Context.
Planet formation theory suggests that planet bulk compositions are likely to reflect the chemical abundance ratios of their host star’s photosphere. Variations in the abundance of particular ...chemical species in stellar photospheres between different galactic stellar populations demonstrate that there are differences among the expected solid planet bulk compositions.
Aims.
We aim to present planetary mass-radius relations of solid planets for kinematically differentiated stellar populations, namely, the thin disc, thick disc, and halo.
Methods.
Using two separate internal structure models, we generated synthetic planets using bulk composition inputs derived from stellar abundances. We explored two scenarios, specifically iron-silicate planets at 0.1 AU and silicate-iron-water planets at 4 AU.
Results.
We show that there is a persistent statistical difference in the expected mass-radius relations of solid planets among the different galactic stellar populations. At 0.1 AU for silicate-iron planets, there is a 1.51–2.04% mean planetary radius difference between the thick and thin disc stellar populations, whilst for silicate-iron-water planets past the ice line at 4 AU, we calculate a 2.93–3.26% difference depending on the models. Between the halo and thick disc, we retrieve at 0.1 AU a 0.53–0.69% mean planetary radius difference, and at 4 AU we find a 1.24–1.49% difference depending on the model.
Conclusions.
Future telescopes (such as PLATO) will be able to precisely characterize solid exoplanets and demonstrate the possible existence of planetary mass-radius relationship variability between galactic stellar populations.
Aims. We extend the models presented by Mordasini and collaborators 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. Methods. The population synthesis calculations that we use are based on the Bernese planet formation model developed by Alibert and collaborators, 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. Results. 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 high mass planets orbiting 0.5 M-circle dot stars can be directly explained as resulting from a only slightly steeper than linear scaling. The observed lack of short period planets orbiting 2.0 M-circle dot stars can also be attributed to this scaling but only if associated with a decrease in the mean disk lifetime for stars more massive than 1.5 M-circle dot. Finally, we show that the distribution of minimum mass and semi-major axis of our synthetic planets are statistically comparable with observations.
The opacity due to grains in the envelope of a protoplanet Kappa subgr regulates the accretion rate of gas during formation, meaning that the final bulk composition of planets with a primordial H/He ...envelope is a function of that opacity. Observationally, for extrasolar planets with known mass and radius it is possible to estimate the bulk composition via internal structure models. We want to study the global effects of Kappa subgr as a poorly known, but important quantity on synthetic planetary populations. We find observational hints that the opacity in protoplanetary atmospheres is much smaller than in the ISM even if the specific value of Kappa subgr cannot be constrained in this first study as Kappa subgr is found by scaling the ISM opacity. Our results for the enrichment of giant planets are also important to distinguish core accretion and gravitational instability.
We present a new model of giant planet formation that extends the core-accretion model of Pollack et al. (1996, Icarus, 124, 62) to include migration, disc evolution and gap formation. We show that ...taking these effects into account can lead to much more rapid formation of giant planets, making it compatible with the typical disc lifetimes inferred from observations of young circumstellar discs. This speed up is due to the fact that migration prevents the severe depletion of the feeding zone as observed in in situ calculations. Hence, the growing planet is never isolated and it can reach cross-over mass on a much shorter timescale. To illustrate the range of planets that can form in our model, we describe a set of simulations in which we have varied some of the initial parameters and compare the final masses and semi-major axes with those inferred from observed extra-solar planets.
Context . Out of the more than 5000 detected exoplanets, a considerable number belong to a category called “mini-Neptunes”. Interior models of these planets suggest that they have primordial ...H–He-dominated atmospheres. As this type of planet is not found in the Solar System, understanding their formation is a key challenge in planet formation theory. Unfortunately, quantifying how much H–He planets have, based on their observed mass and radius, is impossible due to the degeneracy of interior models. Aims . Another approach to estimating the range of possible primordial envelope masses is to use formation theory. As different assumptions in planet formation can heavily influence the nebular gas accretion rate of small planets, it is unclear how large the envelope of a protoplanet should be. We explore the effects that different assumptions regarding planet formation have on the nebular gas accretion rate, particularly by exploring the way in which solid material interacts with the envelope. This allows us to estimate the range of possible post-formation primordial envelopes. Thereby, we demonstrate the impact of envelope enrichment on the initial primordial envelope, which can be used in evolution models. Methods . We applied formation models that include different solid accretion rate prescriptions. Our assumption is that mini-Neptunes form beyond the ice line and migrate inward after formation; thus, we formed planets in situ at 3 and 5 au. We considered that the envelope can be enriched by the accreted solids in the form of water. We studied how different assumptions and parameters influence the ratio between the planet’s total mass and the fraction of primordial gas. Results . The primordial envelope fractions for low- and intermediate-mass planets (total mass below 15 M ⊕ ) can range from 0.1% to 50%. Envelope enrichment can lead to higher primordial mass fractions. We find that the solid accretion rate timescale has the largest influence on the primordial envelope size. Conclusions . Rates of primordial gas accretion onto small planets can span many orders of magnitude. Planet formation models need to use a self-consistent gas accretion prescription.
Context.
Planet formation is sensitive to the conditions in protoplanetary disks, for which scaling laws as a function of stellar mass are known.
Aims.
We aim to test whether the observed population ...of planets around low-mass stars can be explained by these trends, or if separate formation channels are needed.
Methods.
We address this question by confronting a state-of-the-art planet population synthesis model with a sample of planets around M dwarfs observed by the HARPS and CARMENES radial velocity (RV) surveys. To account for detection biases, we performed injection and retrieval experiments on the actual RV data to produce synthetic observations of planets that we simulated following the core accretion paradigm.
Results.
These simulations robustly yield the previously reported high occurrence of rocky planets around M dwarfs and generally agree with their planetary mass function. In contrast, our simulations cannot reproduce a population of giant planets around stars less massive than 0.5 solar masses. This potentially indicates an alternative formation channel for giant planets around the least massive stars that cannot be explained with current core accretion theories. We further find a stellar mass dependency in the detection rate of short-period planets. A lack of close-in planets around the earlier-type stars (
M
*
> 0.4
M
⊙
) in our sample remains unexplained by our model and indicates dissimilar planet migration barriers in disks of different spectral subtypes.
Conclusions.
Both discrepancies can be attributed to gaps in our understanding of planet migration in nascent M dwarf systems. They underline the different conditions around young stars of different spectral subtypes, and the importance of taking these differences into account when studying planet formation.
Aims. We explore the possibility that the stellar relative abundances of different species can be used to constrain the bulk abundances of known transiting rocky planets. Methods. We use high ...resolution spectra to derive stellar parameters and chemical abundances for Fe, Si, Mg, O, and C in three stars hosting low mass, rocky planets: CoRoT-7, Kepler-10, and Kepler-93. These planets follow the same line along the mass-radius diagram, pointing toward a similar composition. The derived abundance ratios are compared with the solar values. With a simple stoichiometric model, we estimate the iron mass fraction in each planet, assuming stellar composition. Results. We show that in all cases, the iron mass fraction inferred from the mass-radius relationship seems to be in good agreement with the iron abundance derived from the host star’s photospheric composition. Conclusions. The results suggest that stellar abundances can be used to add constraints on the composition of orbiting rocky planets.
PDF
