Hot Jupiter HAT-P-13b's Core is Probably 11 Earth Mass

Dynamical Constraints on the Core Mass of Hot Jupiter HAT-P-13b

Authors:

Buhler et al

Abstract:

HAT-P-13b is a Jupiter-mass transiting exoplanet that has settled onto a stable, short-period, and mildly eccentric orbit as a consequence of the action of tidal dissipation and perturbations from a second, highly eccentric, outer companion. Due to the special orbital configuration of the HAT-P-13 system, the magnitude of HAT-P-13b's eccentricity (eb) is in part dictated by its Love number (k2b), which is in turn a proxy for the degree of central mass concentration in its interior. Thus, the measurement of eb constrains k2b and allows us to place otherwise elusive constraints on the mass of HAT-P-13b's core (Mcore,b). In this study we derive new constraints on the value of eb by observing two secondary eclipses of HAT-P-13b with the Infrared Array Camera on board the Spitzer Space Telescope. We fit the measured secondary eclipse times simultaneously with radial velocity measurements and find that eb=0.00700±0.00100. We then use octupole-order secular perturbation theory to find the corresponding k2b=0.31+0.08−0.05. Applying structural evolution models, we then find, with 68\% confidence, that Mcore,b is less than 25 Earth masses (M⊕). The most likely value of Mcore,b=11M⊕, which is similar to the core mass theoretically required for runaway gas accretion. This is the tightest constraint to date on the core mass of a hot Jupiter. Additionally, we find that the measured secondary eclipse depths, which are in the 3.6 μm and 4.5 μm bands, best match atmospheric model predictions with a dayside temperature inversion and relatively efficient day-night circulation.

Hydrogen-Water Mixtures in Giant Planet Interiors

Hydrogen-Water Mixtures in Giant Planet Interiors Studied with Ab Initio Simulations

Authors:

Soubiran et al

Abstract:

We study water-hydrogen mixtures under planetary interior conditions using ab initio molecular dynamics simulations. We determine the thermodynamic properties of various water-hydrogen mixing ratios at temperatures of 2000 and 6000 K for pressures of a few tens of GPa. These conditions are relevant for ice giant planets and for the outer envelope of the gas giants. We find that at 2000 K the mixture is in a molecular regime, while at 6000 K the dissociation of hydrogen and water is important and affects the thermodynamic properties. We study the structure of the liquid and analyze the radial distribution function. We provide estimates for the transport properties, diffusion and viscosity, based on autocorrelation functions. We obtained viscosity estimates of the order of a few tenths of mPa.s for the conditions under consideration. These results are relevant for dynamo simulations of ice giant planets.

Kepler-36b's Planetary Interior is Probably Similar to Earth's

Scaling the Earth: A Sensitivity Analysis of Terrestrial Exoplanetary Interior Models

Authors:

Unterborn et al

Abstract:

An exoplanet's structure and composition are first-order controls of the planet's habitability. We explore which aspects of bulk terrestrial planet composition and interior structure affect the chief observables of an exoplanet: its mass and radius. We apply these perturbations to the Earth, the planet we know best. Using the mineral physics toolkit BurnMan to self-consistently calculate mass-radius models, we find that core radius, presence of light elements in the core and an upper-mantle consisting of low-pressure silicates have the largest effect on the final calculated mass at a given radius, with mantle composition being secondary. We further apply this model to determine the interior composition of Kepler-36b, finding that it is likely structurally similar to the Earth with Si/Fe = 1.14 compared to Earth's Si/Fe = 1 and Sun's Si/Fe = 1.19. We expand these results provide a grid of terrestrial mass-radius models for determining whether exoplanets are indeed "Earth-like" as bound by their composition and structure.

The Impact on Tidal Dissipation of Melted Gas Giant Cores

Melting the core of giant planets: impact on tidal dissipation

Authors:

Mathis et al

Abstract:

Giant planets are believed to host central dense rocky/icy cores that are key actors in the core-accretion scenario for their formation. In the same time, some of their components are unstable in the temperature and pressure regimes of central regions of giant planets and only ab-initio EOS computations can address the question of the state of matter. In this framework, several works demonstrated that erosion and redistribution of core materials in the envelope must be taken into account. These complex mechanisms thus may deeply modify giant planet interiors for which signatures of strong tidal dissipation have been obtained for Jupiter and Saturn. The best candidates to explain this dissipation are the viscoelastic dissipation in the central dense core and turbulent friction acting on tidal inertial waves in their fluid convective envelope. In this work, we study the consequences of the possible melting of central regions for the efficiency of each of these mechanisms.

Implications of the Emission Spectra of Five "Cool" Gas Giants

Spitzer Secondary Eclipse Observations of Five Cool Gas Giant Planets and Empirical Trends in Cool Planet Emission Spectra

Authors:

Kammer et al

Abstract:

In this work we present Spitzer 3.6 and 4.5 micron secondary eclipse observations of five new cool (less than 1200 K) transiting gas giant planets: HAT-P-19b, WASP-6b, WASP-10b, WASP-39b, and WASP-67b. We compare our measured eclipse depths to the predictions of a suite of atmosphere models and to eclipse depths for planets with previously published observations in order to constrain the temperature- and mass-dependent properties of gas giant planet atmospheres. We find that the dayside emission spectra of planets less massive than Jupiter require models with efficient circulation of energy to the night side and/or increased albedos, while those with masses greater than that of Jupiter are consistently best-matched by models with inefficient circulation and low albedos. At these relatively low temperatures we expect the atmospheric methane to CO ratio to vary as a function of metallicity, and we therefore use our observations of these planets to constrain their atmospheric metallicities. We find that the most massive planets have dayside emission spectra that are best-matched by solar metallicity atmosphere models, but we are not able to place strong constraints on metallicities of the smaller planets in our sample. Interestingly, we find that the ratio of the 3.6 and 4.5 micron brightness temperatures for these cool transiting planets is independent of planet temperature, and instead exhibits a tentative correlation with planet mass. If this trend can be confirmed, it would suggest that the shape of these planets' emission spectra depends primarily on their masses, consistent with the hypothesis that lower-mass planets are more likely to have metal-rich atmospheres.

Surface/atmospheric Abundances of Bio-Necessary Materials may be VERY Variable in Terrestrial Exoplanets

Tracing the Ingredients for a Habitable Earth from Interstellar Space through Planet Formation

Authors:

Bergin et al

Abstract:

We use the C/N ratio as a monitor of the delivery of key ingredients of life to nascent terrestrial worlds. Total elemental C and N contents, and their ratio, are examined for the interstellar medium, comets, chondritic meteorites and terrestrial planets; we include an updated estimate for the Bulk Silicate Earth (C/N = 49.0 +/- 9.3). Using a kinetic model of disk chemistry, and the sublimation/condensation temperatures of primitive molecules, we suggest that organic ices and macro-molecular (refractory or carbonaceous dust) organic material are the likely initial C and N carriers. Chemical reactions in the disk can produce nebular C/N ratios of ~1-12, comparable to those of comets and the low end estimated for planetesimals. An increase of the C/N ratio is traced between volatile-rich pristine bodies and larger volatile-depleted objects subjected to thermal/accretional metamorphism. The C/N ratios of the dominant materials accreted to terrestrial planets should therefore be higher than those seen in carbonaceous chondrites or comets. During planetary formation, we explore scenarios leading to further volatile loss and associated C/N variations owing to core formation and atmospheric escape. Key processes include relative enrichment of nitrogen in the atmosphere and preferential sequestration of carbon by the core. The high C/N BSE ratio therefore is best satisfied by accretion of thermally processed objects followed by large-scale atmospheric loss. These two effects must be more profound if volatile sequestration in the core is effective. The stochastic nature of these processes hints that the surface/atmospheric abundances of biosphere-essential materials will likely be variable.

Water, Hydrogen and the Internal Structure of Gas Giants

Miscibility calculations for water and hydrogen in giant planets

Authors:

Soubiran et al

Abstract:

We present results from ab initio simulations of liquid water-hydrogen mixtures in the range from 2 to 70 GPa and from 1000 to 6000 K, covering conditions in the interiors of ice giant planets and parts of the outer envelope of gas giant planets. In addition to computing the pressure and the internal energy, we derive the Gibbs free energy by performing a thermodynamic integration. For all conditions under consideration, our simulations predict hydrogen and water to mix in all proportions. The thermodynamic behavior of the mixture can be well described with an ideal mixing approximation. We suggest a substantial fraction of water and hydrogen in giant planets may occur in homogeneously mixed form rather than in separate layers. The extend of mixing depends on the planet's interior dynamics and its conditions of formation, in particular on how much hydrogen was present when icy planetesimals were delivered. Based on our results, we do not predict water-hydrogen mixtures to phase separate during any stage of the evolution of giant planets. We also show that the hydrogen content of an exoplanet is much higher if the mixed interior is assumed.

Simulating the Interiors of Giant Planets

Does Stellar Metalicity Effect Exoplanet Composition?

What are little worlds made of? Stellar abundances and the building blocks of planets

Author:

Gaidos

Abstract:

If the photospheres of solar-type stars represent the composition of circumstellar disks from which any planets formed, spectroscopic determinations of stellar elemental abundances offer information on the composition of those planets, including smaller, rocky planets. In particular, the C/O ratio is proposed to be a key determinant of the composition of solids that condense from disk gas and are incorporated into planets. Also, planets may leave chemical signatures on the photospheres of their host stars by sequestering heavy elements, or by being accreted by the stars. The presence, absence, and composition of planets could be revealed by small differences in the relative abundances between stars. I critically examine these scenarios and show that (i) a model of Galactic chemical evolution predicts that the C/O ratio is expected to be close to the solar value and vary little between dwarf stars in the solar neighborhood; (ii) spectroscopic surveys of M dwarf stars limit the occurrence of stars with C/O ≳1 to less than 10−3; and (iii) planetesimal chemistry will be controlled by the composition of oxygen-rich dust inherited from the molecular cloud and processed in a dust-rich environment, not a gas with the stellar composition. A second generation of more reduced planetesimals could be produced by re-equilibration of some material with dust-depleted gas. Finally, I discuss how minor differences in relative abundances between stars that correlate with condensation temperature can be explained by dust-gas segregation, perhaps in circumstellar disks, rather than planet formation.

Can we Figure out the Internal Structure of Rocky Exoplanets From Their Mass and Radius?

Can we constrain interior structure of rocky exoplanets from mass and radius measurements?

Authors:

Dorn et al

Abstract:

We present an inversion method based on Bayesian analysis to constrain the interior structure of terrestrial exoplanets, in the form of chemical composition of the mantle and core size. Specifically, we identify what parts of the interior structure of terrestrial exoplanets can be determined from observations of mass, radius, and stellar elemental abundances. We perform a full probabilistic inverse analysis to formally account for observational and model uncertainties and obtain confidence regions of interior structure models. This enables us to characterize how model variability depends on data and associated uncertainties. We test our method on terrestrial solar system planets and find that our model predictions are consistent with independent estimates. Furthermore, we apply our method to synthetic exoplanets up to 10 Earth masses and up to 1.7 Earth radii as well as to exoplanet Kepler-36b. Importantly, the inversion strategy proposed here provides a framework for understanding the level of precision required to characterize the interior of exoplanets. Our main conclusions are: (1) observations of mass and radius are sufficient to constrain core size; (2) stellar elemental abundances (Fe, Si, Mg) are key constraints to reduce degeneracy in interior structure models and to constrain mantle composition; (3) the inherent degeneracy in determining interior structure from mass and radius observations does not only depend on measurement accuracies but also on the actual size and density of the exoplanet. We argue that precise observations of stellar elemental abundances are central in order to place constraints on planetary bulk composition and to reduce model degeneracy. We provide a general methodology of analyzing interior structures of exoplanets that may help to understand how interior models are distributed among star systems. The methodology we propose is sufficiently general to allow its future extension to more complex internal structure
including hydrogen- and water-rich exoplanets.

Delamnination of the Geosphere in SuperEarths

Delamination in super-Earths extrapolated from the Earth model

Authors:

Shoji et al

Abstract:

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 5M⊕5M⊕ 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 5M⊕.

The Beginnings of Exoplanet Geophysics

Exoplanetary Geophysics -- An Emerging Discipline

Authors:

Laughlin et al

Abstract:

Thousands of extrasolar planets have been discovered, and it is clear that the galactic planetary census draws on a diversity greatly exceeding that exhibited by the solar system's planets. We review significant landmarks in the chronology of extrasolar planet detection, and we give an overview of the varied observational techniques that are brought to bear. We then discuss the properties of the currently known distribution, using the mass-period diagram as a guide to delineating hot Jupiters, eccentric giant planets, and a third, highly populous, category that we term "ungiants", planets having masses less than 30 Earth masses and orbital periods less than 100 days. We then move to a discussion of the bulk compositions of the extrasolar planets. We discuss the long-standing problem of radius anomalies among giant planets, as well as issues posed by the unexpectedly large range in sizes observed for planets with masses somewhat greater than Earth's. We discuss the use of transit observations to probe the atmospheres of extrasolar planets; various measurements taken during primary transit, secondary eclipse, and through the full orbital period, can give clues to the atmospheric compositions, structures, and meteorologies. The extrasolar planet catalog, along with the details of our solar system and observations of star-forming regions and protoplanetary disks, provide a backdrop for a discussion of planet formation in which we review the elements of the favored pictures for how the terrestrial and giant planets were assembled. We conclude by listing several research questions that are relevant to the next ten years and beyond.

SuperEarths may Have Long Lived Lava Oceans

Shock compression of stishovite and melting of silica at planetary interior conditions

Authors:

Millot et al

Abstract:

Deep inside planets, extreme density, pressure, and temperature strongly modify the properties of the constituent materials. In particular, how much heat solids can sustain before melting under pressure is key to determining a planet’s internal structure and evolution. We report laser-driven shock experiments on fused silica, α-quartz, and stishovite yielding equation-of-state and electronic conductivity data at unprecedented conditions and showing that the melting temperature of SiO2 rises to 8300 K at a pressure of 500 gigapascals, comparable to the core-mantle boundary conditions for a 5–Earth mass super-Earth. We show that mantle silicates and core metal have comparable melting temperatures above 500 to 700 gigapascals, which could favor long-lived magma oceans for large terrestrial planets with implications for planetary magnetic-field generation in silicate magma layers deep inside such planets.

Paper Link: SuperEarths Retain Their Oceans for Longer (but are less habitable when younger)

The persistence of oceans on Earth-like planets: insights from the deep-water cycle

Authors:

Schaefer et al

Abstract:

In this paper we present a series of models for the deep water cycle on super-Earths experiencing plate tectonics. The deep water cycle can be modeled through parameterized convection models coupled with a volatile recycling model. The convection of the silicate mantle is linked to the volatile cycle through the water-dependent viscosity. Important differences in surface water content are found for different parameterizations of convection. Surface oceans are smaller and more persistent for single layer convection, rather than convection by boundary layer instability. Smaller planets have initially larger oceans but also return that water to the mantle more rapidly than larger planets. Super-Earths may therefore be less habitable in their early years than smaller planets, but their habitability (assuming stable surface conditions), will persist much longer.

link to original post.

SuperEarths Retain Their Oceans for Longer

Persistence of oceans on Earth-like planets

Authors:

Schaefer et al

Abstract:

The habitable zone is an orbital region around a star in which an Earth-like planet can maintain liquid water on its surface given a variety of atmospheric compositions. However, the abundance of water on the Earth's surface is not controlled by the atmosphere, but rather by the deep water/silicate cycle. On the Earth, volcanic outgassing of water from the mantle is balanced by loss of water to the mantle through subduction of water-rich oceanic seafloor. Much of this water is released immediately back to the surface through shallow, water-induced volcanism. However, a small but significant fraction of the water can be transported to deeper levels of the mantle. Mantle convection has therefore played an important role in controlling the size of Earth's surface oceans over the planet's lifetime.The deep water cycle of Earth has been studied with parameterized convection models incorporating a water-dependent viscosity. The abundance of water in the mantle, which lowers the convective viscosity, evolves along with the mantle temperature. Here we present results from a parameterized convection model extended to high pressures to study the deep water cycles of super-Earths. Assuming compositions similar to the Earth, our models indicate that ocean formation will be delayed on 5 MEarth planets by ~1 Gyr after planet formation. Although ocean mass on these planets increases with time, the oceans remain much shallower than for smaller planets, consistent with previous studies. Intermediate mass planets (2-4 MEarth) have immediate, but gradual outgassing and persistent oceans. Small terrestrial planets (<= 1 MEarth) have rapid initial outgassing, but will gradually lose a significant fraction of their surface oceans due to mantle sequestration over their lifetimes.

Implications for 'Diamond' SuperEarths: 2/3s of Earth's Carbon Hidden in Core

Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe7C3

Authors:

Chen et al

Abstract:

Earth’s inner core is known to consist of crystalline iron alloyed with a small amount of nickel and lighter elements, but the shear wave (S wave) travels through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures. The anomalously low S-wave velocity (vS) has been attributed to the presence of liquid, hence questioning the solidity of the inner core. Here we report new experimental data up to core pressures on iron carbide Fe7C3, a candidate component of the inner core, showing that its sound velocities dropped significantly near the end of a pressure-induced spin-pairing transition, which took place gradually between 10 GPa and 53 GPa. Following the transition, the sound velocities increased with density at an exceptionally low rate. Extrapolating the data to the inner core pressure and accounting for the temperature effect, we found that low-spin Fe7C3 can reproduce the observed vS of the inner core, thus eliminating the need to invoke partial melting or a postulated large temperature effect. The model of a carbon-rich inner core may be consistent with existing constraints on the Earth's carbon budget and would imply that as much as two thirds of the planet's carbon is hidden in its center sphere.

Applying Seismology to Giant Planets

Seismology of Giant Planets

Authors:

Gaulme et al

Abstract:

Seismology applied to giant planets could drastically change our understanding of their deep interiors, as it has happened with the Earth, the Sun, and many main-sequence and evolved stars. The study of giant planets' composition is important for understanding both the mechanisms enabling their formation and the origins of planetary systems, in particular our own. Unfortunately, its determination is complicated by the fact that their interior is thought not to be homogeneous, so that spectroscopic determinations of atmospheric abundances are probably not representative of the planet as a whole. Instead, the determination of their composition and structure must rely on indirect measurements and interior models. Giant planets are mostly fluid and convective, which makes their seismology much closer to that of solar-like stars than that of terrestrial planets. Hence, helioseismology techniques naturally transfer to giant planets. In addition, two alternative methods can be used: photometry of the solar light reflected by planetary atmospheres, and ring seismology in the specific case of Saturn. The current decade has been promising thanks to the detection of Jupiter's acoustic oscillations with the ground-based imaging-spectrometer SYMPA and indirect detection of Saturn's f-modes in its rings by the NASA Cassini orbiter. This has motivated new projects of ground-based and space-borne instruments that are under development. In this chapter, we review the science that seismology could help understand about the four giant planets, the instrumental and modeling approaches, and the most recent observational results.

A Collisional Origin to Earth’s Non-chondritic Composition?

A collisional origin to earth’s non-chondritic composition?

Authors:

Bonsor et al

Abstract:

Several lines of evidence indicate a non-chondritic composition for Bulk Earth. If Earth formed from the accretion of chondritic material, its non-chondritic composition, in particular the super-chondritic 142Nd/144Nd142Nd/144Nd and low Mg/Fe ratios, might be explained by the collisional erosion of differentiated planetesimals during its formation. In this work we use an N-body code, that includes a state-of-the-art collision model, to follow the formation of protoplanets, similar to proto-Earth, from differentiated planetesimals (> 100 km) up to isolation mass (> 0.16 M⊕). Collisions between differentiated bodies have the potential to change the core-mantle ratio of the accreted protoplanets. We show that sufficient mantle material can be stripped from the colliding bodies during runaway and oligarchic growth, such that the final protoplanets could have Mg/Fe and Si/Fe ratios similar to that of bulk Earth, but only if Earth is an extreme case and the core is assumed to contain 10% silicon by mass. This may indicate an important role for collisional differentiation during the giant impact phase if Earth formed from chondritic material.

Diversity of Hot Jupiters' internal structure

Embryo impacts and gas giant mergers II: Diversity of Hot Jupiters' internal structure

Authors:

Liu et al

Abstract:

We consider the origin of compact, short-period, Jupiter-mass planets. We propose that their diverse structure is caused by giant impacts of embryos and super-Earths or mergers with other gas giants during the formation and evolution of these hot Jupiters. Through a series of numerical simulations, we show that typical head-on collisions generally lead to total coalescence of impinging gas giants. Although extremely energetic collisions can disintegrate the envelope of gas giants, these events seldom occur. During oblique and moderately energetic collisions, the merger products retain higher fraction of the colliders' cores than their envelopes. They can also deposit considerable amount of spin angular momentum to the gas giants and desynchronize their spins from their orbital mean motion. We find that the oblateness of gas giants can be used to infer the impact history. Subsequent dissipation of stellar tide inside the planets' envelope can lead to runaway inflation and potentially a substantial loss of gas through Roche-lobe overflow. The impact of super-Earths on parabolic orbits can also enlarge gas giant planets' envelope and elevates their tidal dissipation rate over ∼ 100 Myr time scale. Since giant impacts occur stochastically with a range of impactor sizes and energies, their diverse outcomes may account for the dispersion in the mass-radius relationship of hot Jupiters.

The Structure and Evolution of Gas Giant Planets and Their Host Stars

On the structure and evolution of planets and their host stars − effects of various heating mechanisms on the size of giant gas planets

Authors:

Yidiz et al

Abstract:

It is already stated in the previous studies that the radius of the giant planets is affected by stellar irradiation. The confirmed relation between radius and incident flux depends on planetary mass intervals. In this study, we show that there is a single relation between radius and irradiated energy per gram per second (l−), for all mass intervals. There is an extra increase in radius of planets if l− is higher than 1100 times energy received by the Earth (l⊕). This is likely due to dissociation of molecules. The tidal interaction as a heating mechanism is also considered and found that its maximum effect on the inflation of planets is about 15 per cent. We also compute age and heavy element abundances from the properties of host stars, given in the TEPCat catalogue (Southworth 2011). The metallicity given in the literature is as [Fe/H]. However, the most abundant element is oxygen, and there is a reverse relation between the observed abundances [Fe/H] and [O/Fe]. Therefore, we first compute [O/H] from [Fe/H] by using observed abundances, and then find heavy element abundance from [O/H]. We also develop a new method for age determination. Using the ages we find, we analyse variation of both radius and mass of the planets with respect to time, and estimate the initial mass of the planets from the relation we derive for the first time. According to our results, the highly irradiated gas giants lose 5 per cent of their mass in every 1 Gyr.