Planetary formation and water delivery in the habitable zone around solar-type stars in different dynamical environments

Planetary formation and water delivery in the habitable zone around solar-type stars in different dynamical environments 

Authors:

Zain et al

Abstract:

Aims.

We study the formation and water delivery of planets in the habitable zone (HZ) around solar-type stars. In particular, we study different dynamical environments that are defined by the most massive body in the system.

Methods.

First of all, a semi-analytical model was used to define the mass of the protoplanetary disks that produce each of the five dynamical scenarios of our research. Then, we made use of the same semi-analytical model to describe the evolution of embryos and planetesimals during the gaseous phase. Finally, we carried out N-body simulations of planetary accretion in order to analyze the formation and water delivery of planets in the HZ in the different dynamical environments.

Results.

Water worlds are efficiently formed in the HZ in different dynamical scenarios. In systems with a giant planet analog to Jupiter or Saturn around the snow line, super-Earths tend to migrate into the HZ from outside the snow line as a result of interactions with other embryos and accrete water only during the gaseous phase. In systems without giant planets, Earths and super-Earths with high water by mass contents can either be formed in situ in the HZ or migrate into it from outer regions, and water can be accreted during the gaseous phase and in collisions with water-rich embryos and planetesimals.

Conclusions.

The formation of planets in the HZ with very high water by mass contents seems to be a common process around Sun- like stars. Our research suggests that such planets are still very efficiently produced in different dynamical environments. Moreover, our study indicates that the formation of planets in the HZ with masses and water contents similar to those of Earth seems to be a rare process around solar-type stars in the systems under consideration.

Pebble Accretion in Turbulent Protoplanetary Disks

Pebble Accretion in Turbulent Protoplanetary Disks 

Authors: 

Xu et al 

Abstract: 

It has been realized in recent years that the accretion of pebble-sized dust particles onto planetary cores is an important mode of core growth, which enables the formation of giant planets at large distances and assists planet formation in general. The pebble accretion theory is built upon the orbit theory of dust particles in a laminar protoplanetary disk (PPD). For sufficiently large core mass (in the "Hill regime"), essentially all particles of appropriate sizes entering the Hill sphere can be captured. However, the outer regions of PPDs are expected to be weakly turbulent due to the magnetorotational instability (MRI), where turbulent stirring of particle orbits may affect the efficiency of pebble accretion. We conduct shearing-box simulations of pebble accretion with different levels of MRI turbulence (strongly turbulent assuming ideal magnetohydrodynamics, weakly turbulent in the presence of ambipolar diffusion, and laminar) and different core masses to test the efficiency of pebble accretion at a microphysical level. We find that accretion remains efficient for marginally coupled particles (dimensionless stopping time ${\tau }_{s}\sim 0.1\mbox{--}1$) even in the presence of strong MRI turbulence. Though more dust particles are brought toward the core by the turbulence, this effect is largely canceled by a reduction in accretion probability. As a result, the overall effect of turbulence on the accretion rate is mainly reflected in the changes in the thickness of the dust layer. On the other hand, we find that the efficiency of pebble accretion for strongly coupled particles (down to ${\tau }_{s}\sim 0.01$) can be modestly reduced by strong turbulence for low-mass cores.

Do planets remember how they formed?

Do planets remember how they formed?

Author:

Kipping

Abstract:

One of the most directly observable features of a transiting multi-planet system is their size-ordering when ranked in orbital separation. Kepler has revealed a rich diversity of outcomes, from perfectly ordered systems, like Kepler-80, to ostensibly disordered systems, like Kepler-20. Under the hypothesis that systems are born via preferred formation pathways, one might reasonably expect non-random size-orderings reflecting these processes. However, subsequent dynamical evolution, often chaotic and turbulent in nature, may erode this information and so here we ask - do systems remember how they formed? To address this, we devise a model to define the entropy of a planetary system's size-ordering, by first comparing differences between neighboring planets and then extending to accommodate differences across the chain. We derive closed-form solutions for many of the micro state occupancies and provide public code with look-up tables to compute entropy for up to ten-planet systems. All three proposed entropy definitions exhibit the expected property that their credible interval increases with respect to a proxy for time. We find that the observed Kepler multis display a highly significant deficit in entropy compared to a randomly generated population. Incorporating a filter for systems deemed likely to be dynamically packed, we show that this result is robust against the possibility of missing planets too. Put together, our work establishes that Kepler systems do indeed remember something of their younger years and highlights the value of information theory for exoplanetary science.

Observability of Forming Planets and their Circumplanetary Disks I. – Parameter Study for ALMA

Observability of Forming Planets and their Circumplanetary Disks I. – Parameter Study for ALMA
Authors:

Szulágyi et al

Abstract:
We present mock observations of forming planets with ALMA. The possible detections of circumplanetary disks (CPDs) were investigated around planets of Saturn, 1, 3, 5, and 10 Jupiter-masses that are placed at 5.2 AU from their star. The radiative, three dimensional hydrodynamic simulations were then post-processed with RADMC3D and the ALMA Observation Simulator. We found that even though the CPDs are too small to be resolved, they are hot due to the accreting planet in the optically thick limit, therefore the best chance to detect them with continuum observations in this case is at the shortest ALMA wavelengths, such as Band 9 (440 microns). Similar fluxes were found in the case of Saturn and Jupiter-mass planets, as for the 10 MJup gas-giant, due to temperature weighted optical depth effects: when no deep gap is carved, the planet region is blanketed by the optically thick circumstellar disk leading to a less efficient cooling there. A test was made for a 52 AU orbital separation, showed that optically thin CPDs are also detectable in band 7 but they need longer integration times (>5hrs). Comparing the gap profiles of the same simulation at various ALMA bands and the hydro simulation confirmed that they change significantly, first because the gap is wider at longer wavelengths due to decreasing optical depth; second, the beam convolution makes the gap shallower and at least 25% narrower. Therefore, caution has to be made when estimating planet masses based on ALMA continuum observations of gaps.

Transfer, loss and physical processing of water in hit-and-run collisions of planetary embryos

Transfer, loss and physical processing of water in hit-and-run collisions of planetary embryos

Authors:

Burger et al

Abstract:

Collisions between large, similar-sized bodies are believed to shape the final characteristics and composition of terrestrial planets. Their inventories of volatiles such as water, are either delivered or at least significantly modified by such events. Besides the transition from accretion to erosion with increasing impact velocity, similar-sized collisions can also result in hit-and-run outcomes for sufficiently oblique impact angles and large enough projectile-to-target mass ratios. We study volatile transfer and loss focusing on hit-and-run encounters by means of Smooth Particle Hydrodynamics simulations, including all main parameters: impact velocity, impact angle, mass ratio, and also the total colliding mass. We find a broad range of overall water losses, up to 75% in the most energetic hit-and-run events, and confirm the much more severe consequences for the smaller body also for stripping of volatile layers. Transfer of water between projectile and target inventories is found to be mostly rather inefficient, and final water contents are dominated by pre-collision inventories reduced by impact losses, for similar pre-collision water mass fractions. Comparison with our numerical results shows that current collision outcome models are not accurate enough to reliably predict these composition changes in hit-and-run events. To also account for non-mechanical losses we estimate the amount of collisionally vaporized water over a broad range of masses, and find that these contributions are particularly important in collisions of ~Mars-sized bodies, with sufficiently high impact energies, but still relatively low gravity. Our results clearly indicate that the cumulative effect of several (hit-and-run) collisions can efficiently strip protoplanets of their volatile layers, especially the smaller body, as it might be common e.g. for Earth-mass planets in systems with Super-Earths.

Optically Thin Core Accretion: How Planets Get Their Gas in Nearly Gas-Free Disks

Optically Thin Core Accretion: How Planets Get Their Gas in Nearly Gas-Free Disks 

Authors:

Lee et al

Abstract:

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

Inside-Out Planet Formation. IV. Pebble Evolution and Planet Formation Timescales

Inside-Out Planet Formation. IV. Pebble Evolution and Planet Formation Timescales

Authors:

Hu et al

Abstract:
Systems with tightly-packed inner planets (STIPs) are very common. Chatterjee & Tan proposed Inside-Out Planet Formation (IOPF), an in situ formation theory, to explain these planets. IOPF involves sequential planet formation from pebble-rich rings that are fed from the outer disk and trapped at the pressure maximum associated with the dead zone inner boundary (DZIB). Planet masses are set by their ability to open a gap and cause the DZIB to retreat outwards. We present models for the disk density and temperature structures that are relevant to the conditions of IOPF. For a wide range of DZIB conditions, we evaluate the gap opening masses of planets in these disks that are expected to lead to truncation of pebble accretion onto the forming planet. We then consider the evolution of dust and pebbles in the disk, estimating that pebbles typically grow to sizes of a few cm during their radial drift from several tens of AU to the inner, ≲1AU-scale disk. A large fraction of the accretion flux of solids is expected to be in such pebbles. This allows us to estimate the timescales for individual planet formation and entire planetary system formation in the IOPF scenario. We find that to produce realistic STIPs within reasonable timescales similar to disk lifetimes requires disk accretion rates of ∼10−9M⊙yr−1 and relatively low viscosity conditions in the DZIB region, i.e., Shakura-Sunyaev parameter of α∼10−4.

Is There a Temperature Limit in Planet Formation at 1000 K?

Is There a Temperature Limit in Planet Formation at 1000 K?

Authors:

Demirci et al

Abstract:
Dust drifting inward in protoplanetary disks is subject to increasing temperatures. In laboratory experiments, we tempered basaltic dust between 873 K and 1273 K and find that the dust grains change in size and composition. These modifications influence the outcome of self-consistent low speed aggregation experiments showing a transition temperature of 1000\,K. Dust tempered at lower temperatures grows to a maximum aggregate size of 2.02±0.06 mm, which is 1.49±0.08 times the value for dust tempered at higher temperatures. A similar size ratio of 1.75±0.16 results for a different set of collision velocities. This transition temperature is in agreement with orbit temperatures deduced for observed extrasolar planets. Most terrestrial planets are observed at positions equivalent to less than 1000 K. Dust aggregation on the millimeter-scale at elevated temperatures might therefore be a key factor for terrestrial planet formation.

Possible formation pathways for the low density Neptune-mass planet HAT-P-26b

Possible formation pathways for the low density Neptune-mass planet HAT-P-26b  

Authors:

Ali-Dib et al

Abstract:

We investigate possible pathways for the formation of the low density Neptune-mass planet HAT-P-26b. We use two formation different models based on pebbles and planetesimals accretion, and includes gas accretion, disk migration and simple photoevaporation. The models tracks the atmospheric oxygen abundance, in addition to the orbital period, and mass of the forming planets, that we compare to HAT-P-26b. We find that pebbles accretion can explain this planet more naturally than planetesimals accretion that fails completely unless we artificially enhance the disk metallicity significantly. Pebble accretion models can reproduce HAT-P-26b with either a high initial core mass and low amount of envelope enrichment through core erosion or pebbles dissolution, or the opposite, with both scenarios being possible. Assuming a low envelope enrichment factor as expected from convection theory and comparable to the values we can infer from the D/H measurements in Uranus and Neptune, our most probable formation pathway for HAT-P-26b is through pebble accretion starting around 10 AU early in the disk’s lifetime.

How Mini Neptunes Form

The formation of mini-Neptunes

Authors:

Venturini et al

Abstract:

Mini-Neptunes seem to be common planets. In this work we investigate the possible formation histories and predicted occurrence rates of mini-Neptunes assuming the planets form beyond the iceline. We consider pebble and planetesimal accretion accounting for envelope enrichment and two different opacity conditions. We find that the formation of mini-Neptunes is a relatively frequent output when envelope enrichment by volatiles is included, and that there is a "sweet spot" for mini-Neptune formation with a relatively low solid accretion rate of ~10^{-6} Earth masses per year. This rate is typical for low/intermediate-mass protoplanetary disks and/or disks with low metallicities. With pebble accretion, envelope enrichment and high opacity favor the formation of mini-Neptunes, with more efficient formation at large semi-major axes (~30 AU) and low disk viscosity. For planetesimal accretion, such planets can form also without enrichment, with the opacity being a key aspect in the growth history and favorable formation location. Finally, we show that the formation of Neptune-like planets remains a challenge for planet formation theories.

Chemical enrichment of the planet forming region as probed by accretion

Chemical enrichment of the planet forming region as probed by accretion

Authors:

Booth et al

Abstract:

The chemical conditions in the planet forming regions of protoplanetary discs remain difficult to observe directly. Gas accreting from the disc on to the star provides a way to measure the elemental abundances because even refractory species are in an atomic gaseous form. Here we compare the abundance ratios derived from UV lines probing T Tauri accretion streams to simple models of disc evolution. Although the interpretation of line ratios in terms of abundances is highly uncertain, discs with large cavities in mm images tend to have lower Si emission. Since this can naturally be explained by the suppressed accretion of dust, this suggests that abundance variations are at least partially responsible for the variations seen in the line ratios. Our models of disc evolution due to grain growth, radial drift and the flux of volatile species carried as ices on grain surfaces, give rise to a partial sorting of the atomic species based on the volatility of their dominant molecular carriers. This arises because volatiles are left behind at their snow lines while the grains continue to drift. We show that this reproduces the main features seen in the accretion line ratio data, such as C/N ratios that are a few times solar and the correlation between the Si to volatile ratio with mm-flux. We highlight the fact that developing a more robust linkage between line ratios and abundance ratios and acquiring data for larger samples has the potential to cast considerable light on the chemical history of protoplanetary discs.

Planet formation and disk-planet interactions

Planet formation and disk-planet interactions

Authors:

Kley

Abstract:

This review is based on lectures given at the 45th Saas-Fee Advanced Course 'From Protoplanetary Disks to Planet Formation' held in March 2015 in Les Diablerets, Switzerland. Starting with an overview of the main characterictics of the Solar System and extrasolar planets, we describe the planet formation process in terms of the sequential accretion scenario. First the growth processes of dust particles to planetesimals and subsequently to terrestrial planets or planetary cores are presented. This is followed by the formation process of the giant planets either by core accretion or gravitational instability. Finally, the dynamical evolution of the orbital elements as driven by disk-planet interaction and the overall evolution of multi-object systems is presented.

Global Simulations of the Inner Regions of Protoplanetary Disks with Comprehensive Disk Microphysics

Global Simulations of the Inner Regions of Protoplanetary Disks with Comprehensive Disk Microphysics

Author: 

Bai

Abstract:
The gas dynamics of weakly ionized protoplanetary disks (PPDs) is largely governed by the coupling between gas and magnetic fields, described by three non-ideal magnetohydrodynamical (MHD) effects (Ohmic, Hall, ambipolar). Previous local simulations incorporating these processes have revealed that the inner regions of PPDs are largely laminar accompanied by wind-driven accretion. We conduct 2D axisymmetric, fully global MHD simulations of these regions (∼1−20 AU), taking into account all non-ideal MHD effects, with tabulated diffusion coefficients and approximate treatment of external ionization and heating. With net vertical field aligned with disk rotation, the Hall-shear instability strongly amplifies horizontal magnetic field, making the overall dynamics dependent on initial field configuration. Following disk formation, the disk likely relaxes into an inner zone characterized by asymmetric field configuration across the midplane that smoothly transitions to a more symmetric outer zone. Angular momentum transport is driven by both MHD winds and laminar Maxwell stress, with both accretion and decretion flows present at different heights, and modestly asymmetric winds from the two disk sides. With anti-aligned field polarity, weakly magnetized disks settle into an asymmetric field configuration with supersonic accretion flow concentrated at one side of disk surface, and highly asymmetric winds between the two disk sides. In all cases, the wind is magneto-thermal in nature characterized by mass loss rate exceeding the accretion rate. More strongly magnetized disks give more symmetric field configuration and flow structures. Deeper far-UV penetration leads to stronger and less stable outflows. Implications for observations and planet formation are also discussed.

ALMA Observes Exoplanet Formation Around V1247 Orionis

The quest to discover how planets found in the far reaches of the universe are born has taken a new, crucial twist.

A new study by an international team of scientists, led by Stefan Kraus from the University of Exeter, has given a fascinating new insight into one of the most respected theories of how planets are formed.

Young stars start out with a massive disk of gas and dust that over time, astronomers think, either diffuses away or coalesces into planets and asteroids.

However, scientists are still searching for a complete understanding of how these early formations come together to form asteroid-sized objects. One reason has been that drag in the disk produced by surrounding gas makes the grains move inward toward the star - which can in turn deplete the disk rapidly in a process known as "radial drift."

In the new research, the team use high powered telescopes to target the star V1247 Orionis -, a young, hot star surrounded by a dynamic ring of gas and dust.

The team produced a detailed image of the star and its surrounding dust disc, shown in two parts: a clearly defined central ring of matter and a more delicate crescent structure located further out.

The region between the ring and crescent, visible as a dark strip, is thought to be caused by a young planet carving its way through the disc. As the planet moves around in its orbit, its motion creates areas of high pressure on either side of its path, similar to how a ship creates bow waves as it cuts through water.

These areas of high pressure could become protective barriers around sites of planet formation; dust particles are trapped within them for millions of years, allowing them the time and space to clump together and grow.

link.

Generation of inclined protoplanetary discs and misaligned planets through mass accretion I: Coplanar secondary discs

Generation of inclined protoplanetary discs and misaligned planets through mass accretion I: Coplanar secondary discs

Authors:

Xiang-Gruess et al

Abstract:

We study the three-dimensional evolution of a viscous protoplanetary disc which accretes gas material from a second protoplanetary disc during a close encounter in an embedded star cluster. The aim is to investigate the capability of the mass accretion scenario to generate strongly inclined gaseous discs which could later form misaligned planets. We use smoothed particle hydrodynamics to study mass transfer and disc inclination for passing stars and circumstellar discs with different masses. We explore different orbital configurations to find the parameter space which allows significant disc inclination generation. Thies et al. (2011) suggested that significant disc inclination and disc or planetary system shrinkage can generally be produced by the accretion of external gas material with a different angular momentum. We found that this condition can be fullfilled for a large range of gas mass and angular momentum. For all encounters, mass accretion from the secondary disc increases with decreasing mass of the secondary proto-star. Thus, higher disc inclinations can be attained for lower secondary stellar masses. Variations of the secondary disc’s orientation relative to the orbital plane can alter the disc evolution significantly. The results taken together show that mass accretion can change the three-dimensional disc orientation significantly resulting in strongly inclined discs. In combination with the gravitational interaction between the two star-disc systems, this scenario is relevant for explaining the formation of highly inclined discs which could later form misaligned planets.

Hot Start Giant Planets Form With Radiative Interiors

Hot Start Giant Planets Form With Radiative Interiors

Authors:

Berardo et al

Abstract:
In the hot-start core accretion formation model for gas giants, the interior of a planet is usually assumed to be fully convective. By calculating the detailed internal evolution of a planet assuming hot start outer boundary conditions, we show that such a planet will in fact form with a radially increasing internal entropy profile, so that its interior will be radiative instead of convective. For a hot outer boundary, there is a minimum value for the entropy of the internal adiabat Smin below which the accreting envelope does not match smoothly onto the interior, but instead deposits high entropy material onto the growing interior. One implication of this would be to at least temporarily halt the mixing of heavy elements within the planet, which are deposited by planetesimals accreted during formation. The compositional gradient this would impose could subsequently disrupt convection during post-accretion cooling, which would alter the observed cooling curve of the planet. However even with a homogeneous composition, for which convection develops as the planet cools, the difference in cooling timescale will change the inferred mass of directly-imaged gas giants.

NanoRocks: Design and Performance of an Experiment Studying Planet Formation on the International Space Station

NanoRocks: Design and Performance of an Experiment Studying Planet Formation on the International Space Station

Authors:

Brissett et al

Abstract:

In an effort to better understand the early stages of planet formation, we have developed a 1.5U payload that flew on the International Space Station (ISS) in the NanoRacks NanoLab facility between September 2014 and March 2016. This payload, named NanoRocks, ran a particle collision experiment under long-term microgravity conditions. The objectives of the experiment were (a) to observe collisions between mm-sized particles at relative velocities of less than 1~cm/s, and (b) to study the formation and disruption of particle clusters for different particle types and collision velocities. Four types of particles were used: mm-sized acrylic, glass, and copper beads, and 0.75 mm-sized JSC-1 lunar regolith simulant grains. The particles were placed in sample cells carved out of an aluminum tray. This tray was attached to one side of the payload casing with three springs. Every 60~s, the tray was agitated and the resulting collisions between the particles in the sample cells were recorded by the experiment camera.

During the 18 months the payload stayed on ISS, we obtained 158 videos, thus recording a great number of collisions. The average particle velocities in the sample cells after each shaking event were around 1 cm/s. After shaking stopped, the inter-particle collisions damped the particle kinetic energy in less than 20~s, reducing the average particle velocity to below 1 mm/s, and eventually slowing them to below our detection threshold. As the particle velocity decreased, we observed the transition from bouncing to sticking collisions. We recorded the formation of particle clusters at the end of each experiment run. This paper describes the design and performance of the NanoRocks ISS payload.

TW Hydrae is Forming a SubSaturn Gas Giant

Deep imaging search for planets forming in the TW Hya protoplanetary disk with the Keck/NIRC2 vortex coronagraph


Authors:

Ruane et al

Abstract:
Distinct gap features in the nearest protoplanetary disk, TW Hya (distance of 59.5±0.9 pc), may be signposts of ongoing planet formation. We performed long-exposure thermal infrared coronagraphic imaging observations to search for accreting planets especially within dust gaps previously detected in scattered light and submm-wave thermal emission. Three nights of observations with the Keck/NIRC2 vortex coronagraph in L′ (3.4-4.1μm) did not reveal any statistically significant point sources. We thereby set strict upper limits on the masses of non-accreting planets. In the four most prominent disk gaps at 24, 41, 47, and 88 au, we obtain upper mass limits of 1.6-2.3, 1.1-1.6, 1.1-1.5, and 1.0-1.2 Jupiter masses (MJ) assuming an age range of 7-10 Myr for TW Hya. These limits correspond to the contrast at 95\% completeness (true positive fraction of 0.95) with a 1\% chance of a false positive within 1′′ of the star. We also approximate an upper limit on the product of planet mass and planetary accretion rate of MpM˙≲10−8M2J/yr implying that any putative ∼0.1MJ planet, which could be responsible for opening the 24 au gap, is presently accreting at rates insufficient to build up a Jupiter mass within TW Hya's pre-main sequence lifetime.

Magnetospheric Truncation, Tidal Inspiral, and the Creation of Short-period and Ultra-short-period Planets

Magnetospheric Truncation, Tidal Inspiral, and the Creation of Short-period and Ultra-short-period Planets


Authors:

Lee et al

Abstract:
Sub-Neptunes around FGKM dwarfs are evenly distributed in log orbital period down to ~10 days, but dwindle in number at shorter periods. Both the break at ~10 days and the slope of the occurrence rate down to ~1 day can be attributed to the truncation of protoplanetary disks by their host star magnetospheres at corotation. We demonstrate this by deriving planet occurrence rate profiles from empirical distributions of pre-main-sequence stellar rotation periods. Observed profiles are better reproduced when planets are distributed randomly in disks—as might be expected if planets formed in situ—rather than piled up near disk edges, as would be the case if they migrated in by disk torques. Planets can be brought from disk edges to ultra-short (less than 1 day) periods by asynchronous equilibrium tides raised on their stars. Tidal migration can account for how ultra-short-period planets are more widely spaced than their longer-period counterparts. Our picture provides a starting point for understanding why the sub-Neptune population drops at ~10 days regardless of whether the host star is of type FGK or early M. We predict planet occurrence rates around A stars to also break at short periods, but at ~1 day instead of ~10 days because A stars rotate faster than stars with lower masses (this prediction presumes that the planetesimal building blocks of planets can drift inside the dust sublimation radius).

Eccentricity excitation and merging of planetary embryos heated by pebble accretion

Eccentricity excitation and merging of planetary embryos heated by pebble accretion

Authors:

Chrenko et al

Abstract:
Context:

Planetary embryos can continue to grow by pebble accretion until they become giant planet cores. Simultaneously, these embryos mutually interact and also migrate due to torques arising from the protoplanetary disk.

Aims:

Our aim is to investigate how pebble accretion alters the orbital evolution of embryos undergoing the Type-I migration. In particular, we study whether they establish resonant chains, whether these chains are prone to instabilities and if giant planet cores form through embryo merging, thus occurring more rapidly than by pebble accretion alone.

Methods:

For the first time, we perform self-consistent global-scale radiative hydrodynamic simulations of a two-fluid protoplanetary disk consisting of gas and pebbles, the latter being accreted by embedded embryos. Accretion heating, along with other radiative processes, is accounted for to correctly model the Type-I migration.

Results:

We track the evolution of four super-Earth-like embryos, initially located in a region where the disk structure allows for a convergent migration. Generally, embryo merging is facilitated by rapidly increasing embryo masses and breaks the otherwise oligarchic growth. Moreover, we find that the orbital eccentricity of each embryo is considerably excited (≃0.03) due to the presence of an asymmetric underdense lobe of gas, a so-called `hot trail', produced by accretion heating of the embryo's vicinity. Eccentric orbits lead the embryos to frequent close encounters and make resonant locking more difficult.

Conclusions:

Embryo merging typically produces one massive core (≳10ME) in our simulations, orbiting near 10AU. Pebble accretion is naturally accompanied by occurrence of eccentric orbits which should be considered in future efforts to explain the structure of exoplanetary systems.