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We conduct a series of numerical experiments of thermal convection of compressible fluids with temperature-dependent viscosity, in order to study how the adiabatic compression and model geometries affect the mantle convection on super-Earths. A two-dimensional basally heated convection is considered under the truncated anelastic liquid approximation (TALA), either in a rectangular box or in a cylindrical annulus. We varied the magnitude of adiabatic heating and the Rayleigh number as well as the depth profile of thermodynamic properties (thermal expansivity and reference density) in accordance with the planetary sizes. From our calculations by varying the planetary sizes up to 10 times the Earth's mass, we confirmed that the adiabatic compression affects the thermal convection more strongly for larger planets. The activity of hot plumes originating from the core–mantle boundary is significantly suppressed in the terrestrial planets whose mass is larger than the Earth's by a factor of about 3 regardless of the model geometries. We also developed scaling relationships between the vigor of thermal convection and the planetary mass by appropriately incorporating the effect of adiabatic compression into those of Boussinesq (or incompressible) cases. Our scaling relationships suggest that the stress level in the top cold thermal boundary layers is almost independent of the planetary mass, which may further imply that the emergence of plate tectonics is not likely to be enhanced for massive terrestrial planets whose composition is similar to the Earth's. [ABSTRACT FROM AUTHOR]

Many super‐Earths are on very short orbits around their host star and, therefore, more likely to be tidally locked. Because this locking can lead to a strong contrast between the dayside and nightside surface temperatures, these super‐Earths could exhibit mantle convection patterns and tectonics that could differ significantly from those observed in the present‐day solar system. The presence of an atmosphere, however, would allow transport of heat from the dayside toward the nightside and thereby reduce the surface temperature contrast between the two hemispheres. On rocky planets, atmospheric and geodynamic regimes are closely linked, which directly connects the question of atmospheric thickness to the potential interior dynamics of the planet. Here, we study the interior dynamics of super‐Earth GJ 486b (R=1.34 $R=1.34$R⊕ ${R}_{\oplus }$, M=3.0 $M=3.0$M⊕ ${M}_{\oplus }$, Teq≈700 ${\mathrm{T}}_{\text{eq}}\approx 700$ K), which is one of the most suitable M‐dwarf super‐Earth candidates for retaining an atmosphere produced by degassing from the mantle and magma ocean. We investigate how the geodynamic regime of GJ 486b is influenced by different surface temperature contrasts by varying possible atmospheric circulation regimes. We also investigate how the strength of the lithosphere affects the convection pattern. We find that hemispheric tectonics, the surface expression of degree‐1 convection with downwellings forming on one hemisphere and upwelling material rising on the opposite hemisphere, is a consequence of the strong lithosphere rather than surface temperature contrast. Anchored hemispheric tectonics, where downwellings und upwellings have a preferred (day/night) hemisphere, is favored for strong temperature contrasts between the dayside and nightside and higher surface temperatures. Plain Language Summary: The tectonic processes occurring on super‐Earths, which are rocky exoplanets with masses exceeding that of Earth, may differ significantly from those observed on the terrestrial planets in our solar system. Many super‐Earths are also expected to be tidally locked to their host star, so that always the same hemisphere faces the star. Tidal locking can lead to a strong temperature contrast between the dayside and nightside surface. Here, we investigate the influence of such strong surface temperature contrasts on the interior dynamics and tectonic behavior of super‐Earth GJ 486b, which is one of the best characterized Earth‐mass planets to date. We determined the surface temperature contrast between the dayside and nightside by assuming different efficiencies of atmospheric heat transport and ran global climate models for a set of different atmospheric compositions. Our 2D mantle convection simulations reveal that hemispheric tectonics, where cold material sinks on one hemisphere and hot material rises on the other side, emerges regardless of the surface temperature contrast if the planet has a strong lithosphere. However, a significant surface temperature contrast and an overall higher surface temperature tend to anchor the sinking cold material and rising hot material to the dayside and nightside respectively. Key Points: The mantle dynamics of super‐Earth GJ 486b are governed by the strength of the lithosphere and the day‐night surface temperature contrastDegree‐1 convection is a consequence of the strong lithosphere, rather than the temperature contrast between the dayside and nightsideA strong surface temperature contrast between the dayside and nightside can anchor downwellings to one hemisphere [ABSTRACT FROM AUTHOR]

Abstract In this paper, we carried out a series of linear analyses on the onset of thermal convection of highly compressible fluids whose physical properties strongly vary in space in convecting vessels either of a three-dimensional spherical shell or a two-dimensional spherical annulus geometry. The variations in thermodynamic properties (thermal expansivity and reference density) with depth are taken to be relevant for the super-Earths with ten times the Earth’s mass, while the thermal conductivity and viscosity are assumed to exponentially depend on depth and temperature, respectively. Our analysis showed that, for the cases with strong temperature dependence in viscosity and strong depth dependence in thermal conductivity, the critical Rayleigh number is on the order of 108–109, implying that the mantle convection of massive super-Earths is most likely to fall in the stagnant-lid regime very close to the critical condition, if the properties of their mantle materials are quite similar to the Earth’s. Our analysis also demonstrated that the structures of incipient flows of stagnant-lid convection in the presence of strong adiabatic compression are significantly affected by the depth dependence in thermal conductivity and the geometries of convecting vessels, through the changes in the static stability of thermal stratification of the reference state. When the increase in thermal conductivity with depth is sufficiently large, the thermal stratification can be greatly stabilized at depth, further inducing regions of insignificant fluid motions above the bottom hot boundaries in addition to the stagnant lids along the top cold surfaces. We can therefore speculate that the stagnant-lid convection in the mantles of massive super-Earths is accompanied by another motionless regions at the base of the mantles if the thermal conductivity strongly increases with depth (or pressure), even though their occurrence is hindered by the effects the spherical geometries of convecting vessels.

Super‐Earths ranging up to 10 Earth masses (ME) with Earth‐like density are common among the observed exoplanets thus far, but their measured masses and radii do not uniquely elucidate their internal structure. Exploring the phase transitions in the Mg‐silicates that define the mantle‐structure of super‐Earths is critical to characterizing their interiors, yet the relevant terapascal conditions are experimentally challenging for direct structural analysis. Here we investigated the crystal chemistry of Fe3O4 as a low‐pressure analog to Mg2SiO4 between 45–115 GPa and up to 3000 K using powder and single crystal X‐ray diffraction in the laser‐heated diamond anvil cell. Between 60–115 GPa and above 2000 K, Fe3O4 adopts an 8‐fold coordinated Th3P4‐type structure (I‐43d, Z = 4) with disordered Fe2+ and Fe3+ into one metal site. This Fe‐oxide phase is isostructural with that predicted for Mg2SiO4 above 500 GPa in super‐Earth mantles and suggests that Mg2SiO4 can incorporate both ferric and ferrous iron at these conditions. The pressure‐volume behavior observed in this 8‐fold coordinated Fe3O4 indicates a maximum 4% density increase across the 6‐ to 8‐fold coordination transition in the analog Mg‐silicate. Reassessment of the FeO—Fe3O4 fugacity buffer considering the Fe3O4 phase relationships identified in this study reveals that increasing pressure and temperature to 120 GPa and 3000 K in Earth and planetary mantles drives iron toward oxidation. Plain Language Summary: Thousands of exoplanets have been discovered so far, and a sizable portion of them, known as "super‐Earths," are much more massive than Earth but have an Earth‐like density. In order to accurately model super‐Earth interiors, their high pressure‐temperature mineralogy must be determined. Previous studies have predicted that an ultra‐dense Mg2SiO4 phase becomes stable above 500 GPa and is the predominant mineral in the mantles of super‐Earths. The stability of this phase would mark the onset of 8‐fold Si–O coordination—a critical densification boundary. Here we have identified that Fe3O4 undergoes an analogous 6 – to 8 – fold coordination transition and adopts the same structure above 60 GPa and 2000 K. We have employed powder and single‐crystal X‐ray diffraction to explore the stability and structural properties of this 8‐fold coordinated Fe3O4 phase and the results suggest that the analog silicate is likely an iron‐bearing phase and that the onset of its stability is associated with approximately a 4% density increase in super‐Earth mantles. Key Points: Fe3O4 was explored to 115 GPa and 3000 K using powder and single‐crystal X‐ray diffraction as an analog to Mg2SiO4 in super‐Earth mantlesAbove 60 GPa and 2000 K, Fe3O4 transitions to an 8‐fold coordinated, Th3P4‐related structureThe analog coordination increase in silicates would result in a ∼4% density increase in super‐Earth mantles [ABSTRACT FROM AUTHOR]

The hundreds of exoplanets that have been discovered in the past two decades offer a new perspective on planetary structure. Instead of being the archetypal examples of planets, those of our solar system are merely possible outcomes of planetary system formation and evolution, and conceivably not even especially common outcomes (although this remains an open question). Here, we review the diverse range of interior structures that are both known and speculated to exist in exoplanetary systems--from mostly degenerate objects that are more than 10× as massive as Jupiter, to intermediate-mass Neptune-like objects with large cores and moderate hydrogen/helium envelopes, to rocky objects with roughly the mass of Earth.

In this paper, we carried out a series of linear analyses on the onset of thermal convection of highly compressible fluids whose physical properties strongly vary in space in convecting vessels either of a three-dimensional spherical shell or a two-dimensional spherical annulus geometry. The variations in thermodynamic properties (thermal expansivity and reference density) with depth are taken to be relevant for the super-Earths with ten times the Earth's mass, while the thermal conductivity and viscosity are assumed to exponentially depend on depth and temperature, respectively. Our analysis showed that, for the cases with strong temperature dependence in viscosity and strong depth dependence in thermal conductivity, the critical Rayleigh number is on the order of 108–109, implying that the mantle convection of massive super-Earths is most likely to fall in the stagnant-lid regime very close to the critical condition, if the properties of their mantle materials are quite similar to the Earth's. Our analysis also demonstrated that the structures of incipient flows of stagnant-lid convection in the presence of strong adiabatic compression are significantly affected by the depth dependence in thermal conductivity and the geometries of convecting vessels, through the changes in the static stability of thermal stratification of the reference state. When the increase in thermal conductivity with depth is sufficiently large, the thermal stratification can be greatly stabilized at depth, further inducing regions of insignificant fluid motions above the bottom hot boundaries in addition to the stagnant lids along the top cold surfaces. We can therefore speculate that the stagnant-lid convection in the mantles of massive super-Earths is accompanied by another motionless regions at the base of the mantles if the thermal conductivity strongly increases with depth (or pressure), even though their occurrence is hindered by the effects the spherical geometries of convecting vessels. [ABSTRACT FROM AUTHOR]

Surveys have shown that super-Earth and Neptune-mass exoplanets are more frequent than gas giants around low-mass stars, as predicted by the core accretion theory of planet formation. We report the discovery of a giant planet around the very-low-mass star GJ 3512, as determined by optical and near-infrared radial-velocity observations. The planet has a minimum mass of 0.46 Jupiter masses, very high for such a small host star, and an eccentric 204-dayorbit. Dynamical models show that the high eccentricity is most likely due to planet-planet interactions. We use simulations to demonstrate that the GJ 3512 planetary system challenges generally accepted formation theories, and that it puts constraints on the planet accretion and migration rates. Disk instabilities may be more efficient in forming planets than previously thought. [ABSTRACT FROM AUTHOR]

We present a thermodynamic model for liquid iron, based on ab initio molecular dynamics simulations, which is applicable to 2 TPa and beyond 10000 K, conditions that are relevant in the cores of super‐Earths. We combine ab initio results for V‐T‐P‐E with a correction scheme to match experimental properties at ambient pressure, where ab initio results show poor agreement. We explore the performance of our thermodynamic potential and various previously published models for liquid iron over a wide range of conditions: (i) at ambient pressure as a function of temperature, (ii) along the melting curve of Fe to 40 GPa, relevant for the cores of smaller terrestrial bodies in our solar system, (iii) along isentropes in the Earth's outer core, and (iv) for the core of super‐Earth Kepler‐36b. The correction term significantly improves the agreement of computed properties with experiments and other thermodynamic models that are based on an assessment of the phase diagram at ambient and moderate pressure, showing how ab initio molecular dynamics simulations can be used at par with other thermodynamic techniques. For the Earth's core, densities from the various models are similar, but higher‐order derivatives (acoustic velocities and Grüneisen parameter) show significant differences. Evaluated along a core‐temperature profile in Kepler‐36b, differences in density from various models are negligible, for core mass they do not exceed 2%, showing robust extrapolation of all equation of state models. Plain Language Summary: The cores of the inner planets in our solar system and likely other planets outside of it (super‐Earths) contain a partially liquid iron core that is critical in understanding magnetic field generation and their internal structure. In order to characterize their structure, the relation between density, pressure, and temperature (equation of state) must be known. As conditions in the interior of these planets are difficult to access in experiments, we perform computer simulations on the electronic structure of iron. However, results from such simulations have been shown to inadequately represent low‐pressure data and we correct for this shortcoming. By combining simulation results and the correction scheme, we develop an equation of state that is applicable over a wide range of conditions. We compare our results to experimental data and previously published models and find that they generally agree at conditions of the Earth and continue to yield consistent results for super‐Earths. This robust behavior can be used in analyzing their structure once relevant astronomical observations become available. Key Points: We present a thermodynamic model for liquid iron from ab initio simulations that is equally applicable at low pressure and up to 2 TPaDifferent equations of state for liquid iron yield similar results in density for the Earth's core, but higher‐order derivatives differFor the cores of super‐Earths equations of state yield similar results on density and core mass, suggesting that they extrapolate robustly [ABSTRACT FROM AUTHOR]

We test the hypothesis that the sub-millimetre thermal emission and scattered light gaps seen in recent observations of TW Hya are caused by planet–disc interactions. We perform global three-dimensional dusty smoothed particle hydrodynamics simulations, comparing synthetic observations of our models with dust thermal emission, CO emission, and scattered light observations. We find that the dust gaps observed at 24 au and 41 au can be explained by two super-Earths (∼ 4 M⊕). A planet of approximately Saturn-mass can explain the CO emission and the depth and width of the gap seen in scattered light at 94 au. Our model produces a prominent spiral arm while there are only hints of this in the data. To avoid runaway growth and migration of the planets we require a disc mass of |${\lesssim } 10^{-2}\, \mathrm{M}_{\odot }{}$| in agreement with CO observations but 10–100 times lower than the estimate from HD line emission. [ABSTRACT FROM AUTHOR]

In a few years, space telescopes will investigate our Galaxy to detect evidence of life, mainly by observing rocky planets. In the last decade, the observation of exoplanet atmospheres and the theoretical works on biosignature gasses have experienced a considerable acceleration. The most attractive feature of the realm of exoplanets is that 40% of M dwarfs host super-Earths with a minimum mass between 1 and 30 Earth masses, orbital periods shorter than 50 days, and radii between those of the Earth and Neptune (1–3.8 R⊕). Moreover, the recent finding of cyanobacteria able to use far-red (FR) light for oxygenic photosynthesis due to the synthesis of chlorophylls d and f, extending in vivo light absorption up to 750 nm, suggests the possibility of exotic photosynthesis in planets around M dwarfs. Using innovative laboratory instrumentation, we exposed different cyanobacteria to an M dwarf star simulated irradiation, comparing their responses to those under solar and FR simulated lights. As expected, in FR light, only the cyanobacteria able to synthesize chlorophyll d and f could grow. Surprisingly, all strains, both able or unable to use FR light, grew and photosynthesized under the M dwarf generated spectrum in a similar way to the solar light and much more efficiently than under the FR one. Our findings highlight the importance of simulating both the visible and FR light components of an M dwarf spectrum to correctly evaluate the photosynthetic performances of oxygenic organisms exposed under such an exotic light condition.

One of the key goals of exoplanet science is the atmospheric characterization of super-Earths. Atmospheric abundances provide insight on the formation and evolution of those planets and help to put our own rocky planets in context. Observations on 55 Cancri e point towards an N-dominated atmosphere. In this paper we explore this possibility, showing which will be the most abundant gases and observable species in emission and transmission spectroscopy of such an atmosphere. We use analytical arguments and observed parameters to estimate the possible thermal profile of the atmosphere and test three different extreme possibilities. The chemistry is calculated using equilibrium calculations and adopting Titan's elemental abundances as a potential N-dominated atmospheric composition. We also test the effect of different N/O ratios in the atmosphere. Emission and transmission spectra are computed and showed with a resolution relevant to future missions suitable to observe super-Earths (e.g. JWST, ARIEL). We find that even though N2 is the most abundant molecule in the atmosphere followed by H2 and CO, the transmission spectra show strong features of NH3 and HCN, and CO and HCN dominate emission spectra. We also show that a decrease in the N/O ratio leads to stronger H2O, CO and CO2 and weaker NH3 and HCN features. A larger N/O is also more consistent with observations. Our exploration of an N-atmosphere for 55 Cancri e serve as a guide to understand such atmospheres and provide a reference for future observations. [ABSTRACT FROM AUTHOR]

More than a third of all exoplanets can be classified as super-Earths based on radius (1–2 R ⊕ ) and mass (<10 M ⊕ ). Here we model mass-radius relations based on silicate mantle and iron core equations of state to infer to first order the structure and composition range of rocky super-Earths assuming insignificant gas envelopes. As their core pressures exceed those in the Earth by an order of magnitude, significant extrapolations of equations of state for iron are required. We develop a new equation of state of hexagonal close packed (hcp) iron for super-Earth conditions (SEOS) based on density functional theory results for pressures up to 137 TPa. A comparison of SEOS and extrapolated equations of state for iron from the literature reveals differences in density of up to 4% at 1 TPa and up to 20% at 10 TPa. Such density differences significantly affect mass-radius relations. On mass, the effect is as large as 10% for Earth-like super-Earths (core radius fraction of 0.5) and 20% for Mercury-like super-Earths (core radius fraction of 0.8). We also quantify the effects of other modeling assumptions such as temperature and composition by considering extreme cases. We find that the effect of temperature on mass (<5%) is smaller than that resulting from the extrapolation of the equations of state of iron, and lower mantle temperatures are too low to allow for rock and iron miscibility for R <1.75 R ⊕ . Our end-member cases of core and mantle compositions create a spread in mass-radius curves reaching more than 50% in terms of mass for a given planetary radius, implying that modeling uncertainties dominate over observational uncertainties for many observed super-Earths. We illustrate these uncertainties explicitly for Kepler-36b with well-constrained mass and radius. Assuming a core composition of 0.8 ρ Fe (equivalent to 50 mol% S) instead of pure Fe leads to an increase of the core radius fraction from 0.53 to 0.64. Using a mantle composition of Mg 0.5 Fe 0.5 SiO 3 instead of MgSiO 3 leads to a decrease of the core radius fraction to 0.33. Effects of thermal structure and the choice of equation of state for the core material on the core radius of Kepler-36b are small but non-negligible, reaching 2% and 5%, respectively. [ABSTRACT FROM AUTHOR]

A leading model for the origin of super-Earths proposes that planetary embryos migrate inward and pile up on close-in orbits. As large embryos are thought to preferentially form beyond the snowline, this naively predicts that most super-Earths should be very water-rich. Here we show that the shortest period planets formed in the migration model are often purely rocky. The inward migration of icy embryos through the terrestrial zone accelerates the growth of rocky planets via resonant shepherding. We illustrate this process with a simulation that provided a match to the Kepler-36 system of two planets on close orbits with very different densities. In the simulation, two super-Earths formed in a Kepler-36-like configuration; the inner planet was pure rock while the outer one was ice-rich. We conclude from a suite of simulations that the feeding zones of close-in super-Earths are likely to be broad and disconnected from their final orbital radii. [ABSTRACT FROM AUTHOR]

Abstract With the ceaseless progress of detecting technology, over 3500 exo-planets have been discovered. It is interesting but unexpected that the majority of the detected exoplanets are unlike any planets in our solar system. They have a size and mass between the Earth and the Neptune, and thus they are called as Super-Earths or Sub-Neptunes. In this article, I introduce these newly rising planets and review our current knowledge of their physical properties, orbital properties, and origins. Finally, I discuss the promising and exciting prospects. [ABSTRACT FROM AUTHOR]

Models of core accretion assume that in the radiative zones of accreting gas envelopes, radiation diffuses. But super-Earths/sub-Neptunes (1-4 R⊕, 2-20M⊕) point to formation conditions that are optically thin: their modest gas masses are accreted from short-lived and gas-poor nebulae reminiscent of the transparent cavities of transitional discs. Planetary atmospheres born in such environments can be optically thin to both incident starlight and internally generated thermal radiation. We construct time-dependent models of such atmospheres, showing that super-Earths/sub-Neptunes can accrete their ~1 per cent-by-mass gas envelopes, and super-puffs/sub-Saturns their ~20 per cent-by-mass envelopes, over a wide range of nebular depletion histories requiring no fine tuning. Although nascent atmospheres can exhibit stratospheric temperature inversions affected by atomic Fe and various oxides that absorb strongly at visible wavelengths, the rate of gas accretion remains controlled by the radiative-convective boundary (rcb) at much greater pressures. For dusty envelopes, the temperature at the rcb Trcb ≃ 2500 K is still set by H2 dissociation; for dust-depleted envelopes, Trcb tracks the temperature of the visible or thermal photosphere, whichever is deeper, out to at least ~5 au. The rate of envelope growth remains largely unchanged between the old radiative diffusion models and the new optically thin models, reinforcing how robustly super-Earths form as part of the endgame chapter in disc evolution. [ABSTRACT FROM AUTHOR]

First-principles calculations are performed to investigate vacancy formation and migration in the B2 phase of MgO. Defect energetics suggest the importance of intrinsic non-interacting vacancy pairs, even though the extrinsic vacancy concentration might govern atomic diffusion in the B2 phase of MgO. The enthalpies of ionic vacancy migration are generally found to decrease across the B1-B2 phase transition around a pressure of 500 GPa. It is shown that this enthalpy change induces a substantial increase in the rate of vacancy diffusion in MgO of almost four orders of magnitude (∼10 4 ) when the B1 phase transforms into the B2 phase with increasing pressure. If plastic deformation is controlled by vacancy diffusion, mantle viscosity is expected to decrease in relation to this enhanced diffusion rate in MgO across the B1-B2 transition in the interior of Earth-like large exoplanets. Our results of atomic relaxations near the defects suggest that diffusion controlled creep viscosity may generally decrease across high-pressure phase transitions with increasing coordination number. Plastic flow and resulting mantle convection in the interior of these super-Earths may be therefore less sluggish than previously thought. [ABSTRACT FROM AUTHOR]

We conduct a series of numerical experiments of thermal convection of highly compressible fluids in a two-dimensional rectangular box, in order to study the mantle convection on super-Earths. The thermal conductivity and viscosity are assumed to exponentially depend on depth and temperature, respectively, while the variations in thermodynamic properties (thermal expansivity and reference density) with depth are taken to be relevant for the super-Earths with 10 times the Earth’s. From our experiments we identified a distinct regime of convecting flow patterns induced by the interplay between the adiabatic temperature change and the spatial variations in viscosity and thermal conductivity. That is, for the cases with strong temperature-dependent viscosity and depth-dependent thermal conductivity, a “deep stratosphere” of stable thermal stratification is formed at the base of the mantle, in addition to thick stagnant lids at their top surfaces. In the “deep stratosphere”, the fluid motion is insignificant particularly in the vertical direction in spite of smallest viscosity owing to its strong dependence on temperature. Our finding may further imply that some of super-Earths which are lacking in mobile tectonic plates on their top surfaces may have “deep stratospheres” at the base of their mantles. [ABSTRACT FROM AUTHOR]

The highest pressure form of the major Earth-forming mantle silicate is MgSiO 3 post-perovskite (PPv). Understanding the fate of PPv at TPa pressures is the first step for understanding the mineralogy of super-Earths-type exoplanets, arguably the most interesting for their similarities with Earth. Modeling their internal structure requires knowledge of stable mineral phases, their properties under compression, and major element abundances. Several studies of PPv under extreme pressures support the notion that a sequence of pressure induced dissociation transitions produce the elementary oxides SiO 2 and MgO as the ultimate aggregation form at ∼3 TPa. However, none of these studies have addressed the problem of mantle composition, particularly major element abundances usually expressed in terms of three main variables, the Mg/Si and Fe/Si ratios and the Mg#, as in the Earth. Here we show that the critical compositional parameter, the Mg/Si ratio, whose value in the Earth's mantle is still debated, is a vital ingredient for modeling phase transitions and internal structure of super-Earth mantles. Specifically, we have identified new sequences of phase transformations, including new recombination reactions that depend decisively on this ratio. This is a new level of complexity that has not been previously addressed, but proves essential for modeling the nature and number of internal layers in these rocky mantles. [ABSTRACT FROM AUTHOR]

'Hot super-Earths' (or 'mini-Neptunes') between one and four times Earth's size with period shorter than 100 d orbit 30-50 per cent of Sun-like stars. Their orbital configuration - measured as the period ratio distribution of adjacent planets in multiplanet systems - is a strong constraint for formation models. Here, we use N-body simulations with synthetic forces from an underlying evolving gaseous disc to model the formation and long-term dynamical evolution of super-Earth systems. While the gas disc is present, planetary embryos grow and migrate inward to form a resonant chain anchored at the inner edge of the disc. These resonant chains are far more compact than the observed super-Earth systems. Once the gas dissipates, resonant chains may become dynamically unstable. They undergo a phase of giant impacts that spreads the systems out. Disc turbulence has no measurable effect on the outcome. Our simulations match observations if a small fraction of resonant chains remain stable, while most super-Earths undergo a late dynamical instability. Our statistical analysis restricts the contribution of stable systems to less than 25 per cent. Our results also suggest that the large fraction of observed single-planet systems does not necessarily imply any dichotomy in the architecture of planetary systems. Finally, we use the low abundance of resonances in Kepler data to argue that, in reality, the survival of resonant chains happens likely only in ~5 per cent of the cases. This leads to a mystery: in our simulations only 50-60 per cent of resonant chains became unstable, whereas at least 75 per cent (and probably 90-95 per cent) must be unstable to match observations. [ABSTRACT FROM AUTHOR]

The Earth is likely to have acquired most of its water during accretion. Internal heat of planetesimals by short-lived radioisotopes would have caused some water loss, but impacts into planetesimals were insufficiently energetic to produce further drying. Water is thought to be critical for the development of plate tectonics, because it lowers viscosities in the asthenosphere, enabling subduction. The following issue persists: if water is necessary for plate tectonics, but subduction itself hydrates the upper mantle, how is the upper mantle initially hydrated? The giant impacts of late accretion created magma lakes and oceans, which degassed during solidification to produce a heavy atmosphere. However, some water would have remained in the mantle, trapped within crystallographic defects in nominally anhydrous minerals. In this paper, we present models demonstrating that processes associated with magma ocean solidification and overturn may segregate sufficient quantities of water within the upper mantle to induce partial melting and produce a damp asthenosphere, thereby facilitating plate tectonics and, in turn, the habitability of Earth-like extrasolar planets. This article is part of the themed issue ‘The origin, history and role of water in the evolution of the inner Solar System’. [ABSTRACT FROM AUTHOR]

Context: The Kepler mission has discovered that close-in super-Earth planets are common around solar-type stars. They are often seen together in multiplanetary systems, but their period ratios do not show strong pile-ups near mean motion resonances (MMRs). One scenario is that super-Earths form early, in the presence of a gas-rich disk. These planets interact gravitationally with the disk gas, inducing their orbital migration. However, for this scenario disk migration theory predicts that planets will end up at resonant orbits due to their differential migration speed. Aims. Motivated by the discrepancy between observation and theory, we seek a mechanism that moves planets out of resonances. We examine the orbital evolution of planet pairs near the magnetospheric cavity during the gas disk dispersal phase. Our study determines the conditions under which planets can escape resonances. Methods: We extend Type I migration theory by calculating the torque a planet experiences at the interface of the empty magnetospheric cavity and the disk, namely the one-sided torque. We perform two-planet N-body simulations with the new Type I expressions, varying the planet masses, stellar magnetic field strengths, disk accretion rates, and gas disk depletion timescales. Results: As planets migrate outwards with the expanding magnetospheric cavity, their dynamical configurations can be rearranged. Migration of planets is substantial (minor) in a massive (light) disk. When the outer planet is more massive than the inner planet, the period ratio of two planets increases through outward migration. On the other hand, when the inner planet is more massive, the final period ratio tends to remain similar to the initial one. Larger stellar magnetic field strengths result in planets stopping their migration at longer periods. We apply this model to two systems, Kepler-170 and Kepler-180. By fitting their present dynamical architectures, the disk and stellar B-field parameters at the time of disk dispersal can be retrieved. Conclusions: We highlight "magnetospheric" rebound as an important ingredient able to reconcile disk migration theory with observations. Even when planets are trapped into MMRs during the early gas-rich stage, subsequent cavity expansion induces substantial changes to their orbits that move them out of resonance. [ABSTRACT FROM AUTHOR]

Adiabatic compression is a key factor that exerts control over thermal convection in the compressible solid mantle of super-Earths. To discuss the effects of adiabatic compression, we present a numerical model of transient convection in the cooling mantle of a super-Earth that is ten times larger in size than the Earth. The calculations started with the shallow mantle that was hotter than expected by the extrapolation from the deep mantle conditions. This type of initial thermal state of the mantle is expected to naturally occur in real super-Earths due to heating by giant impacts at the time of their formation. With our initial setup conditions, the convection temporarily occurs as a layered convection for the first several to ten billion years of the calculation and then changes its style into a whole layer convection. The long duration of the transient stage suggests that mantle convection currently occurs as a temporal layered convection in many of the super-Earths. A temporal layered convection, if it occurs, can exert control over the tectonic activities of super-Earths. Future studies should clarify how internal heating and complicated rheological properties of mantle materials including their pressure dependence affect the duration of the temporal layered convection. [ABSTRACT FROM AUTHOR]

We describe a 'super-Earth'-size (2.30 ± 0.16 R⊕) planet transiting an early K-type dwarf star in the Campaign 4 field observed by the K2 mission. The host star, EPIC 210363145, was identified as a candidate member of the approximately 120 Myr-old Pleiades cluster based on its kinematics and photometric distance. It is rotationally variable and exhibits near-ultraviolet emission consistent with a Pleiades age, but its rotational period is ≈20 d and its spectrum contains no Hα emission nor the Li I absorption expected of Pleiades K dwarfs. Instead, the star is probably an interloper that is unaffiliated with the cluster, but younger (≲1.3 Gyr) than the typical field dwarf. We ruled out a false positive transit signal produced by confusion with a background eclipsing binary by adaptive optics imaging and a statistical calculation. Doppler radial velocity measurements limit the companion mass to <2 times that of Jupiter. Screening of the light curves of 1014 potential Pleiades candidate stars uncovered no additional planets. An injection-and-recovery experiment using the K2 Pleiades light curves with simulated planets, assuming a planet population like that in the Kepler prime field, predicts only 0.8-1.8 detections (versus ~20 in an equivalent Kepler sample). The absence of Pleiades planet detections can be attributed to the much shorter monitoring time of K2 (80 d versus 4 yr), increased measurement noise due to spacecraft motion, and the intrinsic noisiness of the stars. [ABSTRACT FROM AUTHOR]

Super Earths are the largest population of exoplanets and are seen to exhibit a rich diversity of compositions as inferred through their mean densities. Here we present a model that combines equilibrium chemistry in evolving discs with core accretion that tracks materials accreted on to planets during their formation. In doing so, we aim to explain why super Earths form so frequently and how they acquire such a diverse range of compositions. A key feature of our model is disc inhomogeneities, or planet traps, that act as barriers to rapid type-I migration. The traps we include are the dead zone, which can be caused by either cosmic ray or X-ray ionization, the ice line, and the heat transition. We find that in discs with sufficiently long lifetimes (≳4 Myr), all traps produce Jovian planets. In these discs, planet formation in the heat transition and X-ray dead zone produces hot Jupiters, while the ice line and cosmic ray dead zones produce Jupiters at roughly 1 au. Super Earth formation takes place within short-lived discs (≲2 Myr), whereby the discs are photoevaporated while planets are in a slow phase of gas accretion. We find that super Earth compositions range from dry and rocky (<6 per cent ice by mass) to those with substantial water contents (>30 per cent ice by mass). The traps play a crucial role in our results, as they dictate where in the disc particular planets can accrete from, and what compositions they are able to acquire. [ABSTRACT FROM AUTHOR]

This paper considers secular interactions within multi-planet systems. In particular, we consider dynamical evolution of known planetary systems resulting from an additional hypothetical planet on an eccentric orbit. We start with an analytical study of a general two-planet system, showing that a planet on an elliptical orbit transfers all of its eccentricity to an initially circular planet if the two planets have comparable orbital angular momenta. Application to the single super-Earth system HD38858 shows that an additional hypothetical planet below current radial velocity (RV) constraints with Msini = 3-10 M☉, semi-major axis 1-10 au and eccentricity 0.2-0.8 is unlikely to be present from the eccentricity that would be excited in the known planet (albeit cyclically). However, additional planets in proximity to the known planet could stabilize the system against secular perturbations from outer planets. Moreover, these additional planets can have an Msini below RV sensitivity and still affect their neighbours. For example, application to the two super-Earth system 61 Vir shows that an additional hypothetical planet cannot excite high eccentricities in the known planets, unless its mass and orbit lie in a restricted area of parameter space. Inner planets in HD38858 below RV sensitivity would also modify conclusions above about excluded parameter space. This suggests that it may be possible to infer the presence of additional stabilizing planets in systems with an eccentric outer planet and an inner planet on an otherwise suspiciously circular orbit. This reinforces the point that the full complement of planets in a system is needed to assess its dynamical state. [ABSTRACT FROM AUTHOR]

The study of exoplanets is an exploding field in astronomy. Recent discoveries have made possible the development of a new research field, the spectroscopic characterization of the exoplanetary atmospheres, using both primary and eclipse transits. A dedicated space mission will make possible the characterization of many classes of exoplanets, from the hot Jupiters to the temperate super-Earths. In this paper, we discuss how the spectral range and the spectral resolving power can be optimized for identifying a maximum number of candidate atmospheric species. Spectral modeling shows that the simultaneous observation of the whole spectral range, from 0.55 to 16 μm is ideal for (1) capturing all types of planets at different temperatures, (2) detecting the variety of chemical atmospheric compounds with some redundancy, and (3) enabling an optimal retrieval of the chemical abundances and thermal profile. Limiting the spectral interval to 11 μm would make the retrieval more difficult in the case of cold exoplanets. In the visible range, the extension down to 0.4 s at different temperatures, (2) detecting the variety of chemical atmospheric compounds with some redundancy, and (3) enabling an optimal retrieval of the chemical abundances andst candidate molecules. [ABSTRACT FROM AUTHOR]

We predict by first-principles methods a phase transition in TiO2 at 6.5 Mbar from the Fe2P-type polymorph to a ten-coordinated structure with space group I4/mmm. This is the first report, to our knowledge, of the pressure-induced phase transition to the I4/mmm structure among all dioxide compounds. The I4/mmm structure was found to be up to 3.3% denser across all pressures investigated. Significant differences were found in the electronic properties of the two structures, and themetallization of TiO2 was calculated to occur concomitantly with the phase transition to I4/mmm. The implications of our findings were extended to SiO2, and an analogous Fe2P-type to I4/mmm transition was found to occur at 10 TPa. This is consistent with the lower-pressure phase transitions of TiO2, which are well-established models for the phase transitions in other AX2 compounds, including SiO2. As in TiO2, the transition to I4/mmm corresponds to the metallization of SiO2. This transformation is in the pressure range reached in the interiors of recently discovered extrasolar planets and calls for a reformulation of the equations of state used to model them. [ABSTRACT FROM AUTHOR]

Context. A large fraction of stars host one or multiple close-in super-Earth planets. There is an active debate about whether these planets formed in situ or at greater distances from the central star and migrated to their current position. It has been shown that part of their observed properties (e.g., eccentricity distribution) can be reproduced by N-body simulations of in situ formation starting with a population of protoplanets of high masses and neglecting the effects of the disk gas. Aims. We plan to reassess the in situ formation of close-in super-Earths through more complete simulations. Methods. We performed N-body simulations of a population of small planetary embryos and planetesimals that include the effects of disk-planet interactions (e.g., eccentricity damping, Type I migration). In addition, we also consider the accretion of a primitive atmosphere from a protoplanetary disk. Results. We find that planetary embryos grow very quickly well before the gas dispersal, and thus undergo rapid inward migration, which means that one cannot neglect the effects of a gas disk when considering the in-situ formation of close-in super-Earths. Owing to their rapid inward migration, super-Earths reach a compact configuration near the disk's inner edge whose distribution of orbital parameters matches the observed close-in super-Earths population poorly. On the other hand, simulations including eccentricity damping, but no Type I migration, reproduce the observed distributions better. Including the accretion of an atmosphere does not help reproduce the bulk architecture of observations. Interestingly, we find that the massive embryos can migrate inside the disk edge while capturing only a moderately massive hydrogen/helium atmosphere. By this process they avoid becoming giant planets. Conclusions. The bulk of close-in super-Earths cannot form in situ, unless Type I migration is suppressed in the entire disk inside 1 AU. [ABSTRACT FROM AUTHOR]

We present new observations of the Kuiper belt analogues around HD 38858 and HD 20794, hosts of super-Earth mass planets within 1 au. As two of the four nearby G-type stars (with HD 69830 and 61 Vir) that form the basis of a possible correlation between low-mass planets and debris disc brightness, these systems are of particular interest. The disc around HD 38858 is well resolved with Herschel and we constrain the disc geometry and radial structure. We also present a probable James Clerk Maxwell Telescope sub-mm continuum detection of the disc and a CO J = 2-1 upper limit. The disc around HD 20794 is much fainter and appears marginally resolved with Herschel, and is constrained to be less extended than the discs around 61 Vir and HD 38858. We also set limits on the radial location of hot dust recently detected around HD 20794 with near-IR interferometry. We present High Accuracy Radial velocity Planet Searcher upper limits on unseen planets in these four systems, ruling out additional super-Earths within a few au, and Saturn-mass planets within 10 au. We consider the disc structure in the three systems with Kuiper belt analogues (HD 69830 has only a warm dust detection), concluding that 61 Vir and HD 38858 have greater radial disc extent than HD 20794. We speculate that the greater width is related to the greater minimum planet masses (10-20 M⊕ versus 3-5 M⊕), arising from an eccentric planetesimal population analogous to the Solar system's scattered disc. We discuss alternative scenarios and possible means to distinguish among them. [ABSTRACT FROM AUTHOR]

It is suggested that the delamination process, in which the mantle lithosphere is peeled into the asthenosphere, contributes to the topographies and magmatism of the Earth. We investigated the vigorousness of the delamination in super-Earths by applying the Earth model to planets of heavy mass. Delamination is induced in planets of mass 5 M ⊕ by the negative buoyancy of the mantle lithosphere. However, assuming pressure dependent rheology, the thermal Rayleigh number decreases due to the high pressure in super-Earths and thus the magnitude of convection in the Moho decreases. Because reduced convection in the Moho weakens the peeling of the mantle lithosphere, the delaminated area is narrower. The magnitude of the heat flux caused by the delamination process is also reduced in planets large in size compared with Earth. Although further work is needed, our model indicates that delamination can transfer more heat than the conduction of the lithosphere if the planet׳s mass is less than 5 M ⊕ . [ABSTRACT FROM AUTHOR]

We calculate and analyse the distribution of period ratios observed in systems of Kepler exoplanet candidates including studies of both adjacent planet pairs and all planet pairs. These distributions account for both the geometrical bias against detecting more distant planets and the effects of incompleteness due to planets missed by the data reduction pipeline. In addition to some of the known features near first-order mean-motion resonances (MMRs), there is a significant excess of planet pairs with period ratios near 2.2. The statistical significance of this feature is assessed using Monte Carlo simulation. We also investigate the distribution of period ratios near first-order MMR and compare different quantities used to measure this distribution. We find that beyond period ratios of ~2.5, the distribution of all period ratios follows a power law with an exponent -- 1.26±0.05.We discuss implications that these results may have on the formation and dynamical evolution of Kepler-like planetary systems--systems of sub-Neptune/super-Earth planets with relatively short orbital periods. [ABSTRACT FROM AUTHOR]

The majority of discovered exoplanetary systems harbour a new class of planets, bodies that are typically several times more massive than the Earth but that orbit their host stars well inside the orbit of Mercury. The origin of these close-in super-Earths and mini-Neptunes is one of the major unanswered questions in planet formation. Unlike the Earth, whose atmosphere contains less than 10-6 of its total mass, a large fraction of close-in planets have significant gaseous envelopes, containing 1-10 per cent or more of their total mass. It has been proposed that close-in super-Earths and mini-Neptunes formed in situ either by delivery of 50-100 Me of rocky material to the inner regions of the protoplanetary disc or in a disc enhanced relative to the minimum mass solar nebula. In both cases, the final assembly of the planets occurs via giant impacts. Here we test the viability of these scenarios. We show that atmospheres that can be accreted by isolation masses are small (typically 10-3-10-2 of the core mass) and that the atmospheric mass-loss during giant impacts is significant, resulting in typical post-giant impact atmospheres that are 8 x 10-4 of the core mass. Such values are consistent with terrestrial planet atmospheres but more than an order of magnitude below atmospheric masses of 1-10 per cent inferred for many close-in exoplanets. In the most optimistic scenario in which there is no core luminosity from giant impacts and/or planetesimal accretion, we find that post-giant impact envelope accretion from a depleted gas disc can yield atmospheric masses that are several per cent the core mass. If the gravitational potential energy resulting from the last mass doubling of the planet by giant impacts is released over the disc dissipation time-scale as core luminosity, then the accreted envelope masses are reduced by about an order of magnitude. Finally we show that, even in the absence of type I migration, radial drift time-scales due to gas drag for many isolation masses are shorter than typical disc lifetimes for standard gas-to-dust ratios. Given these challenges, we conclude that most of the observed close-in planets with envelopes larger than several per cent of their total mass likely formed at larger separations from their host stars. [ABSTRACT FROM AUTHOR]