Theory of planet formation (original) (raw)
Related papers
From Dust to Terrestrial Planets — Introduction
Space Sciences Series of ISSI, 2000
Terrestrial planets are accreted in a disk orbiting a central star. The basic challenge of their formation consists of assembling micron-sized or smaller dust grains to bodies with over 104 km in diameter. This formation process, ultimately based on collisions, occurs in three very different physical regimes depending upon the size of the bodies present: 1) Early on, micron- to
Formation of Giant Planets by Fragmentation of Protoplanetary Disks
Science, 2002
The evolution of gravitationally unstable protoplanetary gaseous disks has been studied using three dimensional smoothed particle hydrodynamics (SPH) simulations with unprecedented resolution. We have considered disks with initial masses and temperature profiles consistent with those inferred for the protosolar nebula and for other protoplanetary disks. We show that long-lasting, self-gravitating protoplanets arise after a few disk orbital times if cooling is efficient enough to maintain the temperature close to 50 K. The resulting bodies have masses and orbital eccentricities remarkably similar to those of observed extrasolar planets.
Planet formation models: the interplay with the planetesimal disc
Astronomy & Astrophysics, 2012
Context. According to the sequential accretion model (or core-nucleated accretion model), giant planet formation is based first on the formation of a solid core which, when massive enough, can gravitationally bind gas from the nebula to form the envelope. The most critical part of the model is the formation time of the core: to trigger the accretion of gas, the core has to grow up to several Earth masses before the gas component of the protoplanetary disc dissipates. Aims. We calculate planetary formation models including a detailed description of the dynamics of the planetesimal disc, taking into account both gas drag and excitation of forming planets. Methods. We computed the formation of planets, considering the oligarchic regime for the growth of the solid core. Embryos growing in the disc stir their neighbour planetesimals, exciting their relative velocities, which makes accretion more difficult. Here we introduce a more realistic treatment for the evolution of planetesimals' relative velocities, which directly impact on the formation timescale. For this, we computed the excitation state of planetesimals, as a result of stirring by forming planets, and gas-solid interactions. Results. We find that the formation of giant planets is favoured by the accretion of small planetesimals, as their random velocities are more easily damped by the gas drag of the nebula. Moreover, the capture radius of a protoplanet with a (tiny) envelope is also larger for small planetesimals. However, planets migrate as a result of disc-planet angular momentum exchange, with important consequences for their survival: due to the slow growth of a protoplanet in the oligarchic regime, rapid inward type I migration has important implications on intermediate-mass planets that have not yet started their runaway accretion phase of gas. Most of these planets are lost in the central star. Surviving planets have masses either below 10 M ⊕ or above several Jupiter masses. Conclusions. To form giant planets before the dissipation of the disc, small planetesimals (∼0.1 km) have to be the major contributors of the solid accretion process. However, the combination of oligarchic growth and fast inward migration leads to the absence of intermediate-mass planets. Other processes must therefore be at work to explain the population of extrasolar planets that are presently known.
Collisional Growth of Planetesimals and the Formation of Terrestrial Planets in Binary Star Systems
2010
While recent simulations of the accretion of planetesimals in circumprimary disks in moderately close binary star systems point to the inefficiency of the growth of these objects to larger bodies, the detection of planets around the primaries of binary systems with stellar separation smaller than 20 AU, suggests that planet formation in such binaries may be as efficient as around single stars. We have carried out an expansive numerical study of the collision and interaction of planetesimals, and their growth to planetary embryos and terrestrial planet in such binary systems. By including non-linear gas drag, stemming from an eccentric gas disk with a finite precession rate, we have been able to show that the disk precession decreases the velocity dispersion between different-size planetesimals and facilitates their accretional collisions in particular near the outer parts of the disk. Our results also indicate that terrestrial planet formation is more efficient in binaries with peri...
Rapid planetesimal formation in turbulent circumstellar disks
Nature, 2007
The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies 1 . How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem 2 : boulders stick together poorly 3 , and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas 4 . Gravitational collapse of the solid component has been suggested to overcome this barrier 1, 5, 6 . Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer 2, 7 , but turbulence must be present to explain observed gas accretion in protostellar discs 8 . Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas 9 , and these concentra-1 arXiv:0708.3890v1 [astro-ph]
Formation of planetary populations − II. Effects of initial disc size and radial dust drift
Monthly Notices of the Royal Astronomical Society
Recent ALMA observations indicate that while a range of disc sizes exist, typical disc radii are small, and that radial dust drift affects the distribution of solids in discs. Here, we explore the consequences of these features in planet population synthesis models. A key feature of our model is planet traps – barriers to otherwise rapid type-I migration of forming planets – for which we include the ice line, heat transition, and outer edge of the dead zone. We find that the ice line plays a fundamental role in the formation of warm Jupiters. In particular, the ratio of super Earths to warm Jupiters formed at the ice line depends sensitively on the initial disc radius. Initial gas disc radii of ∼50 au results in the largest super Earth populations, while both larger and smaller disc sizes result in the ice line producing more gas giants near 1 au. This transition between typical planet class formed at the ice line at various disc radii confirms that planet formation is fundamentally...
Dust Resurgence in Protoplanetary Disks Due to Planetesimal–Planet Interactions
The astrophysical journal, 2022
Observational data on the dust content of circumstellar disks show that the median dust content in disks around pre-main sequence stars in nearby star forming regions seem to increase from ∼1 Myr to ∼2 Myr, and then decline with time. This behaviour challenges the models where the small dust grains steadily decline by accumulating into larger bodies and drifting inwards on a short timescale (≤1 Myr). In this Letter we explore the possibility to reconcile this discrepancy in the framework of a model where the early formation of planets dynamically stirs the nearby planetesimals and causes high energy impacts between them, resulting in the production of second-generation dust. We show that the observed dust evolution can be naturally explained by this process within a suite of representative disk-planet architectures.
Hybrid methods in planetesimal dynamics: formation of protoplanetary systems and the mill condition
Monthly Notices of the Royal Astronomical Society, 2014
The formation and evolution of protoplanetary discs remains a challenge from both a theoretical and numerical standpoint. In this work we first perform a series of tests of our new hybrid algorithm presented in Glaschke, Amaro-Seoane and Spurzem 2011 (henceforth Paper I) that combines the advantages of high accuracy of directsummation N −body methods with a statistical description for the planetesimal disc based on Fokker-Planck techniques. We then address the formation of planets, with a focus on the formation of protoplanets out of planetesimals. We find that the evolution of the system is driven by encounters as well as direct collisions and requires a careful modelling of the evolution of the velocity dispersion and the size distribution over a large range of sizes. The simulations show no termination of the protoplanetary accretion due to gap formation, since the distribution of the planetesimals is only subjected to small fluctuations. We also show that these features are weakly correlated with the positions of the protoplanets. The exploration of different impact strengths indicates that fragmentation mainly controls the overall mass loss, which is less pronounced during the early runaway growth. We prove that the fragmentation in combination with the effective removal of collisional fragments by gas drag sets an universal upper limit of the protoplanetary mass as a function of the distance to the host star, which we refer to as the mill condition. (RS) 1 http://exoplanet.eu/catalog-all.php Understanding planet formation comprises many challenges, such as hydrodynamics of the protoplanetary disc, chemical evolution of the embedded dust grains, migration of planets and planetesimals and even star-star interactions in dense young star clusters (see Armitage 2010,for a review and references therein, and also the introduction of Paper I, for a brief summary). All these components constitute the frame for the essential process of planet formation: An enormous growth from dust-sized particles to the final planets, accompanied by a steady decrease of the number of particles which contain most of the mass over many orders of magnitude. The particle number changes over many orders of magnitude as planetary growth proceeds. There is active research on each of the different aspects of planet formation, but the current efforts are far from a unified model of planet formation .
Gravoturbulent Formation of Planetesimals
The Astrophysical Journal, 2006
We explore the effect of magnetorotational turbulence on the dynamics and concentrations of boulders in local box simulations of a sub-Keplerian protoplanetary disc. The solids are treated as particles each with an independent space coordinate and velocity. We find that the turbulence has two effects on the solids. 1) Meter and decameter bodies are strongly concentrated, locally up to a factor 100 times the average dust density, whereas decimeter bodies only experience a moderate density increase. The concentrations are located in large scale radial gas density enhancements that arise from a combination of turbulence and shear. 2) For meter-sized boulders, the concentrations cause the average radial drift speed to be reduced by 40%. We find that the densest clumps of solids are gravitationally unstable under physically reasonable values for the gas column density and for the dust-to-gas ratio due to sedimentation. We speculate that planetesimals can form in a dust layer that is not in itself dense enough to undergo gravitational fragmentation, and that fragmentation happens in turbulent density fluctuations in this sublayer. Subject headings: instabilities -MHD -planetary systems: formation -planetary systems: protoplanetary disks -turbulence 1 The code is available at
Proceedings of the National Academy of Sciences, 2011
Advances in our understanding of terrestrial planet formation have come from a multidisciplinary approach. Studies of the ages and compositions of primitive meteorites with compositions similar to the Sun have helped to constrain the nature of the building blocks of planets. This information helps to guide numerical models for the three stages of planet formation from dust to planetesimals (∼10 6 y), followed by planetesimals to embryos (lunar to Mars-sized objects; few × 10 6 y), and finally embryos to planets (10 7 –10 8 y). Defining the role of turbulence in the early nebula is a key to understanding the growth of solids larger than meter size. The initiation of runaway growth of embryos from planetesimals ultimately leads to the growth of large terrestrial planets via large impacts. Dynamical models can produce inner Solar System configurations that closely resemble our Solar System, especially when the orbital effects of large planets (Jupiter and Saturn) and damping mechani...