homogeneous gaseous envelope, we derive new models with an inhomogeneous distribution of heavy elements, i.e. a gradient of composition, within these planets. Such a compositional stratification hampers large scale convection which turns into double-diffusive convection, yielding an inner thermal profile which departs from the traditionally assumed adiabatic interior, affecting these planet heat content and cooling history.
To address this problem, we develop an analytical approach of layered double-diffusive convection and apply this formalism to Solar System gaseous giant planet interiors. These models satisfy all observational constraints and yield a metal enrichment for our gaseous giants up to 30 to 60% larger than previously thought. The models also constrain the size of the convective layers within the planets. As the heavy elements tend to be redistributed within the gaseous envelope, the models predict smaller than usual central cores inside Saturn and Jupiter, with possibly no core for this latter.
These models open a new window and raise new challenges on our understanding of the internal structure of giant (solar and extrasolar) planets, in particular on the determination of their heavy material content, a key diagnostic for planet formation theories.

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Title: Predictions on the core mass of Jupiter and of giant planets in general
Authors: Nadine Nettelmann

More than 80 giant planets are known by mass and radius. Their interior structure in terms of core mass, number of layers, and composition however is still poorly known. An overview is presented about the core mass Mcore and envelope mass of metals MZ in Jupiter as predicted by various equations of state. It is argued that the uncertainty about the true H/He EOS in a pressure regime where the gravitational moments J2 and J4 are most sensitive, i.e. between 0.5 and 4 Mbar, is in part responsible for the broad range Mcore=0-18 Mearth, MZ=0-38 Mearth, and Mcore+MZ=14-38 Mearth currently offered for Jupiter. We then compare the Jupiter models obtained when we only match J2 with the range of solutions for the exoplanet GJ436b, when we match an assumed tidal Love number k2 value.

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Title: Erosion of icy cores in giant gas planets
Authors: Hugh F. Wilson, Burkhard Militzer

Using ab initio simulations we investigate whether water ice is stable in the cores of giant planets, or whether it dissolves into the layer of metallic hydrogen above. By Gibbs free energy calculations we find that for pressures between 10 and 40 Mbar the ice-hydrogen interface is unstable at temperatures above approximately 3000 K, far below the temperature of the core-mantle boundaries in Jupiter and Saturn that are of the order of 10000 K. This implies that the cores of solar and extrasolar giant planets are at least partially eroded.

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Title: The HARPS search for southern extra-solar planets: XXVIII. Two giant planets around M0 dwarfs
Authors: Thierry Forveille (Grenoble), Xavier Bonfils (Grenoble, Geneva), Gaspare Lo Curto (ESO), Xavier Delfosse (Grenoble), Stephane Udry (Geneva), Francois Bouchy (IAP Paris, Haute Provence), Christophe Lovis (Geneva), Michel Mayor (Geneva), Claire Moutou (Marseille), Dominique Naef (Geneva, ESO), Francesco Pepe (Geneva), Christian Perrier (Geneva), Didier Queloz (Geneva), Nuno Santos (Porto)

Fewer giants planets are found around M dwarfs than around more massive stars, and this dependence of planetary characteristics on the mass of the central star is an important observational diagnostic of planetary formation theories. In part to improve on those statistics, we are monitoring the radial velocities of nearby M dwarfs with the HARPS spectrograph on the ESO 3.6 m telescope. We present here the detection of giant planets around two nearby M0 dwarfs: planets, with minimum masses of respectively 5 Jupiter masses and 1 Saturn mass, orbit around Gl 676A and HIP 12961. The latter is, by over a factor of two, the most massive planet found by radial velocity monitoring of an M dwarf, but its being found around an early M-dwarf is in approximate line with the upper envelope of the planetary vs stellar mass diagram. HIP 12961 ([Fe/H]=-0.07) is slightly more metal-rich than the average solar neighbourhood ([Fe/H]=-0.17), and Gl 676A ([Fe/H=0.18) significantly so. The two stars together therefore reinforce the growing trend for giant planets being more frequent around more metal-rich M dwarfs, and the 5~Jupiter mass Gl 676Ab being found around a metal-rich star is consistent with the expectation that the most massive planets preferentially form in disks with large condensate masses.

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Humans are not alone in struggling to stay slim. Some planets go through a "fat" stage that swells their waistlines temporarily, which possibly explains why some gas giants are unexpectedly large.

"Astronomers have found a lot of planets whose sizes cannot be explained by standard theory" - Laurent Ibgui of Princeton University. The difference between predicted and measured widths of so-called "hot Jupiters" can be 30 per cent or more.

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