The recent detection of planets around very low mass stars raises the question of the formation, composition and potential habitability of these objects. We use planetary system formation models to infer the properties, in particular their radius distribution and water content, of planets that may form around stars ten times less massive than the Sun. Our planetary system formation and composition models take into account the structure and evolution of the protoplanetary disk, the planetary mass growth by accretion of solids and gas, as well as planet-planet, planet-star and planet-disk interactions. We show that planets can form at small orbital period in orbit about low mass stars. We show that the radius of the planets is peaked at about 1 rearth and that they are, in general, volatile rich especially if proto-planetary discs orbiting this type of stars are long-lived. Close-in planets orbiting low-mass stars similar in terms of mass and radius to the ones recently detected can be formed within the framework of the core accretion paradigm as modeled here. The properties of protoplanetary disks, and their correlation with the stellar type, are key to understand their composition.
Prior work has found that a variety of terrestrial planetary compositions are expected to occur within known extrasolar planetary systems. However, such studies ignored the effects of giant planet migration, which is thought to be very common in extra-solar systems. Here we present calculations of the compositions of terrestrial planets that formed in dynamical simulations incorporating varying degrees of giant planet migration. We used chemical equilibrium models of the solid material present in the disks of five known planetary host stars: the Sun, GJ 777, HD4203, HD19994 and HD213240. Giant…
Extrasolar planet host stars have been found to be enriched in key planet-building elements. These enrichments have the potential to drastically alter the composition of material available for terrestrial planet formation. Here we report on the combination of dynamical models of late-stage terrestrial planet formation within known extrasolar planetary systems with chemical equilibrium models of the composition of solid material within the disk. This allows us to determine the bulk elemental composition of simulated extrasolar terrestrial planets. A wide variety of resulting planetary compositi…
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 …
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 com…
The composition of planets is largely determined by the chemical and dynamical evolution of the disk during planetesimal formation and growth. To predict the diversity of exoplanet compositions, previous works modeled planetesimal composition as the equilibrium chemical composition of a proto- planetary disk at a single time. However, planetesimals form over an extended period of time, during which, elements sequentially condense out of the gas as the disk cools and are accreted onto planetesi- mals. To account for the evolution of the disk during planetesimal formation, we couple models of di…
The two current models for giant planet formation are core accretion and disk instability. We discuss the core masses and overall planetary enrichment in heavy elements predicted by the two formation models, and show that both models could lead to a large range of final compositions. For example, both can form giant planets with nearly stellar compositions. However, low-mass giant planets, enriched in heavy elements compared to their host stars, are more easily explained by the core accretion model. The final structure of the planets, i.e., the distribution of heavy elements, is not firmly con…
We model the evolution of planets with various masses and compositions. We investigate the effects of the composition and its depth dependence on the long-term evolution of the planets. The effects of opacity and stellar irradiation are also considered. It is shown that the change in radius due to various compositions can be significantly smaller than the change in radius caused by the opacity. Irradiation also affects the planetary contraction but is found to be less important than the opacity effects. We suggest that the mass-radius relationship used for characterization of observed extrasol…
Constraining planet formation based on the atmospheric composition of exoplanets is a fundamental goal of the exoplanet community. Existing studies commonly try to constrain atmospheric abundances, or to analyze what abundance patterns a given description of planet formation predicts. However, there is also a pressing need to develop methodologies that investigate how to transform atmospheric compositions into planetary formation inferences. In this study we summarize the complexities and uncertainties of state-of-the-art planet formation models and how they influence planetary atmospheric com…
Models of planet formation are mainly focused on the accretion and dynamical processes of the planets, neglecting their chemical composition. In this work, we calculate the condensation sequence of the different chemical elements for a low-mass protoplanetary disk around a solar-type star. We incorporate this sequence of chemical elements (refractory and volatile elements) in our semi-analytical model of planet formation which calculates the formation of a planetary system during its gaseous phase. The results of the semi-analytical model (final distributions of embryos and planetesimals) are …
A planetary mass scale and a system of composition codes are presented for describing the geophysical characteristics of exoplanets and Solar System planets, dwarf planets, and spherical moons. The composition classes characterize the rock, ice, and gas properties of planetary bodies. The planetary mass scale includes five mass classes with upper and lower mass limits derived from recent studies of the exoplanet mass radius and mass density relationships and the physical characteristics of planets, dwarf planets, and spherical moons in the Solar System. The combined mass and composition codes …
The chemical composition of stars that have orbiting planets provides important clues about the frequency, architecture, and composition of exoplanet systems. We explore the possibility that stars from different galactic populations that have different intrinsic abundance ratios may produce planets with a different overall composition. We compiled abundances for Fe, O, C, Mg, and Si in a large sample of solar neighbourhood stars that belong to different galactic populations. We then used a simple stoichiometric model to predict the expected iron-to-silicate mass fraction and water mass fractio…
The melt productivity of a differentiated planet's mantle is primarily controlled by its iron content, which is itself approximated by the planet's core mass fraction (CMF). Here we show that estimates of an exo-planet's CMF allows robust predictions of the thickness, composition and mineralogy of the derivative crust. These predicted crustal compositions allow constraints to be placed on volatile cycling between surface and the deep planetary interior, with implications for the evolution of habitable planetary surfaces. Planets with large, terrestrial-like, CMFs ($\geq$0.32) will exhibit thin…
Low-mass planets have an extraordinarily diverse range of bulk compositions, from primarily rocky worlds to those with deep gaseous atmospheres. As techniques for measuring the masses of exoplanets are advancing the field towards the terrestrial regime, from ultra-short orbital periods to Venus-like distances, we identify the bounds on planet compositions, where sizes and incident fluxes inform bulk planet properties. In some cases, measurement precisions of planet masses and sizes are approaching the theoretical uncertainties in planet models. An emerging picture explains aspects of the diver…
TRAPPIST-1 is a nearby ultra-cool dwarf that is host to a remarkable planetary system consisting of seven transiting planets. The orbital properties and radii of the planets have been well-constrained, and recently the masses of the inner six planets have been measured with additional ground and space-based photometric observations. Large uncertainties in these mass measurements have prevented a robust analysis of the planetary compositions. Here we perform many thousands of N-body dynamical simulations with planet properties perturbed from the observed values and identify those that are stabl…
Transit surveys combined with Doppler data have revealed a class of gas giant planets that are massive and highly enriched in heavy elements (e.g., HD149026b, GJ436b, and HAT-P-20b). It is tempting to consider these planets as validation of core accretion plus gas capture because it is often assumed that disk instability planets should be of nebular composition. We show in this paper, to the contrary, that gas giants that form by disk instability can have a variety of heavy element compositions, ranging from sub- to super-nebular values. High levels of enrichment can be achieved through one or…
Most of planet formation models that incorporate planetesimal fragmentation consider a catastrophic impact energy threshold for basalts at a constant velocity of 3 km/s during all the process of the formation of the planets. However, as planets grow the relative velocities of the surrounding planetesimals increase from velocities of the order of m/s to a few km/s. In addition, beyond the ice line where giant planets are formed, planetesimals are expected to be composed roughly by 50 percentage of ices. We aim to study the role of planetesimal fragmentation on giant planet formation considering…
The requirement that a planet must orbit outside of its Roche limit gives a lower limit on the planet's mean density. The minimum density depends almost entirely on the orbital period and is immune to systematic errors in the stellar properties. We consider the implications of this density constraint for the newly-identified class of small planets with periods shorter than half a day. When the planet's radius is known accurately, this lower limit to the density can be used to restrict the possible combinations of iron and rock within the planet. Applied to KOI 1843.03, with a radius of 0.6 Ear…
Toward Reliable Interpretations of Small Exoplanet Compositions: Comparisons and Considerations of Equations of State and Materials Used in Common Rocky Planet Models
Louisiana State University（路易斯安那州立大学）；National Science Foundation（美国国家科学基金会）；Southwest Research Institute（西南研究院）
We use an end-to-end model of planet formation, thermodynamic evolution, and atmospheric escape to investigate how the statistical imprints of evaporation depend on the bulk composition of planetary cores (rocky vs. icy). We find that the population-wide imprints like the location of the "evaporation valley" in the distance-radius plane and the corresponding bimodal radius distribution clearly differ depending on the bulk composition of the cores. Comparison with the observed position of the valley (Fulton et al. 2017) suggests that close-in low-mass Kepler planets have a predominately Earth-l…
We present a basic, fast, and flexible planet formation model, called SimAb (Simulating Abundances), to form giant planets and study their primary atmospheric composition soon after their formation. In SimAb we introduce parameters to simplify the assumptions about the complex physics involved in the formation of a planet. This approach allows us to trace and understand the influence of complex physical processes on the formed planets. We focus on the C/O ratio and the metallicity of the planetary atmosphere as an indicator of their compositions. We show that the initial protoplanet core mass …
The exoplanet mass radius diagram reveals that super Earths display a wide range of radii, and therefore mean densities, at a given mass. Using planet population synthesis models, we explore the key physical factors that shape this distribution: planets' solid core compositions, and their atmospheric structure. For the former, we use equilibrium disk chemistry models to track accreted minerals onto planetary cores throughout formation. For the latter, we track gas accretion during formation, and consider photoevaporation-driven atmospheric mass loss to determine what portion of accreted gas es…
Most of the properties of the Earth-Moon system can be explained by a collision between a planetary embryo and the growing Earth late in the accretion process. Simulations show that most of the material that eventually aggregates to form the Moon originates from the impactor. However, analysis of the terrestrial and lunar isotopic composition show them to be highly similar. In contrast, the compositions of other solar system bodies are significantly different than the Earth and Moon. This poses a major challenge to the giant impact scenario since the Moon-forming impactor is then thought to al…
Transiting planet surveys like Kepler have provided a wealth of information on the distribution of planetary radii, particularly for the new populations of super-Earth and sub-Neptune sized planets. In order to aid in the physical interpretation of these radii, we compute model radii for low-mass rocky planets with hydrogen-helium envelopes. We provide model radii for planets 1-20 Earth masses, with envelope fractions from 0.01-20%, levels of irradiation 0.1-1000x Earth's, and ages from 100 Myr to 10 Gyr. In addition we provide simple analytic fits that summarize how radius depends on each of …
We explore the possible Proxima Centauri b's interiors assuming the planet belongs to the class of dense solid planets (rocky with possible addition of water) and derive the corresponding radii. To do so, we use an internal structure model that computes the radius of the planet along with the locations of the different layers of materials, assuming that its mass and bulk composition are known. Lacking detailed elementary abundances of the host star to constrain the planet's composition, we base our model on solar system values. We restrained the simulations to the case of solid planets without…
Partial condensation of dust from the Solar nebula is likely responsible for the diverse chemical compositions of chondrites and rocky planets/planetesimals in the inner Solar system. We present a forward physical-chemical model of a protoplanetary disc to predict the chemical compositions of planetary building blocks that may form from such a disc. Our model includes the physical evolution of the disc and the condensation, partial advection, and decoupling of the dust within it. The chemical composition of the condensate changes with time and radius. We compare the results of two dust condens…
Modeling the interior of a planet is difficult because the small number of measured parameters is insufficient to constrain the many variables involved in describing the interior structure and composition. One solution is to invoke additional constraints based on arguments about how the planet formed. However, a planet's actual structure and composition may hold clues to its formation which would be lost if this structure were not allowed by the initial assumptions. It is therefore interesting to explore the space of allowable compositions and structures in order to better understand which cos…
A crucial phase during planetary growth is the migration when the planetary core has been assembled, but the planet did not open a deep gap yet. During this phase the planet is subject to fast type-I migration, which is mostly directed inwards, and the planet can lose a significant fraction of its semi-major axis. The duration of this phase is set by how long the planetary envelope needs to contract until it reaches a mass similar to the mass of the planetary core, which is when runaway gas accretion can set in and the planet can open a deeper gap in the disc, transitioning into the slower typ…
The compositions of stars and planets are connected, yet, the definition of "habitability" and the "habitable zone" only take into account the physical relationship between the star and planet. Planets, however, are made truly habitable by both chemical and physical processes which regulate climatic and geochemical cycling between atmosphere, surface, and interior reservoirs. Despite this, "Earth-like" is often defined as a planet made of a mixture of rock and Fe that is roughly 1 Earth-density. To understand the interior of a terrestrial planet, the stellar abundances of planet-building eleme…
The Kepler Mission has found thousands of planetary candidates with radii between 1 and 4 R$_\oplus$. These planets have no analogues in our own Solar System, providing an unprecedented opportunity to understand the range and distribution of planetary compositions allowed by planet formation and evolution. A precise mass measurement is usually required to constrain the possible composition of an individual super-Earth-sized planet, but these measurements are difficult and expensive to make for the majority of Kepler planet candidates. Fortunately, adopting a statistical approach helps us to ad…
Potentially habitable planets within the habitable zone of M-dwarfs are affected by tidal interaction. We studied the tidal evolution in GJ 667C using a numerical code we call TIDEV. We reviewed the problem of the dynamical evolution focusing on the effects that a rheological treatment, different compositions and the inclusion of orbital perturbations, have on the spin-down time and the probability to be trapped in a low spin-orbit resonance. Composition have a strong effect on the spin-down time, changing, in some cases, by almost a factor of 2 with respect to the value estimated for a refere…
The discovery of multiple transiting planetary systems offers new possibilities for characterising exoplanets and understanding their formation. The Kepler-9 system contains two Saturn-mass planets, Kepler-9b and 9c. Using evolution models of gas giants that reproduce the sizes of known transiting planets and accounting for all sources of uncertainties, we show that Kepler-9b (respectively 9c) contains $45^{+17}_{-12}$\,\mearth\ (resp. $31^{+13}_{-10}$\,\mearth) of hydrogen and helium and $35^{+10}_{-15}$\,\mearth (resp. $24^{+10}_{-12}$\,\mearth) of heavy elements. More accurate constraints a…
Planets with sizes between that of Earth (with radius R$_\oplus$) and Neptune (about 4 R$_\oplus$) are now known to be common around Sun-like stars. Most such planets have been discovered through the transit technique, by which the planet's size can be determined from the fraction of starlight blocked by the planet as it passes in front of its star. Measuring the planet's mass-and hence its density, which is a clue to its composition-is more difficult. Planets of size 2-4 R$_\oplus$ have proven to have a wide range of densities, implying a diversity of compositions, but these measurements did …
Multiple planet systems provide an ideal laboratory for probing exoplanet composition, formation history and potential habitability. For the TRAPPIST-1 planets, the planetary radii are well established from transits (Gillon et al., 2016, Gillon et al., 2017), with reasonable mass estimates coming from transit timing variations (Gillon et al., 2017, Wang et al., 2017) and dynamical modeling (Quarles et al., 2017). The low bulk densities of the TRAPPIST-1 planets demand significant volatile content. Here we show using mass-radius-composition models, that TRAPPIST-1f and g likely contain substant…
The rapid advances in observations of the different populations of exoplanets, the characterization of their host stars and the links to the properties of their planetary systems, the detailed studies of protoplanetary disks, and the experimental study of the interiors and composition of the massive planets in our solar system provide a firm basis for the next big question in planet formation theory. How do the elemental and chemical compositions of planets connect with their formation? The answer to this requires that the various pieces of planet formation theory be linked together in an end-…
Over the last twenty years, the search for extrasolar planets revealed us the rich diversity of the outcomes of the formation and evolution of planetary systems. In order to fully understand how these extrasolar planets came to be, however, the orbital and physical data we possess are not enough, and they need to be complemented with information on the composition of the exoplanets. Ground-based and space-based observations provided the first data on the atmospheric composition of a few extrasolar planets, but a larger and more detailed sample is required before we can fully take advantage of …
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,…
Evidence is now compelling that most externally-polluted white dwarfs derive their heavy atoms by accretion from asteroids - the building blocks of rocky planets. Optical and ultraviolet spectroscopy of a small sample of suitable white dwarf stars shows that to zeroth order, the accreted extrasolar parent bodies compositionally resemble bulk Earth. (1) Extrasolar planetesimals are at least 85% by mass composed of O, Mg, Si and Fe. (2) Compared to the Sun, C is often deficient, usually by at least a factor of 10 and therefore comprises less than 1% of an extrasolar planetesimal's mass. At least…
Future space missions will improve considerably our understanding of the formation and history of planetary systems. Currently, observations show that the presence of planetary companions is closely linked to the metallicity and the chemical abundances of the host stars. We aim to build an integrated tool to predict the planet building blocks composition as a function of the stellar populations, for the interpretation of the ongoing and future large surveys. We synthesize stellar populations with the Besançon Galaxy model (BGM) which includes stellar evolutionary tracks computed with the stell…
The hot Neptune orbiting around GJ 436 is a unique example of an intermediate mass planet. Its close-in orbit suggests that the planet has undergone migration and its study is fundamental to understanding planet formation and evolution. As it transits its parent star, it is the only Neptune-mass extrasolar planet of known mass and radius, being slightly larger and more massive than Neptune (M=22.6 M_Earth, R=4.19R_Earth). In this regime, several bulk compositions are possible: from an Earth-like core with a thick hydrogen envelope to a water-rich planet with a thin hydrogen envelope comprising…
The Radius, Composition, Albedo, and Absolute Magnitude of Planet Nine Based on Exoplanets with Teq less than 600 K and the Planet Nine Reference Population 3.0
We present a model of the early chemical composition and elemental abundances of planetary atmospheres based on the cumulative gaseous chemical species that are accreted onto planets forming by core accretion from evolving protoplanetary disks. The astrochemistry of the host disk is computed using an ionization driven, non-equilibrium chemistry network within viscously evolving disk models. We accrete gas giant planets whose orbital evolution is controlled by planet traps using the standard core accretion model and track the chemical composition of the material that is accreted onto the protop…
We present the next step in a series of papers devoted to connecting the composition of the atmospheres of forming planets with the chemistry of their natal evolving protoplanetary disks. The model presented here computes the coupled chemical and dust evolution of the disk and the formation of three planets per disk model. Our three canonical planet traps produce a Jupiter near 1 AU, a Hot Jupiter and a Super-Earth. We study the dependency of the final orbital radius, mass, and atmospheric chemistry of planets forming in disk models with initial disk masses that vary by 0.02 $M_\odot$ above an…
Several transiting super-Earths are expected to be discovered in the coming few years. While tools to model the interior structure of transiting planets exist, inferences about the composition are fraught with ambiguities. We present a framework to quantify how much we can robustly infer about super-Earth and Neptune-size exoplanet interiors from radius and mass measurements. We introduce quaternary diagrams to illustrate the range of possible interior compositions for planets with four layers (iron core, silicate mantles, water layers, and H/He envelopes). We apply our model to CoRoT-7b, GJ 4…
Outgassing Composition of the Murchison Meteorite: Implications for Volatile Depletion of Planetesimals and Interior-atmosphere Connections for Terrestrial Exoplanets
Chondrites, the building blocks of the terrestrial planets, have mass and atomic proportions of oxygen, iron, magnesium, and silicon totaling $\geq$90\% and variable Mg/Si ($\sim$25\%), Fe/Si (factor of $\geq$2), and Fe/O (factor of $\geq$3). The Earth and terrestrial planets (Mercury, Venus, and Mars) are differentiated into three layers: a metallic core, a silicate shell (mantle and crust), and a volatile envelope of gases, ices, and, for the Earth, liquid water. Each layer has different dominant elements (e.g., increasing Fe content with depth and increasing oxygen content to the surface). …
Extrasolar planetary host stars are enriched in key planet-building elements. These enrichments have the potential to drastically alter the building blocks available for terrestrial planet formation. Here we report on the combination of dynamical models of late-stage terrestrial planet formation within known extrasolar planetary systems with chemical equilibrium models of the composition of solid material within the disk. This allows us to constrain the bulk elemental composition of extrasolar terrestrial planets. A wide variety of resulting planetary compositions exist, ranging from those tha…
Disintegrating planets allow for the unique opportunity to study the composition of the interiors of small, hot, rocky exoplanets because the interior is evaporating and that material is condensing into dust, which is being blown away and then transiting the star. Their transit signal is dominated by dusty effluents forming a comet-like tail trailing the host planet (or leading it, in the case of K2-22b), making these good candidates for transmission spectroscopy. To assess the ability of such observations to diagnose the dust composition, we simulate the transmission spectra from 5-14 $μ$m fo…
Spectroscopic transiting observations of the atmospheres of hot Jupiters around other stars, first with Hubble Space Telescope and then Spitzer, opened the door to compositional studies of exoplanets. The James Webb Space Telescope will provide such a profound improvement in signal-to-noise ratio that it will enable detailed analysis of molecular abundances, including but not limited to determining abundances of all the major carbon- and oxygen-bearing species in hot Jupiter atmospheres. This will allow determination of the carbon-to-oxygen ratio, an essential number for planet formation model…
