2.1 Protoplanetary disk formation
11 min read•august 21, 2024
Protoplanetary disks are the birthplaces of planets, consisting of gas and dust around young stars. Understanding their composition, formation, and evolution is crucial for explaining the diversity of exoplanetary systems we observe today.
These disks undergo significant changes over their lifetimes, typically spanning a few million years. Processes like , photoevaporation, and dust growth shape the disk's structure and set the stage for , influencing the types of planets that can form in different regions.
Protoplanetary disk composition
- Protoplanetary disks serve as the birthplaces of planets, consisting of gas and dust surrounding young stars
- Understanding disk composition provides crucial insights into the raw materials available for planet formation in exoplanetary systems
- Composition variations within disks influence the types and characteristics of planets that can form in different regions
Gas and dust distribution
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- Gas dominates disk mass, typically accounting for 99% of the total material
- Dust particles comprise the remaining 1%, but play a critical role in planet formation processes
- Radial distribution of gas and dust varies, with density decreasing as distance from the central star increases
- Vertical distribution shows settling of larger dust particles towards the disk midplane
Chemical composition variations
- Inner disk regions contain refractory materials (silicates, metals) due to high temperatures
- Outer disk regions harbor volatile species (ices, organic compounds) in frozen form
- Elemental abundances reflect the composition of the molecular cloud from which the star-disk system formed
- Chemical reactions and transport processes create gradients in molecular abundances across the disk
Temperature gradients
- Temperature decreases radially outward from the central star, following a power-law distribution
- Inner disk regions can reach temperatures of 1000-2000 K, while outer regions may be as cold as 10-20 K
- Vertical temperature gradients exist, with the disk surface being hotter than the midplane due to stellar irradiation
- Temperature variations lead to the formation of condensation fronts (snowlines) for different molecular species
Disk formation mechanisms
- Protoplanetary disks form as a natural consequence of star formation processes
- Understanding disk formation mechanisms provides context for the initial conditions of planet formation
- These mechanisms set the stage for the subsequent evolution of the disk and its potential to form planets
Gravitational collapse
- Molecular cloud cores undergo gravitational collapse, initiating the formation of protostars
- As the core contracts, it begins to rotate faster due to conservation of angular momentum
- Infalling material with higher angular momentum forms a flattened structure around the protostar
- This flattened structure evolves into a rotationally supported disk as collapse continues
Angular momentum conservation
- Conservation of angular momentum prevents all infalling material from directly accreting onto the protostar
- Leads to the formation of a centrifugally supported disk around the central object
- Initial disk size determined by the specific angular momentum of the collapsing cloud core
- Disk formation efficiency depends on the initial rotation rate and magnetic field strength of the cloud core
Accretion processes
- Material from the surrounding envelope continues to accrete onto the disk
- Viscous processes within the disk transport angular momentum outward, allowing matter to spiral inward
- Magnetorotational instability (MRI) serves as a primary mechanism for generating disk turbulence and driving accretion
- Accretion rates typically range from 10^-8 to 10^-6 solar masses per year for T Tauri stars
Disk structure
- Protoplanetary disks exhibit complex structures that evolve over time
- Understanding disk structure provides insights into the environment in which planets form
- Structural features of disks can influence planet formation processes and the resulting planetary system architectures
Inner and outer regions
- Inner disk (< 1 AU) characterized by high temperatures and densities, dominated by gas and refractory dust
- Outer disk (> 1 AU) contains cooler material, including ices and more volatile compounds
- Transition zone between inner and outer regions marks the location of the water snowline
- Disk structure varies with stellar mass, with more massive stars typically hosting larger disks
Vertical stratification
- Disk material settles into distinct layers due to gravity and gas pressure
- Upper layers exposed to stellar radiation, creating a hot, tenuous atmosphere
- Intermediate layer (warm molecular layer) contains a rich chemistry of gas-phase molecules
- Cold midplane harbors the bulk of the disk mass and serves as the primary site for planet formation
Density profiles
- Surface density typically follows a power-law distribution, decreasing with increasing distance from the star
- Minimum mass solar nebula (MMSN) model suggests a surface density profile of
- Vertical density structure approximated by a Gaussian distribution, with scale height increasing with radius
- Local density variations can arise due to , planet-disk interactions, or dust concentration mechanisms
Disk evolution
- Protoplanetary disks undergo significant changes over their lifetimes, typically spanning a few million years
- Evolution of disk properties directly impacts the planet formation process and the resulting planetary systems
- Understanding disk evolution helps constrain the timescales available for planet formation in different disk regions
Viscous evolution
- Angular momentum transport drives the radial spreading of disk material over time
- Viscous timescale varies with disk radius, leading to faster evolution in the inner regions
- Alpha-disk model parameterizes , with typical values of α ranging from 10^-4 to 10^-2
- Viscous evolution results in a gradual decrease in disk mass and surface density over time
Photoevaporation
- High-energy radiation (UV, X-rays) from the central star heats the disk surface, causing gas to escape
- Creates a gap in the disk at the gravitational radius where thermal energy exceeds gravitational binding energy
- Inner disk can rapidly clear once the photoevaporation rate exceeds the viscous accretion rate
- Photoevaporation plays a crucial role in the final stages of , setting an upper limit on the time available for giant planet formation
Dust growth and settling
- Dust particles grow through collisions and sticking, forming larger aggregates over time
- Larger particles decouple from the gas and settle towards the disk midplane, enhancing the local dust-to-gas ratio
- Settling timescale depends on particle size and local , with larger particles settling faster
- Growth beyond centimeter sizes faces challenges (radial drift, fragmentation) known as the "meter-size barrier"
Disk observations
- Observational techniques provide crucial data on protoplanetary disk properties and evolution
- Advances in observational capabilities have revolutionized our understanding of disk structure and dynamics
- Multi-wavelength observations offer complementary insights into different aspects of disk physics and chemistry
Infrared spectroscopy
- Probes the warm inner regions of protoplanetary disks (< 10 AU)
- Reveals the presence and composition of small through their emission features
- Silicate emission features at 10 and 20 μm provide information on dust grain size and crystallinity
- PAH (polycyclic aromatic hydrocarbon) emission traces the presence of small organic molecules in the disk atmosphere
Millimeter-wave interferometry
- Allows high-resolution imaging of disk structure at scales of tens to hundreds of AU
- Traces the distribution of large dust grains (mm-sized) in the disk midplane
- Provides constraints on disk mass, size, and radial density profile
- ALMA (Atacama Large Millimeter/submillimeter Array) has revolutionized our view of disk substructures (rings, gaps, spirals)
Direct imaging techniques
- Enables detection of scattered light from small dust grains in the disk surface layers
- Requires high-contrast imaging to overcome the brightness of the central star
- Reveals large-scale disk structures such as spiral arms, asymmetries, and shadows
- Instruments like SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) on the VLT have provided stunning images of protoplanetary disks
Planet formation in disks
- Protoplanetary disks serve as the birthplaces of planets, providing the raw materials and environment for their formation
- Understanding planet formation processes in disks is crucial for explaining the diversity of observed exoplanetary systems
- Different formation mechanisms may operate in different disk regions or at different stages of disk evolution
Core accretion vs disk instability
- Core accretion involves gradual growth of solid cores followed by gas accretion for giant planets
- Begins with dust coagulation, progresses through planetesimal formation, and culminates in oligarchic growth
- Disk instability proposes direct collapse of disk material into giant planets in massive, cool disks
- Core accretion favored for most planet formation scenarios, but disk instability may operate in some cases
Planetesimal formation
- Represents a critical step in the planet formation process, bridging the gap between dust and protoplanets
- Streaming instability concentrates particles in dense filaments, potentially leading to gravitational collapse
- Bouncing barrier and fragmentation limit direct growth of particles beyond centimeter sizes
- Formation of the first planetesimals likely occurs in localized regions of enhanced particle concentration
Migration of planetary embryos
- Gravitational interactions between forming planets and the gas disk lead to orbital migration
- Type I migration affects low-mass planets embedded in the disk, typically resulting in rapid inward motion
- Type II migration occurs when massive planets open gaps in the disk, leading to slower migration
- Migration can significantly alter the architectures of forming planetary systems, explaining features like hot Jupiters
Disk lifetimes
- Disk lifetime sets the available time for planet formation processes to occur
- Understanding factors affecting disk dispersal helps constrain planet formation models
- Observations of disk populations in young stellar clusters provide statistical constraints on disk lifetimes
Observational constraints
- Disk fraction in young stellar clusters decreases with cluster age, providing a statistical measure of disk lifetimes
- Typical disk lifetimes range from 2-5 million years, with significant scatter
- Inner disk clearing often occurs more rapidly than outer disk dissipation
- Transition disks with inner holes represent an intermediate stage between full disks and debris disks
Factors affecting disk dispersal
- Photoevaporation by high-energy radiation from the central star drives disk mass loss
- Accretion onto the central star gradually depletes the disk material
- External environmental factors (stellar encounters, external photoevaporation) can accelerate disk dispersal
- Planet formation itself can contribute to disk clearing by accreting or shepherding disk material
Implications for planet formation
- Limited disk lifetimes constrain the timescales available for giant planet formation via core accretion
- Rapid formation of planetary cores may be necessary to allow sufficient time for gas accretion
- Variation in disk lifetimes may contribute to the diversity of observed planetary systems
- Late stages of terrestrial planet formation likely occur after the dissipation of the gas disk
Disk-planet interactions
- Forming planets interact gravitationally with their natal disks, leading to mutual evolution
- Disk-planet interactions play a crucial role in shaping planetary system architectures
- Understanding these interactions helps explain observed features of both disks and exoplanetary systems
Gap opening
- Massive planets can clear material from their orbital vicinity, creating visible gaps in the disk
- Gap opening occurs when the planet's Hill radius exceeds the local disk scale height
- Multiple gaps can form in the presence of multiple planets or at orbital resonances
- Gap structure provides indirect evidence for the presence of forming planets in protoplanetary disks
Spiral density waves
- Planets excite spiral density waves in the surrounding disk material
- Inner spiral arm leads the planet, while the outer arm trails behind
- Wave amplitude increases with planet mass, potentially leading to shock formation
- Spiral waves can transport angular momentum, contributing to planet migration and disk evolution
Resonant trapping
- Convergent migration of multiple planets can lead to capture into mean motion resonances
- Resonant configurations stabilize planetary orbits and slow down further migration
- Common resonances include 2:1, 3:2, and 4:3 period ratios
- Resonant trapping explains the architecture of some observed multi-planet systems (Kepler-223)
Disk chemistry
- Chemical processes in protoplanetary disks set the initial composition of forming planets
- Understanding disk chemistry is crucial for interpreting observations and modeling planet formation
- Chemical evolution of disks influences the potential for life in resulting planetary systems
Molecular abundances
- Gas-phase chemistry dominated by ion-molecule reactions in cold, dense regions
- Photochemistry plays a significant role in the disk atmosphere and inner regions
- Grain surface reactions important for forming complex organic molecules
- Abundance gradients develop due to variations in temperature, density, and radiation field across the disk
Snowlines and condensation fronts
- Snowlines mark the radial locations where specific molecular species condense from gas to solid phase
- Major snowlines include those of water (H2O), carbon dioxide (CO2), and carbon monoxide (CO)
- Condensation fronts influence the composition of forming planets and their atmospheres
- Snowline locations evolve over time as the disk cools and material is processed
Organic compound formation
- Complex organic molecules (COMs) can form through both gas-phase and grain surface reactions
- Photochemistry in the disk atmosphere can produce simple organics like HCN and C2H2
- Grain surface chemistry allows for the buildup of larger organic molecules through radical-radical reactions
- Delivery of organics to forming planets may provide prebiotic ingredients relevant to the origin of life
Magnetic fields in disks
- Magnetic fields play a crucial role in disk dynamics, accretion processes, and angular momentum transport
- Understanding magnetic effects is essential for developing accurate models of disk evolution and planet formation
- Magnetic fields can influence disk structure and potentially impact the formation and migration of planets
Magnetorotational instability
- MRI serves as a primary mechanism for generating turbulence in weakly magnetized, ionized disks
- Requires a weak vertical magnetic field and sufficient ionization fraction to operate
- Drives angular momentum transport, enabling accretion of material onto the central star
- Efficiency of MRI varies across the disk due to changes in ionization state and field strength
Disk winds and outflows
- Large-scale magnetic fields can launch disk winds from the surface layers
- Magnetocentrifugal acceleration drives material along open field lines, removing angular momentum
- Disk winds contribute to mass loss and may play a role in disk dispersal
- Outflows can interact with the surrounding environment, potentially triggering star formation in nearby regions
Field-driven accretion
- Magnetic fields can facilitate accretion through non-turbulent mechanisms
- Magnetic braking in the disk atmosphere can remove angular momentum, driving accretion
- Magnetically induced disk warps can lead to enhanced angular momentum transport
- Field-driven accretion may dominate in regions where MRI is suppressed (dead zones)
Debris disks
- Debris disks represent a later stage of disk evolution, dominated by collisional processes rather than gas dynamics
- Studying debris disks provides insights into the late stages of planet formation and the architecture of mature planetary systems
- Observations of debris disks around other stars offer clues about the evolution of our own solar system
Transition from protoplanetary disks
- Gas-rich protoplanetary disks evolve into gas-poor debris disks over a few million years
- Transition involves dispersal of primordial gas and small dust through various mechanisms
- Larger bodies (planetesimals) remain and serve as the source of debris dust through collisions
- Timescale of transition varies, with some systems showing both gas-rich and debris disk characteristics
Dust production mechanisms
- Collisional cascades of planetesimals generate small dust particles visible in debris disks
- Radiation pressure removes the smallest particles, while Poynting-Robertson drag causes larger grains to spiral inward
- Steady-state between dust production and removal processes determines the observed disk structure
- Episodic collisions of large bodies can lead to temporary increases in dust production (debris disk variability)
Exoplanet detection in debris disks
- Structures in debris disks can reveal the presence of unseen planets
- Gaps, rings, and asymmetries may be sculpted by gravitational interactions with planets
- Resonant structures (clumps, arcs) can form due to planets trapping dust in mean motion resonances
- Combined observations of debris disk structure and direct planet detection provide powerful constraints on planetary system architecture
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.
Angular momentum conservation: Angular momentum conservation refers to the principle that the total angular momentum of a closed system remains constant if no external torques act on it. This concept is crucial in understanding the behavior of rotating systems, as it explains how the distribution of mass and rotation speed can change while the overall angular momentum remains unchanged. It plays an important role in various astrophysical phenomena, connecting ideas like orbital motion, interactions in binary systems, and the formation of celestial bodies.
Collapse phase: The collapse phase refers to the stage in the formation of a protoplanetary disk where the molecular cloud that creates it undergoes gravitational contraction, leading to a dense core that eventually forms a star. This process is crucial because as the material collapses, it not only initiates star formation but also sets the stage for the creation of a surrounding disk of gas and dust, which is essential for planet formation. The collapse phase is influenced by factors such as turbulence within the cloud and external forces like nearby supernovae or stellar winds.
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.
Differentiation: Differentiation refers to the process by which a planet or planetary body separates into distinct layers based on density and composition. This process results in the formation of a differentiated structure, where heavier materials like metals sink towards the center while lighter materials such as silicates rise to form the crust. Understanding differentiation is crucial for interpreting mass-radius relationships and the evolution of planetary bodies within protoplanetary disks.
Disk dispersal: Disk dispersal refers to the process by which the material in a protoplanetary disk, primarily composed of gas and dust, is gradually removed or cleared away over time. This phenomenon plays a crucial role in the evolution of star systems, as the dispersal of the disk can influence planet formation and migration, affecting the final architecture of planetary systems.
Dust grains: Dust grains are small solid particles, typically measuring between 0.1 micrometers to a few hundred micrometers, found in various astronomical environments, including protoplanetary disks. These grains play a crucial role in the formation of planetary systems by serving as the building blocks for larger bodies through processes like coagulation and accretion. They are also essential in the gravitational instability model, where their presence can lead to local density enhancements that trigger the formation of planets.
Gas density: Gas density refers to the mass of gas per unit volume, typically expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). In the context of protoplanetary disk formation, gas density is crucial as it influences the processes of accretion, the cooling and condensation of material, and ultimately the formation of planets from the surrounding gas and dust.
Gravitational instability model: The gravitational instability model is a theory that explains the formation of celestial bodies, particularly planets, through the rapid collapse of dense regions within a protoplanetary disk under their own gravity. This model suggests that when parts of the disk accumulate enough mass, they can overcome internal pressure and collapse to form solid structures. This process plays a significant role in understanding the early stages of star and planet formation, linking it to the dynamics of protoplanetary disks and the evolutionary paths of stellar systems.
Hans Bethe: Hans Bethe was a prominent physicist known for his groundbreaking work in nuclear physics and astrophysics, particularly in understanding the processes that fuel stars. His research on stellar nucleosynthesis provided crucial insights into how elements are formed within stars, which is essential for understanding protoplanetary disk formation and the life cycles of stars.
Infrared Spectroscopy: Infrared spectroscopy is a technique used to identify and analyze the composition of materials by measuring how they absorb infrared light. This method helps scientists understand the molecular structure and chemical bonds of substances, making it particularly useful in studying protoplanetary disks, where dust and gas are present. By observing the infrared signatures of different compounds, researchers can infer the physical and chemical processes occurring in these disks during planet formation.
Instabilities: Instabilities refer to perturbations or disruptions in a system that can lead to significant changes or the breakdown of that system. In the context of protoplanetary disk formation, instabilities are crucial because they can trigger the clumping of material, influencing how planets and other celestial bodies form within these disks. These instabilities can arise from various factors such as gravitational interactions, thermal fluctuations, and angular momentum distribution, ultimately affecting the evolution and structure of the protoplanetary disk.
J.C. McKee: J.C. McKee is a prominent astrophysicist known for his work on the dynamics of protoplanetary disks, contributing significantly to our understanding of the processes involved in planet formation. His research focuses on the physical and chemical properties of these disks, helping to clarify how dust and gas coalesce to form planetary bodies. McKee's insights are foundational in the field, bridging observational astronomy with theoretical models to explain the complex environments where new planets emerge.
Molecular cloud collapse: Molecular cloud collapse is the process in which a dense region within a molecular cloud undergoes gravitational instabilities, leading to the formation of stars and planetary systems. This collapse is critical as it marks the beginning of star formation, where gravitational forces cause gas and dust to coalesce, eventually forming protostars surrounded by protoplanetary disks. Understanding this phenomenon is essential in studying stellar evolution and the development of planetary systems.
Planet formation: Planet formation is the process by which planets develop from a rotating disk of gas and dust surrounding a newly formed star. This involves several stages, including the condensation of solid particles, the accumulation of these particles into larger bodies called planetesimals, and eventually the coalescence of planetesimals into full-sized planets. Understanding this process is crucial for unraveling the origins and characteristics of planetary systems.
Radio interferometry: Radio interferometry is a technique used in radio astronomy that combines signals from multiple radio telescopes to achieve higher resolution imaging of astronomical objects. By effectively acting as a single, larger telescope, this method allows astronomers to study distant celestial phenomena with remarkable precision, providing insights into the structure and dynamics of protoplanetary disks and other astrophysical structures.
Shock waves: Shock waves are sudden, sharp disturbances in a medium that move faster than the speed of sound in that medium, causing abrupt changes in pressure, temperature, and density. They can be generated by various phenomena, such as stellar winds or explosive events, and play a crucial role in shaping the environments around stars and in the formation of protoplanetary disks.
Viscosity: Viscosity is a measure of a fluid's resistance to flow and deformation. In the context of protoplanetary disk formation, viscosity plays a crucial role in determining how material within the disk moves and interacts, influencing the dynamics of accretion and the formation of planets. The viscosity of a fluid can affect how quickly dust and gas can coalesce into larger bodies, impacting the overall evolution of the protoplanetary disk.