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Register a new userTerrestrial Planet Formation in a protoplanetary disk with a local mass depletion: A successful scenario for the formation of Mars
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Models of terrestrial planet formation for our solar system have been successful in producing planets with masses and orbits similar to those of Venus and Earth. However, these models have generally failed to produce Mars-sized objects around 1.5 AU. The body that is usually formed around Mars semimajor axis is, in general, much more massive than Mars. Only when Jupiter and Saturn are assumed to have initially very eccentric orbits (e $sim$ 0.1), which seems fairly unlikely for the solar system, or alternately, if the protoplanetary disk is truncated at 1.0 AU, simulations have been able to produce Mars-like bodies in the correct location. In this paper, we examine an alternative scenario for the formation of Mars in which a local depletion in the density of the protosolar nebula results in a non-uniform formation of planetary embryos and ultimately the formation of Mars-sized planets around 1.5 AU. We have carried out extensive numerical simulations of the formation of terrestrial planets in such a disk for different scales of the local density depletion, and for different orbital configurations of the giant planets. Our simulations point to the possibility of the formation of Mars-sized bodies around 1.5 AU, specifically when the scale of the disk local mass-depletion is moderately high (50-75%) and Jupiter and Saturn are initially in their current orbits. In these systems, Mars-analogs are formed from the protoplanetary materials that originate in the regions of disk interior or exterior to the local mass-depletion. Results also indicate that Earth-sized planets can form around 1 AU with a substantial amount of water accreted via primitive water-rich planetesimals and planetary embryos. We present the results of our study and discuss their implications for the formation of terrestrial planets in our solar system.
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Measured disk masses seem to be too low to form the observed population of planetary systems. In this context, we develop a population synthesis code in the pebble accretion scenario, to analyse the disk mass dependence on planet formation around low mass stars. We base our model on the analytical sequential model presented in Ormel et al. 2017 and analyse the populations resulting from varying initial disk mass distributions. Starting out with seeds the mass of Ceres near the ice-line formed by streaming instability, we grow the planets using the Pebble Accretion process and migrate them inwards using Type-I migration. The next planets are formed sequentially after the previous planet crosses the ice-line. We explore different initial distributions of disk masses to show the dependence of this parameter with the final planetary population. Our results show that compact close-in resonant systems can be pretty common around M-dwarfs between $0.09-0.2$ $M_{odot}$ only when the disks considered are more massive than what is being observed by sub-mm disk surveys. The minimum disk mass to form a Mars-like planet is found to be about $2 times 10^{-3}$ $M_{odot}$. Small variation in the disk mass distribution also manifest in the simulated planet distribution. The paradox of disk masses might be caused by an underestimation of the disk masses in observations, by a rapid depletion of mass in disks by planets growing within a million years or by deficiencies in our current planet formation picture.
Recent observations of protoplanetary disks have revealed ring-like structures that can be associated to pressure maxima. Pressure maxima are known to be dust collectors and planet migration traps. Most of planet formation works are based either on the pebble accretion model or on the planetesimal accretion model. However, recent studies proposed the possible formation of Jupiter by the hybrid accretion of pebbles and planetesimals. We aim to study the full process of planet formation consisting of dust evolution, planetesimal formation and planet growth at a pressure maximum in a protoplanetary disk. We compute, through numerical simulations, the gas and dust evolution, including dust growth, fragmentation, radial drift and particle accumulation at a pressure bump. We also consider the formation of planetesimals by streaming instability and the formation of a moon-size embryo that grows into a giant planet by the hybrid accretion of pebbles and planetesimals. We find that pressure maxima in protoplanetary disks are efficient collectors of dust drifting inwards. The condition of planetesimal formation by streaming instability is fulfilled due to the large amount of dust accumulated at the pressure bump. Then, a massive core is quickly formed (in $sim 10^4$ yr) by the accretion of pebbles. After the pebble isolation mass is reached, the growth of the core slowly continues by the accretion of planetesimals. The energy released by planetesimal accretion delays the onset of runaway gas accretion, allowing a gas giant to form after $sim$1 Myr of disk evolution. The pressure maximum also acts as a migration trap. Pressure maxima in protoplanetary disks are preferential locations for dust traps, planetesimal formation by streaming instability and planet migration traps. All these conditions allow the fast formation of a giant planet by the hybrid accretion of pebbles and planetesimals.
Rings and radial gaps are ubiquitous in protoplanetary disks, yet their possible connection to planet formation is currently subject to intense debates. In principle, giant planet formation leads to wide gaps which separate the gas and dust mass reservoir in the outer disk, while lower mass planets lead to shallow gaps which are manifested mainly on the dust component. We used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the star HD169142, host to a prominent disk with deep wide gaps that sever the disk into inner and outer regions. The new ALMA high resolution images allow for the outer ring to be resolved as three narrow rings. The HD169142 disk thus hosts both the wide gaps trait of transition disks and a narrow ring system similar to those observed in the TW Hya and HL Tau systems. The mass reservoir beyond a deep gap can thus host ring systems. The observed rings are narrow in radial extent (width/radius of 1.5/57.3, 1.8/64.2 and 3.4/76.0, in au) and have asymmetric mutual separations: the first and middle ring are separated by 7 au while the middle and outermost ring are distanced by ~12 au. Using hydrodynamical modeling we found that a simple explanation, involving a single migrating low mass planet (10 M$_oplus$), entirely accounts for such an apparently complex phenomenon. Inward migration of the planet naturally explains the rings asymmetric mutual separation. The isolation of HD169142s outer rings thus allows a proof of concept to interpret the detailed architecture of the outer region of protoplanetary disks with low mass planet formation of mini-Neptunes size, i.e. as in the protosolar nebula.
While it is generally accepted that the magnetic field and its non-ideal effects play important roles during the stellar formation, simple models of pure hydrodynamics and angular momentum conservation are still widely employed in the studies of disk assemblage in the framework of the so-called alpha-disk model due to their simplicity. There has only been a few efforts trying to bridge the gap between a collapsing prestellar core and a developed disk. The goal of the present work is to revisit the assemblage of the protoplanetary disk (PPD), by performing 3D MHD simulations with ambipolar diffusion and full radiative transfer. We follow the global evolution of the PPD from the prestellar core collapse for 100 kyr, with resolution of one AU. The formed disk is more realistic and is in agreement with recent observations of disks around class-0 young stellar objects. The mass flux arriving onto the disk and the radial mass accretion rate within the disk are measured and compared to analytical self-similar models. The surface mass flux is very centrally peaked, implying that most of the mass falling onto the star does not transit through the mid-plane of the disk. The disk mid-plane is almost dead to turbulence, whereas upper layers and the disk outer edge are very turbulent. The snow-line is significantly further away than in a passive disk. We developed a zoomed rerun technique to quickly obtain a reasonable disk that is highly stratified, weakly magnetized inside, and strongly magnetized outside. During the class-0 phase of PPD formation, the interaction between the disk and the infalling envelope is important and ought not be neglected. Accretion onto the star is found to mostly depend on dynamics of the collapsing envelope, rather than the detailed disk structure.
The growth and composition of Earth is a direct consequence of planet formation throughout the Solar System. We discuss the known history of the Solar System, the proposed stages of growth and how the early stages of planet formation may be dominated by pebble growth processes. Pebbles are small bodies whose strong interactions with the nebula gas lead to remarkable new accretion mechanisms for the formation of planetesimals and the growth of planetary embryos. Many of the popular models for the later stages of planet formation are presented. The classical models with the giant planets on fixed orbits are not consistent with the known history of the Solar System, fail to create a high Earth/Mars mass ratio, and, in many cases, are also internally inconsistent. The successful Grand Tack model creates a small Mars, a wet Earth, a realistic asteroid belt and the mass-orbit structure of the terrestrial planets. In the Grand Tack scenario, growth curves for Earth most closely match a Weibull model. The feeding zones, which determine the compositions of Earth and Venus follow a particular pattern determined by Jupiter, while the feeding zones of Mars and Theia, the last giant impactor on Earth, appear to randomly sample the terrestrial disk. The late accreted mass samples the disk nearly evenly.
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