arXiv subjects

Yann Alibert

Publications and source records attributed to Yann Alibert.

At least 91 records · Page 5Linked to original sources

arXiv2016

Probing TRAPPIST-1-like systems with K2

The search for small planets orbiting late M dwarfs holds the promise of detecting Earth-size planets for which their atmospheres could be characterised within the next decade. The recent discovery of TRAPPIST-1 entertains hope that these systems are common around hosts located at the bottom of the main sequence. In this Letter, we investigate the ability of the repurposed Kepler mission (K2) to probe planetary systems similar to TRAPPIST-1. We perform a consistent data analysis of 189 spectroscopically confirmed M5.5 to M9 late M dwarfs from campaigns 1-6 to search for planet candidates and inject transit signals with properties matching TRAPPIST-1b and c. We find no transiting planet candidates across our K2 sample. Our injection tests show that K2 is able to recover both TRAPPIST-1 planets for 10% of the sample only, mainly because of the inefficient throughput at red wavelengths resulting in Poisson-limited performance for these targets. Increasing injected planetary radii to match GJ1214b's size yields a recovery rate of 70%. The strength of K2 is its ability to probe a large number of cool hosts across the different campaigns, out of which the recovery rate of 10% may turn into bona-fide detections of TRAPPIST-1 like systems within the next two years.

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arXiv2016

Constraining the volatile fraction of planets from transit observations

The determination of the abundance of volatiles in extrasolar planets is very important as it can provide constraints on transport in protoplanetary disks and on the formation location of planets. However, constraining the internal structure of low-mass planets from transit measurements is known to be a degenerate problem. Using planetary structure and evolution models, we show how observations of transiting planets can be used to constrain their internal composition, in particular the amount of volatiles in the planetary interior, and consequently the amount of gas (defined in this paper to be only H and He) that the planet harbors. We show for low-mass gas-poor planets that are located close to their central star that assuming evaporation has efficiently removed the entire gas envelope, it is possible to constrain the volatile fraction of close-in transiting planets. We illustrate this method on the example of 55 Cnc e and show that under the assumption of the absence of gas, the measured mass and radius imply at least 20 % of volatiles in the interior. For planets at larger distances, we show that the observation of transiting planets at different evolutionary ages can be used to set statistical constraints on the volatile content of planets. These results can be used in the context of future missions like PLATO to better understand the internal composition of planets.

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arXiv2015

Elemental ratios in stars vs planets

Context. The chemical composition of planets is an important constraint for planet formation and subsequent differentiation. While theoretical studies try to derive the compositions of planets from planet formation models in order to link the composition and formation process of planets, other studies assume that the elemental ratios in the formed planet and in the host star are the same. Aims. Using a chemical model combined with a planet formation model, we aim to link the composition of stars with solar mass and luminosity with the composition of the hosted planets. For this purpose, we study the three most important elemental ratios that control the internal structure of a planet: Fe/Si, Mg/Si, and C/O. Methods. A set of 18 different observed stellar compositions was used to cover a wide range of these elemental ratios. The Gibbs energy minimization assumption was used to derive the composition of planets, taking stellar abundances as proxies for nebular abundances, and to generate planets in a self-consistent planet formation model. We computed the elemental ratios Fe/Si, Mg/Si and C/O in three types of planets (rocky, icy, and giant planets) formed in different protoplanetary discs, and compared them to stellar abundances. Results. We show that the elemental ratios Mg/Si and Fe/Si in planets are essentially identical to those in the star. Some deviations are shown for planets that formed in specific regions of the disc, but the relationship remains valid within the ranges encompassed in our study. The C/O ratio shows only a very weak dependence on the stellar value.

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arXiv2015

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

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. [...]

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arXiv2014

From stellar nebula to planetesimals

Solar and extrasolar comets and extrasolar planets are the subject of numerous studies in order to determine their chemical composition and internal structure. In the case of planetesimals, their compositions are important as they govern in part the composition of future planets. The present works aims at determining the chemical composition of icy planetesimals, believed to be similar to present day comets, formed in stellar systems of solar chemical composition. The main objective of this work is to provide valuable theoretical data on chemical composition for models of planetesimals and comets, and models of planet formation and evolution. We have developed a model that calculates the composition of ices formed during the cooling of the stellar nebula. Coupled with a model of refractory element formation, it allows us to determine the chemical composition and mass ratio of ices to rocks in icy planetesimals throughout in the protoplanetary disc. We provide relationships for ice line positions (for different volatile species) in the disc, and chemical compositions and mass ratios of ice relative to rock for icy planetesimals in stellar systems of solar chemical composition. From an initial homogeneous composition of the nebula, a wide variety of chemical compositions of planetesimals were produced as a function of the mass of the disc and distance to the star. Ices incorporated in planetesimals are mainly composed of H2O, CO, CO2, CH3OH, and NH3. The ice/rock mass ratio is equal to 1+-0.5 in icy planetesimals following assumptions. This last value is in good agreement with observations of solar system comets, but remains lower than usual assumptions made in planet formation models, taking this ratio to be of 2-3.

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arXiv2014

From planetesimals to planets: volatile molecules

Solar and extrasolar planets are the subject of numerous studies aiming to determine their chemical composition and internal structure. In the case of extrasolar planets, the composition is important as it partly governs their potential habitability. Moreover, observational determination of chemical composition of planetary atmospheres are becoming available, especially for transiting planets. The present works aims at determining the chemical composition of planets formed in stellar systems of solar chemical composition. The main objective of this work is to provide valuable theoretical data for models of planet formation and evolution, and future interpretation of chemical composition of solar and extrasolar planets. We have developed a model that computes the composition of ices in planets in different stellar systems with the use of models of ice and planetary formation. We provide the chemical composition, ice/rock mass ratio and C:O molar ratio for planets in stellar systems of solar chemical composition. From an initial homogeneous composition of the nebula, we produce a wide variety of planetary chemical compositions as a function of the mass of the disk and distance to the star. The volatile species incorporated in planets are mainly composed of H2O, CO, CO2, CH3OH, and NH3. Icy or ocean planets have systematically higher values of molecular abundances compared to giant and rocky planets. Gas giant planets are depleted in highly volatile molecules such as CH4, CO, and N2 compared to icy or ocean planets. The ice/rock mass ratio in icy or ocean and gas giant planets is, respectively, equal at maximum to 1.01+-0.33 and 0.8+-0.5, and is different from the usual assumptions made in planet formation models, which suggested this ratio to be 2-3. The C:O molar ratio in the atmosphere of gas giant planets is depleted by at least 30% compared to solar value.

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arXiv2014

From stellar nebula to planets: the refractory components

We computed the abundance of refractory elements in planetary bodies formed in stellar systems with solar chemical composition by combining models for chemical composition and planet formation. We also consider the formation of refractory organic compounds, which have been ignored in previous studies on this topic. We used the commercial software package HSC Chemistry in order to compute the condensation sequence and chemical composition of refractory minerals incorporated into planets. The problem of refractory organic material is approached with two distinct model calculations: the first considers that the fraction of atoms used in the formation of organic compounds is removed from the system (i.e. organic compounds are formed in the gas phase and are nonreactive); and the second assumes that organic compounds are formed by the reaction between different compounds that had previously condensed from the gas phase. Results show that refractory material represents more than 50 wt % of the mass of solids accreted by the simulated planets, with up to 30 wt % of the total mass composed of refractory organic compounds. Carbide and silicate abundances are consistent with C/O and Mg/Si elemental ratios of 0.5 and 1.02 for the Sun. Less than 1 wt % of carbides; pyroxene and olivine in similar quantities are formed. The model predicts planets that are similar in composition to those of the Solar system. It also shows that, starting from a common initial nebula composition, a wide variety of chemically different planets can form, which means that the differences in planetary compositions are due to differences in the planetary formation process. We show that a model in which refractory organic material is absent from the system is more compatible with observations. The use of a planet formation model is essential to form a wide diversity of planets in a consistent way.

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arXiv2013

On the radius of habitable planets

The conditions that a planet must fulfill to be habitable are not precisely known. However, it is comparatively easier to define conditions under which a planet is very likely not habitable. Finding such conditions is important as it can help select, in an ensemble of potentially observable planets, which ones should be observed in greater detail for characterization studies. Assuming, as in the Earth, that the presence of a C-cycle is a necessary condition for long-term habitability, we derive, as a function of the planetary mass, a radius above which a planet is likely not habitable. We compute the maximum radius a planet can have to fulfill two constraints: surface conditions compatible with the existence of liquid water, and no ice layer at the bottom of a putative global ocean. We demonstrate that, above a given radius, these two constraints cannot be met. We compute internal structure models of planets, using a five-layer model (core, inner mantle, outer mantle, ocean, and atmosphere), for different masses and composition of the planets (in particular, the Fe/Si ratio of the planet). Our results show that for planets in the Super-Earth mass range (1-12 Mearth), the maximum that a planet, with a composition similar to that of the Earth, can have varies between 1.7 and 2.2 Rearth. This radius is reduced when considering planets with higher Fe/Si ratios and taking radiation into account when computing the gas envelope structure. These results can be used to infer, from radius and mass determinations using high-precision transit observations like those that will soon be performed by the CHaracterizing ExOPlanet Satellite (CHEOPS), which planets are very likely not habitable, and therefore which ones should be considered as best targets for further habitability studies.}

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arXiv2013

Giant Planet Formation, Evolution, and Internal Structure

The large number of detected giant exoplanets offers the opportunity to improve our understanding of the formation mechanism, evolution, and interior structure of gas giant planets. The two main models for giant planet formation are core accretion and disk instability. There are substantial differences between these formation models, including formation timescale, favorable formation location, ideal disk properties for planetary formation, early evolution, planetary composition, etc. First, we summarize the two models including their substantial differences, advantages, and disadvantages, and suggest how theoretical models should be connected to available (and future) data. We next summarize current knowledge of the internal structures of solar- and extrasolar- giant planets. Finally, we suggest the next steps to be taken in giant planet exploration.

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arXiv2011

High precision astrometry mission for the detection and characterization of nearby habitable planetary systems with the Nearby Earth Astrometric Telescope (NEAT)

(abridged) A complete census of planetary systems around a volume-limited sample of solar-type stars (FGK dwarfs) in the Solar neighborhood with uniform sensitivity down to Earth-mass planets within their Habitable Zones out to several AUs would be a major milestone in extrasolar planets astrophysics. This fundamental goal can be achieved with a mission concept such as NEAT - the Nearby Earth Astrometric Telescope. NEAT is designed to carry out space-borne extremely-high-precision astrometric measurements sufficient to detect dynamical effects due to orbiting planets of mass even lower than Earth's around the nearest stars. Such a survey mission would provide the actual planetary masses and the full orbital geometry for all the components of the detected planetary systems down to the Earth-mass limit. The NEAT performance limits can be achieved by carrying out differential astrometry between the targets and a set of suitable reference stars in the field. The NEAT instrument design consists of an off-axis parabola single-mirror telescope, a detector with a large field of view made of small movable CCDs located around a fixed central CCD, and an interferometric calibration system originating from metrology fibers located at the primary mirror. The proposed mission architecture relies on the use of two satellites operating at L2 for 5 years, flying in formation and offering a capability of more than 20,000 reconfigurations (alternative option uses deployable boom). The NEAT primary science program will encompass an astrometric survey of our 200 closest F-, G- and K-type stellar neighbors, with an average of 50 visits. The remaining time might be allocated to improve the characterization of the architecture of selected planetary systems around nearby targets of specific interest (low-mass stars, young stars, etc.) discovered by Gaia, ground-based high-precision radial-velocity surveys.

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arXiv2011

Extrasolar planet population synthesis III. Formation of planets around stars of different masses

We extend the models presented in Mordasini et al. (2009) to the formation of planets orbiting stars of different masses. We discuss the properties of the resulting synthetic planet population in terms of mass, orbit, and metallicity distributions. The population synthesis calculations we use are based on the planet formation model developed in Alibert et al. (2005a), which self-consistently takes into account planetary growth and migration in an evolving proto-planetary disk. Using this model, we generate synthetic populations of planets by following their growth in a large number of proto-planetary disks, whose properties (mass and lifetime) are selected in a Monte Carlo fashion using probability distributions derived from observations. We show that the scaling of the proto-planetary disk mass with the mass of the central star has a direct and large influence on the properties of the resulting planet population. In particular, the observed paucity of large mass planets orbiting 0.5 Msun stars can directly be explained as resulting from a only slightly steeper than linear scaling. The observed lack of short period planet orbiting 2.0 Msun stars can also be attributed to this scaling but only if associated with a decrease of the mean disk lifetime for stars more massive than 1.5 Msun. Finally, we show that the distribution of minimum mass and semi-major axis of our synthetic planets are statistically comparable with observations.

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arXiv2010

Theory of planet formation

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

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arXiv2009

Elemental abundances and minimum mass of heavy elements in the envelope of HD 189733b

Oxygen (O) and carbon (C) have been inferred recently to be subsolar in abundance from spectra of the atmosphere of the transiting hot Jupiter HD 189733b. Yet, the mass and radius of the planet coupled with structure models indicate a strongly supersolar abundance of heavy elements in the interior of this object. Here we explore the discrepancy between the large amount of heavy elements suspected in the planet's interior and the paucity of volatiles measured in its atmosphere. We describe the formation sequence of the icy planetesimals formed beyond the snow line of the protoplanetary disk and calculate the composition of ices ultimately accreted in the envelope of HD 189733b on its migration pathway. This allows us to reproduce the observed volatile abundances by adjusting the mass of ices vaporized in the envelope. The predicted elemental mixing ratios should be 0.15--0.3 times solar in the envelope of HD 189733b if they are fitted to the recent O and C determinations. However, our fit to the minimum mass of heavy elements predicted by internal structure models gives elemental abundances that are 1.2--2.4 times oversolar in the envelope of HD189733b. We propose that the most likely cause of this discrepancy is irradiation from the central star leading to development of a radiative zone in the planet's outer envelope which would induce gravitational settling of elements. Hence, all strongly irradiated extrasolar planets should present subsolar abundances of volatiles. We finally predict that the abundances of nitrogen (N), sulfur (S) and phosphorus (P) are of $\sim$ $2.8 \times 10^{-5}$, $5.3 \times 10^{-6}$ and $1.8 \times 10^{-7}$ relative to H$_2$, respectively in the atmosphere of HD 189733b.

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arXiv2009

Extrasolar planet population synthesis I: Method, formation tracks and mass-distance distribution

With the high number of extrasolar planets discovered by now, it becomes possible to constrain theoretical formation models in a statistical sense. This paper is the first in a series in which we carry out a large number of planet population synthesis calculations. We begin the series with a paper mainly dedicated to the presentation of our approach, but also the discussion of a representative synthetic planetary population of solar like stars. Based as tightly as possible on observational data, we have derived probability distributions for the most important initial conditions for the planetary formation process. We then draw sets of initial conditions from these distributions and obtain the corresponding synthetic planets with our formation model. Although the main purpose of this paper is the description of our methods, we present some key results: We find that the variation of the initial conditions in the limits occurring in nature leads to the formation of planets of large diversity. This formation process is best visualized in planetary formation tracks, where different phases of concurrent growth and migration can be identified. These phases lead to the emergence of sub-populations of planets distinguishable in a mass-semimajor axis diagram. The most important ones are the "failed cores", a vast group of core-dominated low mass planets, the "horizontal branch", a sub-population of Neptune mass planets extending out to 6 AU, and the "main clump", a concentration of giant gaseous giants planets at around 0.3-2 AU.

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arXiv2009

Extrasolar planet population synthesis II: Statistical comparison with observation

This is the second paper in a series of papers showing the results of extrasolar planet population synthesis calculations. In the companion paper (Paper I), we have presented in detail our methods. By applying an observational detection bias for radial velocity surveys, we identify the potentially detectable synthetic planets. The properties of these planets are compared in quantitative statistical tests with the properties of a carefully selected sub-population of actual exoplanets. We use a two dimensional Kolmogorov-Smirnov test to compare the mass-distance distributions of synthetic and observed planets, as well as 1D KS tests to compare the mass, the semimajor axis and the [Fe/H] distributions. We find that some models can account to a reasonable degree of significance for the observed properties. We concurrently account for many other observed features, e.g. the "metallicity effect". This gives us confidence that our model captures several essential features of giant planet formation. Our simulations allow us also to extract the properties of the underlying exoplanet population that are not yet detectable. For example, we have derived the planetary initial mass function (PIMF) and have been led to conclude that the planets detected so far represent only the tip of the iceberg. The PIMF can also be used to predict how the detectable extrasolar planet population will change as the precision of radial velocity surveys improves to an extreme precision of 0.1 m/s.

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arXiv2008

Determination of the minimum masses of heavy elements in the envelopes of Jupiter and Saturn

We calculate the minimum mass of heavy elements required in the envelopes of Jupiter and Saturn to match the observed oversolar abundances of volatiles. Because the clathration efficiency remains unknown in the solar nebula, we have considered a set of sequences of ice formation in which the fraction of water available for clathration is varied between 0 and 100 %. In all the cases considered, we assume that the water abundance remains homogeneous whatever the heliocentric distance in the nebula and directly derives from a gas phase of solar composition. Planetesimals then form in the feeding zones of Jupiter and Saturn from the agglomeration of clathrates and pure condensates in proportions fixed by the clathration efficiency. A fraction of Kr and Xe may have been sequestrated by the H3+ ion in the form of stable XeH3+ and KrH3+ complexes in the solar nebula gas phase, thus implying the formation of at least partly Xe- and Kr-impoverished planetesimals in the feeding zones of Jupiter and Saturn. These planetesimals were subsequently accreted and vaporized into the hydrogen envelopes of Jupiter and Saturn, thus engendering volatiles enrichments in their atmospheres, with respect to hydrogen. Taking into account both refractory and volatile components, and assuming plausible molecular mixing ratios in the gas phase of the outer solar nebula, we show that it is possible to match the observed enrichments in Jupiter and Saturn, whatever the clathration efficiency. Our calculations predict that the O/H enrichment decreases from 6.7 to 5.6 times solar (O/H) in the envelope of Jupiter and from 18.1 to 15.4 times solar (O/H) in the envelope of Saturn with the growing clathration efficiency in the solar nebula.

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arXiv2008

Clathration of Volatiles in the Solar Nebula and Implications for the Origin of Titan's atmosphere

We describe a scenario of Titan's formation matching the constraints imposed by its current atmospheric composition. Assuming that the abundances of all elements, including oxygen, are solar in the outer nebula, we show that the icy planetesimals were agglomerated in the feeding zone of Saturn from a mixture of clathrates with multiple guest species, so-called stochiometric hydrates such as ammonia hydrate, and pure condensates. We also use a statistical thermodynamic approach to constrain the composition of multiple guest clathrates formed in the solar nebula. We then infer that krypton and xenon, that are expected to condense in the 20-30 K temperature range in the solar nebula, are trapped in clathrates at higher temperatures than 50 K. Once formed, these ices either were accreted by Saturn or remained embedded in its surrounding subnebula until they found their way into the regular satellites growing around Saturn. In order to explain the carbon monoxide and primordial argon deficiencies of Titan's atmosphere, we suggest that the satellite was formed from icy planetesimals initially produced in the solar nebula and that were partially devolatilized at a temperature not exceeding 50 K during their migration within Saturn's subnebula. The observed deficiencies of Titan's atmosphere in krypton and xenon could result from other processes that may have occurred both prior or after the completion of Titan. Thus, krypton and xenon may have been sequestrated in the form of XH3+ complexes in the solar nebula gas phase, causing the formation of noble gas-poor planetesimals ultimately accreted by Titan. Alternatively, krypton and xenon may have also been trapped efficiently in clathrates located on the satellite's surface or in its atmospheric haze.

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arXiv2008

Constraints from deuterium on the formation of icy bodies in the Jovian system and beyond

We consider the role of deuterium as a potential marker of location and ambient conditions during the formation of small bodies in our Solar system. We concentrate in particular on the formation of the regular icy satellites of Jupiter and the other giant planets, but include a discussion of the implications for the Trojan asteroids and the irregular satellites. We examine in detail the formation of regular planetary satellites within the paradigm of a circum-Jovian subnebula. Particular attention is paid to the two extreme potential subnebulae - "hot" and "cold". In particular, we show that, for the case of the "hot" subnebula model, the D:H ratio in water ice measured from the regular satellites would be expected to be near-Solar. In contrast, satellites which formed in a "cold" subnebula would be expected to display a D:H ratio that is distinctly over-Solar. We then compare the results obtained with the enrichment regimes which could be expected for other families of icy small bodies in the outer Solar system - the Trojan asteroids and the irregular satellites. In doing so, we demonstrate how measurements by Laplace, the James Webb Space Telescope, HERSCHEL and ALMA will play an important role in determining the true formation locations and mechanisms of these objects.

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