Planet formation: The case for large efforts on the computational side (original) (raw)

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.

HOW DO MOST PLANETS FORM?—CONSTRAINTS ON DISK INSTABILITY FROM DIRECT IMAGING

The Astrophysical Journal, 2012

Core accretion and disk instability have traditionally been regarded as the two competing possible paths of planet formation. In recent years, evidence have accumulated in favor of core accretion as the dominant mode, at least for close-in planets. However, it might be hypothesized that a significant population of wide planets formed by disk instabilities could exist at large separations, forming an invisible majority. In previous work, we addressed this issue through a direct imaging survey of B2-A0-type stars, and concluded that <30% of such stars form and retain planets and brown dwarfs through disk instability, leaving core accretion as the likely dominant mechanism. In this paper, we extend this analysis to FGKMtype stars by applying a similar analysis to the Gemini Deep Planet Survey (GDPS) sample. The results strengthen the conclusion that substellar companions formed and retained around their parent stars by disk instabilities are rare. Specifically, we find that the frequency of such companions is <8% for FGKM-type stars under our most conservative assumptions, for an outer disk radius of 300 AU, at 99% confidence. Furthermore, we find that the frequency is always <10% at 99% confidence independently of outer disk radius, for any radius from 5 to 500 AU. We also simulate migration at a wide range of rates, and find that the conclusions hold even if the companions move substantially after formation. Hence, core accretion remains the likely dominant formation mechanism for the total planet population, for every type of star from M-type through B-type.

A reexamination of early numerical simulations of planetary accretion

A Bibliography of Planetary Geology and Geophysics Principal Investigators and Their Associates 1990 1991, 1991

The intermediate stage of protoplanetary accretion, where sub-km sized bodies accrete in 500-1000 km diameter bodies, is critical in determining the character of subsequent evolution. Greenberg et al. discovered that, at least through the formation of 500 km planetary embryos, the small particle dominate the population and control the velocity distribution, keeping velocities much smaller and setting up the possibility of runaway accretion, where the largest body is able to accumulate quickly a much greater mass and separate itself from the continuum of the size distribution. However, some controversy remains as to the exact methods used by Greenberg et al. I reviewed that planet formation model in detail to address these criticisms and better explain the algorithm used in the study. In particular, the algorithms for mutual velocity stirring among planetesimals and mass shifting between adjacent diameter bins are rederived in detail. I also examined the likelihood of artificial acceleration of the runaway growth phase of the intermediate stage of planetesimal formation.

Dynamics and accretion of planetesimals

Progress of Theoretical and Experimental Physics, 2012

In the standard scenario of planet formation, planets are formed from a protoplanetary disk that consists of gas and dust. The building blocks of solid planets are called planetesimals; they are formed by coagulation of dust. We review the basic dynamics and accretion of planetesimals by showing N-body simulations. The orbits of planetesimals evolve through two-body gravitational relaxation: viscous stirring increases the random velocity and dynamical friction realizes the equiparation of the random energy. In the early stage of planetesimal accretion, the growth mode of planetesimals is runaway growth, where larger planetesimals grow faster than smaller ones. When a protoplanet (a runaway-growing planetesimal) exceeds a critical mass, the growth mode shifts to oligarchic growth, where similar-sized protoplanets grow while keeping a certain orbital separation. The final stage of terrestrial planet formation is collision among protoplanets, known as giant impacts. We also summarize the dynamical effects of disk gas on planets and the core accretion model for the formation of gas giants, and discuss the diversity of planetary systems.

Disk-Planet Interactions During Planet Formation, 2007

The discovery of close orbiting extrasolar giant planets led to extensive studies of disk planet interactions and the forms of migration that can result as a means of accounting for their location. Early work established the type I and type II migration regimes for low mass embedded planets and high mass gap forming planets respectively. While providing an attractive means of accounting for close orbiting planets intially formed at several AU, inward migration times for objects in the earth mass range were found to be disturbingly short, making the survival of giant planet cores an issue. Recent progress in this area has come from the application of modern numerical techniques wich make use of up to date supercomputer resources. These have enabled higher resolution studies of the regions close to the planet and the initiation of studies of planets interacting with disks undergoing MHD turbulence. This work has led to indications of how the inward migration of low to intermediate mass planets could be slowed down or reversed. In addition, the possibility of a new very fast type III migration regime, that can be directed inwards or outwards, that is relevant to partial gap forming planets in massive disks has been investigated.

Disk-Planet Interactions During Planet Formation

Protostars and Planets V, 2007

The discovery of close orbiting extrasolar giant planets led to extensive studies of diskplanet interactions and the forms of migration that can result as a means of accounting for their location. Early work established the type I and type II migration regimes for low-mass embedded planets and high-mass gap-forming planets respectively. While providing an attractive means of accounting for close orbiting planets initially formed at several AU, inward migration times for objects in the Earth-mass range were found to be disturbingly short, making the survival of giant planet cores an issue. Recent progress in this area has come from the application of modern numerical techniques that make use of up-to-date supercomputer resources. These have enabled higher-resolution studies of the regions close to the planet and the initiation of studies of planets interacting with disks undergoing magnetohydrodynamic turbulence. This work has led to indications of how the inward migration of low- to intermediate-mass planets could be slowed down or reversed. In addition, the possibility of a new very fast type III migration regime, which can be directed inward or outward, that is relevant to partial gap-forming planets in massive disks has been investigated.

N-body simulations of planet formation via pebble accretion

Astronomy and Astrophysics, 2021

Aims. The connection between initial disc conditions and final orbital and physical properties of planets is not well-understood. In this paper, we numerically study the formation of planetary systems via pebble accretion and investigate the effects of disc properties such as masses, dissipation timescales, and metallicities on planet formation outcomes. Methods. We improved the N-body code SyMBA that was modified for our Paper I by taking account of new planet-disc interaction models and type II migration. We adopted the 'two-α' disc model to mimic the effects of both the standard disc turbulence and the mass accretion driven by the magnetic disc wind. Results. We successfully reproduced the overall distribution trends of semi-major axes, eccentricities, and planetary masses of extrasolar giant planets. There are two types of giant planet formation trends, depending on whether or not the disc's dissipation timescales are comparable to the planet formation timescales. When planet formation happens fast enough, giant planets are fully grown (Jupiter mass or higher) and are distributed widely across the disc. On the other hand, when planet formation is limited by the disc's dissipation, discs generally form low-mass cold Jupiters. Our simulations also naturally explain why hot Jupiters (HJs) tend to be alone and how the observed eccentricity-metallicity trends arise. The low-metallicity discs tend to form nearly circular and coplanar HJs in situ, because planet formation is slower than high-metallicity discs, and thus protoplanetary cores migrate significantly before gas accretion. The high-metallicity discs, on the other hand, generate HJs in situ or via tidal circularisation of eccentric orbits. Both pathways usually involve dynamical instabilities, and thus HJs tend to have broader eccentricity and inclination distributions. When giant planets with very wide orbits ("super-cold Jupiters") are formed via pebble accretion followed by scattering, we predict that they belong to metal-rich stars, have eccentric orbits, and tend to have (∼80%) companions interior to their orbits.

N-body simulations of planet formation via pebble accretion

Astronomy & Astrophysics, 2017

Context. Planet formation with pebbles has been proposed to solve a couple of long-standing issues in the classical formation model. Some sophisticated simulations have been performed to confirm the efficiency of pebble accretion. However, there has not been any global N-body simulations that compare the outcomes of planet formation via pebble accretion to observed extrasolar planetary systems. Aims. In this paper, we study the effects of a range of initial parameters of planet formation via pebble accretion, and present the first results of our simulations. Methods. We incorporate a published pebble-accretion model into the N-body code SyMBA, along with the effects of gas accretion, eccentricity and inclination damping, and planet migration in the disc. Results. We confirm that pebble accretion leads to a variety of planetary systems, but have difficulty in reproducing observed properties of exoplanetary systems, such as planetary mass, semimajor axis, and eccentricity distributions. The main reason behind this is an overly efficient type-I migration, which closely depends on the disc model. However, our simulations also lead to a few interesting predictions. First, we find that formation efficiencies of planets depend on the stellar metallicities, not only for giant planets, but also for Earths (Es) and Super-Earths (SEs). The dependency for Es/SEs is subtle. Although higher metallicity environments lead to faster formation of a larger number of Es/SEs, they also tend to be lost later via dynamical instability. Second, our results indicate that a wide range of bulk densities observed for Es and SEs is a natural consequence of dynamical evolution of planetary systems. Third, the ejection trend of our simulations suggest that one free-floating E/SE may be expected for two smaller-mass planets. Key words. planets and satellites: formation-planets and satellites: dynamical evolution and stability-planetary systemsplanets and satellites: general-protoplanetary disks 1 Recent studies of protostellar discs suggest that the MRI turbulence may not be efficient in the planet-forming region (∼1-10 AU), and that the angular momentum transfer may be largely done by magnetocentrifugal disc winds (Turner et al. 2014). If this were the case, the pressure bumps would need to be created by other mechanisms, or pebbles would need to be trapped by other means such as vortices (e.g. Barge & Sommeria 1995; Raettig et al. 2015).

Measuring Protoplanetary Disk Accretion with H I Pfund Β

The Astrophysical Journal, 2013

In this work, we introduce the use of H i Pfund β (Pfβ; 4.6538 µm) as a tracer of mass accretion from protoplanetary disks onto young stars. Pfβ was serendipitously observed in NIRSPEC and CRIRES surveys of CO fundamental emission, amounting to a sample size of 120 young stars with detected Pfβ emission. Using a subsample of disks with previously measured accretion luminosities, we show that Pfβ line luminosity is well correlated with accretion luminosity over a range of at least three orders of magnitude. We use this correlation to derive accretion luminosities for all 120 targets, 65 of which are previously unreported in the literature. The conversion from accretion luminosity to accretion rate is limited by the availability of stellar mass and radius measurements; nevertheless, we also report accretion rates for 67 targets, 16 previously unmeasured. Our large sample size and our ability to probe high extinction values allow for relatively unbiased comparisons between different types of disks. We find that the transitional disks in our sample have lower than average Pfβ line luminosities, and thus accretion luminosities, at a marginally significant level. We also show that high Pfβ equivalent width is a signature of transitional disks with high inner disk gas/dust ratios. In contrast, we find that disks with signatures of slow disk winds have Pfβ luminosities comparable to those of other disks in our sample. Finally, we investigate accretion rates for stage I disks, including significantly embedded targets. We find that stage I and stage II disks have statistically indistinguishable Pfβ line luminosities, implying similar accretion rates, and that the accretion rates of stage I disks are too low to be consistent with quiescent accretion. Our results are instead consistent with both observational and theoretical evidence that stage I objects experience episodic, rather than quiescent, accretion.

Formation, Orbital and Internal Evolutions of Young Planetary Systems

Space Science Reviews, 2016

The growing body of observational data on extrasolar planets and protoplanetary disks has stimulated intense research on planet formation and evolution in the past few years. The extremely diverse, sometimes unexpected physical and orbital characteristics of exoplanets lead to frequent updates on the mainstream scenarios for planet formation and evolution, but also to the exploration of alternative avenues. The aim of this review is to bring together classical pictures and new ideas on the formation, orbital and internal evolutions of planets, highlighting the key role of the protoplanetary disk in the various parts of the theory. We begin by briefly reviewing the conventional mechanism of core accretion by the growth of planetesimals, and discuss a relatively recent model of core growth through the accretion of pebbles. We review the basic physics of planet-disk interactions, recent progress in this area, and discuss their role in observed planetary systems. We address the most important effects of planets internal evolution, like cooling and contraction, the mass-luminosity relation, and the bulk composition expressed in the mass-radius and mass-mean density relations. Keywords planets and satellites: formation • planets and satellites: interiors • protoplanetary disks • planet-disk interactions 1 Introduction Planet formation and evolution is a fast-moving field, stimulated by the rapid increase in the number of exoplanets and their great diversity. Despite the wealth of observational data on planetary systems, including our own, it is difficult to have a general theory for planet formation and evolution as it involves a broad range of physical processes that happen at