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UA Astronomers Track the Birth of a 'Super-Earth'

A new model giving rise to young planetary systems offers a fresh solution to a puzzle that has vexed astronomers ever since new detection technologies and planet-hunting missions such as NASA's Kepler space telescope have revealed thousands of planets orbiting other stars: While the majority of these exoplanets fall into a category called super-Earths - bodies with a mass somewhere between Earth and Neptune - most of the features observed in nascent planetary systems were thought to require much more massive planets, rivaling or dwarfing Jupiter, the gas giant in our solar system.
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Title: Formation of Super-Earth Mass Planets at 125-250 AU from a Solar-type Star
Author: S. J. Kenyon, B. C. Bromley

We investigate pathways for the formation of icy super-Earth mass planets orbiting at 125-250 AU around a 1 solar mass star. An extensive suite of coagulation calculations demonstrates that swarms of 1 cm to 10 m planetesimals can form super-Earth mass planets on time scales of 1-3 Gyr. Collisional damping of 0.01-100 cm particles during oligarchic growth is a highlight of these simulations. In some situations, damping initiates a second runaway growth phase where 100-3000 km protoplanets grow to super-Earth sizes. Our results establish the initial conditions and physical processes required for in situ formation of super-Earth planets at large distances from the host star. For nearby dusty disks in HD 107146, HD 202628, and HD 207129, ongoing super-Earth formation at 80-150 AU could produce gaps and other structures in the debris. In the solar system, forming a putative planet X at a < 300 AU (a > 1000 AU) requires a modest (very massive) protosolar nebula.

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'Super-Earths' may be dead worlds

Scientists have looked at how these worlds form and suggest that many of them may be a lot less clement than was though. They find that planets that form from less massive cores can become benign habitats for life, whereas the larger objects instead end up as mini-Neptunes with thick atmospheres and probably stay sterile. The researchers, led by Dr. Helmut Lammer of the Space Research Institute (IWF) of the Austrian Academy of Sciences, publish their results in Monthly Notices of the Royal Astronomical Society.
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Earth's super-siblings grew up very differently

True clones of Earth may be rare indeed. It seems that many of the super-Earths spotted in our galaxy so far formed in a very different way to our own celestial home.
Super-Earths - rocky planets 1 to 10 times the size of ours - are just one type of weird world uncovered in recent years. Hot Jupiters - enormous balls of gas that sit closer to their stars than Mercury is to the sun - are another. Understanding how these types of worlds form should help our search for other inhabited worlds.

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Title: Super Earths and Dynamical Stability of Planetary Systems: First Parallel GPU Simulations Using GENGA
Authors: S.Elser, S.L.Grimm, J.G.Stadel

We report on the stability of hypothetical Super-Earths in the habitable zone of known multi-planetary systems. Most of them have not yet been studied in detail concerning the existence of additional low-mass planets. The new N-body code GENGA developed at the UZH allows us to perform numerous N-body simulations in parallel on GPUs. With this numerical tool, we can study the stability of orbits of hypothetical planets in the semi-major axis and eccentricity parameter space in high resolution. Massless test particle simulations give good predictions on the extension of the stable region and show that HIP 14180 and HD 37124 do not provide stable orbits in the habitable zone. Based on these simulations, we carry out simulations of 10 Earth mass planets in several systems (HD 11964, HD 47186, HD 147018, HD 163607, HD 168443, HD 187123, HD 190360, HD 217107 and HIP 57274). They provide more exact information about orbits at the location of mean motion resonances and at the edges of the stability zones. Beside the stability of orbits, we study the secular evolution of the planets to constrain probable locations of hypothetical planets. Assuming that planetary systems are in general closely packed, we find that apart from HD 168443, all of the systems can harbour 10 Earth mass planets in the habitable zone.

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Are super-Earths really mini-Neptunes?

In the last two decades astronomers have found hundreds of planets in orbit around other stars. One type of these so-called 'exoplanets' is the super-Earths that are thought to have a high proportion of rock but at the same time are significantly bigger than our own world. Now a new study led by Helmut Lammer of the Space Research Institute (IWF) of the Austrian Academy of Sciences suggests that these planets are actually surrounded by extended hydrogen-rich envelopes and that they are unlikely to ever become Earth-like. Rather than being super-Earths, these worlds are more like mini-Neptunes. The scientists publish their work in the journal Monthly Notices of the Royal Astronomical Society.
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Vaporising the Earth

Simulations of the vaporisation of Earth-like planets tell planet-hunting astronomers what to look for in the atmospheres of candidate super-Earths.
Bruce Fegley, PhD, professor of earth and planetary sciences at Washington University in St. Louis, and his colleagues Katharina Loddersm, a research professor of earth and planetary sciences who is currently on assignment at the National Science Foundation, and Laura Schaefer, currently a graduate student at Harvard University, have vaporised the Earth - if only by simulation, that is mathematically and inside a computer.

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Title: Dynamics of Rotation of Super-Earths
Authors: Nelson Callegari Jr, Adrian Rodríguez

We numerically investigate the dynamics of rotation of several close-in terrestrial exoplanets candidates. In our model, the rotation of the planet is disturbed by the torque of the central star due to the asymmetric equilibrium figure of the planet. We use surfaces of section to explore numerically the rotation phase space of the systems adopting different sets of parameters and initial conditions close to the main spin-orbit resonant states. We show that, depending on some parameters of the system like the radius and mass of the planet, orbital eccentricity etc, the rotation can be strongly perturbed and a chaotic layer around the synchronous state may occupy a significant region of the phase space. 55 Cnc e is an example.

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Title: The silicate model and carbon rich model of CoRoT-7b, Kepler-9d and Kepler-10b
Authors: Yan-Xiang Gong, Ji-Lin Zhou

Possible bulk compositions of the super-Earth exoplanets, CoRoT-7b, Kepler-9d, and Kepler-10b are investigated by applying a commonly used silicate and a non-standard carbon model. Their internal structures are deduced using the suitable equation of state of the materials. The degeneracy problems of their compositions can be partly overcome, based on the fact that all three planets are extremely close to their host stars. By analysing the numerical results, we conclude: 1) The iron core of CoRoT-7b is not more than 27% of its total mass within 1 \sigma mass-radius error bars, so an Earth-like composition is less likely, but its carbon rich model can be compatible with an Earth-like core/mantle mass fraction; 2) Kepler-10b is more likely with a Mercury-like composition, its old age implies that its high iron content may be a result of strong solar wind or giant impact; 3) the transiting-only super-Earth Kepler-9d is also discussed. Combining its possible composition with the formation theory, we can place some constraints on its mass and bulk composition.

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Title: Mantle Dynamics in Super-Earths: Post-Perovskite Rheology and Self-Regulation of Viscosity
Authors: Paul J. Tackley, Michael Ammann, John P. Brodholt, David P. Dobson, Diana Valencia

Simple scalings suggest that super-Earths are more likely than an equivalent Earth-sized planet to be undergoing plate tectonics. Generally, viscosity and thermal conductivity increase with pressure while thermal expansivity decreases, resulting in lower convective vigour in the deep mantle. According to conventional thinking, this might result in no convection in a super-Earth's deep mantle. Here we evaluate this. First, we here extend the density functional theory (DFT) calculations of post-perovskite activation enthalpy of to a pressure of 1 TPa. The activation volume for diffusion creep becomes very low at very high pressure, but nevertheless for the largest super-Earths the viscosity along an adiabat may approach 1030 Pa s in the deep mantle. Second, we use these calculated values in numerical simulations of mantle convection and lithosphere dynamics of planets with up to ten Earth masses. The models assume a compressible mantle including depth-dependence of material properties and plastic yielding induced plate tectonics. Results confirm the likelihood of plate tectonics and show a novel self-regulation of deep mantle temperature. The deep mantle is not adiabatic; instead internal heating raises the temperature until the viscosity is low enough to facilitate convective loss of the radiogenic heat, which results in a super-adiabatic temperature profile and a viscosity increase with depth of no more than ~3 orders of magnitude, regardless of the viscosity increase that is calculated for an adiabat. Convection in large super-Earths is characterised by large upwellings and small, time-dependent downwellings. If a super-Earth was extremely hot/molten after its formation, it is thus likely that even after billions of years its deep interior is still extremely hot and possibly substantially molten with a "super basal magma ocean" - a larger version of (Labrosse et al., 2007).

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