2.2 Core accretion model
8 min read•august 21, 2024
The explains how planets form in protoplanetary disks around young stars. It describes the gradual buildup of solid particles into planetesimals, then planetary cores, and finally full-fledged planets through various stages of growth and gas .
This model is crucial for understanding the diverse exoplanets we observe. It provides a framework for explaining the formation of both rocky terrestrial planets and gas giants, accounting for their different compositions, sizes, and locations within planetary systems.
Fundamentals of core accretion
- Describes primary mechanism for planet formation in protoplanetary disks around young stars
- Crucial for understanding the diversity of exoplanets observed in various planetary systems
- Provides framework for explaining formation of both terrestrial and planets
Definition and basic principles
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- Gradual accumulation of solid particles in leads to planet formation
- Begins with micron-sized dust grains and progresses to kilometer-sized planetesimals
- Gravity plays increasingly important role as objects grow larger
- precedes gas envelope accumulation for gas giants
Historical development of model
- Proposed by Viktor Safronov in 1969 as part of solar system formation theory
- Refined by Goldreich and Ward in 1973 introducing concept of runaway growth
- Pollack et al. (1996) developed modern version incorporating gas accretion for giant planets
- Continuous refinements based on new exoplanet discoveries and improved computational models
Key stages in process
- Dust forms initial larger particles
- Planetesimal formation through gravitational instabilities or streaming instabilities
- Runaway growth creates planetary embryos
- Oligarchic growth produces protoplanetary cores
- Gas accretion for giant planet formation
Physical processes involved
Dust grain accumulation
- Brownian motion causes initial collisions between dust particles
- Van der Waals forces enable sticking of small grains
- Differential settling in disk midplane concentrates particles
- Fractal growth produces fluffy aggregates up to millimeter sizes
- Bouncing barrier and fragmentation limit direct growth beyond centimeter scale
Planetesimal formation
- Streaming instability concentrates pebbles in dense filaments
- of pebble clumps forms kilometer-sized planetesimals
- Turbulence in disk affects efficiency of planetesimal formation
- Size distribution of initial planetesimals impacts subsequent growth stages
- Planetesimal formation rate influences overall timescale of planet formation
Runaway growth phase
- Gravitational focusing enhances collision cross-section of larger bodies
- Growth rate increases with mass, leading to rapid size increase of largest objects
- Velocity dispersion of smaller bodies remains low, facilitating efficient accretion
- Oligarchs emerge as dominant bodies in their feeding zones
- Duration of runaway growth depends on initial planetesimal size distribution and disk properties
Oligarchic growth phase
- Largest bodies (oligarchs) dominate gravitational interactions in their orbital regions
- Oligarchs grow at similar rates, maintaining relative spacing
- Increased velocity dispersion of planetesimals slows growth rate
- Embryos reach masses of approximately 0.1 Earth masses
- Transition to giant impact phase for terrestrial planet formation
Role of gas in accretion
Gas capture initiation
- Core mass reaches critical value (typically 5-10 Earth masses) to retain gas envelope
- Envelope mass initially increases slowly through Kelvin-Helmholtz contraction
- Radiative energy transport in outer envelope balances gravitational contraction
- Core continues to grow through planetesimal accretion during early envelope phase
- Opacity of accreted material affects efficiency of gas capture
Envelope contraction
- As envelope mass increases, gravitational compression heats gas
- Radiative cooling allows envelope to contract and accrete more gas
- Contraction rate depends on opacity and core luminosity
- Crossover mass reached when envelope mass equals core mass
- Marks transition to rapid gas accretion phase
Rapid gas accretion
- Hydrodynamic collapse of surrounding gas onto protoplanet
- Accretion rate limited by available gas in feeding zone and disk supply
- Forms extended envelope that contracts over time
- Final mass determined by disk dispersal or gap opening in disk
- Differentiation of planet interior occurs during this phase
Timescales and constraints
Formation timescales vs disk lifetimes
- Typical protoplanetary disk lifetimes range from 1-10 million years
- Terrestrial planet formation can occur within this timeframe
- Gas giant formation challenging to complete before disk dissipation
- Ice giant formation intermediate between terrestrial and gas giant timescales
- Rapid pebble accretion proposed to accelerate core growth for giant planets
Mass of solid material required
- Minimum mass solar nebula (MMSN) provides baseline estimate for solid content
- Typically requires 100-300 Earth masses of solids in disk for giant planet formation
- Dust-to-gas ratio in disk affects available solid material
- Radial drift of solids can concentrate material in certain disk regions
- Pressure bumps or planet traps may aid in retaining solid material locally
Temperature and pressure conditions
- Temperature gradient in disk affects composition of accreted material
- Snow line marks transition where water ice can condense, enhancing solid surface density
- Pressure affects gas density and thus gas accretion rates
- Higher temperatures near star challenge formation of close-in giant planets
- Disk evolution changes temperature and pressure profiles over time
Application to different planet types
Terrestrial planet formation
- Occurs primarily through accretion of rocky planetesimals
- Final assembly involves giant impacts between planetary embryos
- Water delivery may occur through accretion of icy bodies from outer disk
- Atmospheric acquisition through outgassing and late veneer accretion
- Timescale of 10-100 million years for complete formation
Gas giant formation
- Requires rapid core growth to reach critical mass before disk dissipation
- Enhanced solid surface density beyond snow line aids core formation
- Gas accretion dominates final stages of formation
- Jupiter-mass planets typically form at or beyond 5 AU in solar-like systems
- Formation location affects final composition and atmospheric properties
Ice giant formation
- Intermediate between terrestrial and gas giant formation processes
- Core accretion occurs in region of disk rich in icy materials
- Limited gas accretion due to lower core masses or disk dissipation
- Explains composition of Uranus and Neptune in our solar system
- Challenges in forming ice giants rapidly enough in standard core accretion model
Observational evidence
Protoplanetary disk observations
- ALMA observations reveal detailed disk structures (rings, gaps)
- Detection of dust traps and pressure bumps consistent with planetesimal formation sites
- Measurements of disk masses and lifetimes constrain formation timescales
- Evidence for gas and dust evolution in disks supports core accretion scenario
- Direct imaging of young planets embedded in disks (PDS 70 system)
Exoplanet population statistics
- Mass-radius relationships of exoplanets support core accretion model predictions
- Correlation between stellar metallicity and giant planet occurrence
- Prevalence of super-Earths and mini-Neptunes aligns with core accretion outcomes
- Period-mass distribution of exoplanets consistent with formation and migration scenarios
- Composition estimates from transit spectroscopy inform formation conditions
Solar system formation indicators
- Isotopic compositions of meteorites provide timeline for solar system formation
- Giant planet core masses inferred from gravitational measurements
- Compositional gradients in solar system reflect formation locations
- Kuiper Belt and asteroid belt structures shaped by planet formation processes
- Moon-forming impact on Earth exemplifies late-stage terrestrial planet formation
Challenges and limitations
Migration during formation
- Type I migration can rapidly move low-mass cores inward
- Type II migration occurs for massive planets opening gaps in disk
- Migration can disrupt orderly growth of planets in situ
- Explains presence of hot Jupiters and compact multi-planet systems
- Requires mechanisms to slow or halt migration (planet traps, disk winds)
Pebble accretion modifications
- Efficient accretion of mm-cm sized particles can accelerate core growth
- Addresses issue of forming giant planets within disk lifetimes
- Sensitive to disk turbulence and particle sizes
- May lead to different predicted core masses for giant planets
- Interactions between multiple growing planets affect pebble flow
Disk instability vs core accretion
- Disk instability proposes direct collapse of gas disk to form giant planets
- Can potentially form giant planets more rapidly than core accretion
- Difficult to explain intermediate-mass planets through disk instability alone
- Core accretion remains favored for majority of observed exoplanet population
- Hybrid models incorporating both mechanisms under investigation
Computational modeling
N-body simulations
- Track gravitational interactions between large numbers of particles
- Essential for modeling late stages of terrestrial planet formation
- Can incorporate effects of gas drag and dynamical friction
- Reveal chaotic nature of planet formation process
- Computationally intensive for full system simulations over long timescales
Hydrodynamic simulations
- Model gas dynamics in protoplanetary disks
- Crucial for understanding planet-disk interactions and migration
- Can resolve disk structures like spiral arms and gaps
- Include treatment of thermodynamics and radiative transfer
- Limited by computational power in resolving full range of scales involved
Population synthesis models
- Combine various aspects of planet formation into single framework
- Generate synthetic populations of planets for comparison with observations
- Incorporate probabilistic treatment of initial conditions and processes
- Useful for exploring parameter space and identifying key factors in formation
- Continuously refined based on new observational constraints and theoretical insights
Implications for exoplanet diversity
Mass-radius relationships
- Core accretion predicts range of compositions based on formation location and history
- Explains transition from rocky to gaseous planets with increasing mass
- Allows for diversity in internal structures (iron cores, water layers, H/He envelopes)
- Informs interpretation of observed mass-radius relationships in exoplanet populations
- Suggests possibility of "super-puffs" as extremely low-density planets formed beyond snow line
Composition predictions
- Predicts gradient in bulk composition with formation distance from star
- Allows for water-rich planets formed beyond snow line and migrated inward
- Explains presence of carbon-rich planets around stars with high C/O ratios
- Suggests possibility of helium-dominated atmospheres for some close-in exoplanets
- Informs expectations for atmospheric metallicities of giant planets
Atmospheric retention
- Core accretion model informs likelihood of primordial atmosphere retention
- Predicts mass threshold for significant H/He envelope retention
- Explains atmospheric loss for close-in low-mass planets due to stellar irradiation
- Allows for secondary atmosphere formation through outgassing on rocky planets
- Suggests possibilities for exotic atmospheric compositions based on formation conditions
Future research directions
Improving model accuracy
- Incorporating more realistic dust physics and coagulation models
- Better treatment of disk thermodynamics and chemistry
- Improved modeling of gas accretion processes for giant planets
- More accurate treatment of planet-disk interactions and migration
- Integration of N-body and hydrodynamic simulations for full system modeling
Integration with other formation theories
- Exploring hybrid models combining core accretion and disk instability
- Investigating role of pebble accretion in different stages of planet formation
- Incorporating effects of stellar clusters and external environment on planet formation
- Studying influence of binary stars and multiple star systems on formation processes
- Considering impact of planetary system architecture on long-term stability and
Exoplanet characterization goals
- Detailed atmospheric composition measurements to constrain formation conditions
- Improved mass and radius measurements to refine internal structure models
- Direct imaging of young forming planets to test core accretion predictions
- Expanding sample of characterized planets around diverse stellar types
- Searching for signatures of formation process in exoplanet orbital architectures
Key Terms to Review (18)
Accretion: Accretion is the process by which particles in space, such as dust and gas, come together under the influence of gravity to form larger bodies, like planets or stars. This process plays a critical role in the formation of celestial structures and influences the evolution of planetary systems over time.
Alfred W. Becklin: Alfred W. Becklin is a prominent figure in the study of exoplanets, particularly known for his contributions to the understanding of the core accretion model. This model explains how planets form from the gradual accumulation of solid materials, allowing scientists to better comprehend the processes that lead to the formation of planetary systems.
Atmospheric Escape: Atmospheric escape refers to the process by which particles from a planet's atmosphere are lost to space, often influenced by factors like gravity, thermal energy, and stellar radiation. This phenomenon plays a crucial role in shaping the evolution of a planet's atmosphere, especially for different types of exoplanets, affecting their potential habitability and atmospheric composition.
Coagulation: Coagulation is the process by which particles in a fluid, such as gas or liquid, clump together to form larger aggregates. This phenomenon is essential in understanding how small particles can come together in various environments, leading to the formation of clouds and hazes in the atmosphere, as well as playing a critical role in the core accretion model of planet formation.
Condensation: Condensation is the process by which water vapor or other gases transform into liquid droplets when they cool down. This process is crucial in the formation of clouds and hazes in planetary atmospheres and also plays a significant role in the core accretion model, where it helps in forming solid bodies in protoplanetary disks as materials gather together.
Core accretion model: The core accretion model is a widely accepted theory for the formation of planets, proposing that a solid core forms first by the accumulation of dust and ice in a protoplanetary disk, which then attracts gas to create a larger planetary body. This model helps explain various aspects of planet formation, including the presence of gas giants and terrestrial planets within different regions of a solar system.
Core formation: Core formation is the process by which a planet's core is created, typically through the differentiation of materials based on density, with heavier elements sinking to the center while lighter materials rise. This process is crucial for understanding a planet's overall structure and its thermal evolution, connecting to how planets form and evolve over time.
Disk Instability Model: The disk instability model is a theoretical framework that explains the formation of giant planets through rapid gravitational collapse of a massive protoplanetary disk. This model suggests that under certain conditions, regions of the disk can become gravitationally unstable, leading to the formation of clumps that quickly evolve into planets, particularly gas giants like Jupiter.
Gas Giant: A gas giant is a large planet that is primarily composed of hydrogen and helium, with a deep atmosphere and no well-defined solid surface. These planets are characterized by their massive sizes and low densities, which contribute to their unique formation and development in the context of planetary systems. Gas giants often have thick atmospheres and may possess complex weather systems, including storms and winds, influenced by their rapid rotation and heat from internal processes.
Gravitational Collapse: Gravitational collapse refers to the process where an object, such as a cloud of gas and dust, experiences a gravitational force that causes it to contract and eventually form a denser structure, like a star or a planet. This phenomenon is fundamental in the formation of celestial bodies, as the gravitational attraction pulls matter together, leading to an increase in temperature and pressure in the core until nuclear fusion ignites or a planet forms around a central mass.
Gregory W. Laughlin: Gregory W. Laughlin is an influential astrophysicist known for his research in planetary formation, particularly the core accretion model. His work has provided insights into the processes that govern the formation of gas giant planets and the environments where they develop, which are critical to understanding exoplanetary systems and their characteristics.
Habitability: Habitability refers to the potential of an environment to support life, specifically the conditions that allow for the presence of liquid water, essential elements, and a stable climate. This concept connects with various processes and features that influence the existence of life beyond Earth, such as water delivery mechanisms, adaptations of extreme life forms on our planet, tidal interactions affecting planetary climates, formation theories like core accretion, and biosignatures indicating biological activity over time.
Planetary Migration: Planetary migration refers to the process by which planets move from their original formation locations to different orbits around their parent star, often due to interactions with the surrounding protoplanetary disk or other celestial bodies. This phenomenon can significantly impact a planetary system's architecture, influencing the positions of planets, their compositions, and their potential habitability.
Protoplanetary Disk: A protoplanetary disk is a rotating disk of dense gas and dust surrounding a newly formed star, where planet formation occurs. These disks are essential in understanding the processes that lead to the creation of planets, moons, and other celestial bodies within a solar system.
Radial velocity method: The radial velocity method is an observational technique used to detect exoplanets by measuring the changes in a star's spectrum caused by the gravitational pull of an orbiting planet. As a planet orbits, it exerts a gravitational influence on its host star, causing the star to wobble slightly, which can be observed as shifts in the star's light spectrum toward red or blue wavelengths.
Super-Earth: Super-Earths are a class of exoplanets that have a mass larger than Earth's but significantly less than that of Uranus or Neptune, typically ranging from about 1 to 10 times the mass of Earth. These planets often possess unique characteristics that influence their potential for habitability and their formation processes, making them key targets in the study of planetary systems.
Thermal Evolution: Thermal evolution refers to the changes in temperature and thermal structure of a planetary body over time due to various internal and external processes. It encompasses how a planet's heat dissipates, influencing geological activity, atmosphere retention, and potential habitability. Understanding thermal evolution is essential to grasp how planetary resonances can affect heating through tidal forces and how the core accretion model outlines the initial heat generation during planet formation.
Transit Photometry: Transit photometry is a method used to detect exoplanets by observing the dimming of a star's light as a planet passes in front of it. This technique allows scientists to gather information about the size, orbit, and atmospheric characteristics of the exoplanet, making it essential for studying various planetary phenomena and evolution.