🪐Exoplanetary Science Unit 2 – Planetary Formation and Evolution

Key Concepts and Theories

• Nebular hypothesis proposes that the Solar System formed from the gravitational collapse of a large molecular cloud of gas and dust
• Protoplanetary disk is a rotating circumstellar disk of dense gas and dust surrounding a young newly formed star
• Accretion is the process by which dust and gas accumulate into larger bodies through collisions and gravitational attraction
• Differentiation is the process by which a planetary body develops compositionally distinct layers (core, mantle, crust) due to density differences and heat
• Kepler's laws of planetary motion describe the orbits of planets around the Sun (or other stars)
  • First law states that planets orbit in ellipses with the star at one focus
  • Second law states that a line connecting a planet to the star sweeps out equal areas in equal times
  • Third law relates a planet's orbital period to its semi-major axis ($P^2 \propto a^3$)
• Tidal forces are gravitational forces exerted by one body on another, causing deformation and potentially affecting orbital dynamics (moons, close-in planets)
• Resonances occur when two orbiting bodies exert regular, periodic gravitational influence on each other (mean-motion resonances, spin-orbit resonances)

Formation of Planetary Systems

• Stellar nurseries are molecular clouds where stars and planetary systems form, containing mostly hydrogen and helium with traces of heavier elements
• Gravitational instability within a molecular cloud leads to fragmentation and collapse, forming protostars and protoplanetary disks
• Conservation of angular momentum causes the collapsing cloud to flatten into a disk perpendicular to its rotation axis
• Dust settles to the midplane of the protoplanetary disk due to gravity and gas drag, increasing the dust-to-gas ratio
• Streaming instability is a process in which gas drag and self-gravity cause dust particles to clump together, aiding in the formation of planetesimals
• Snowline (frost line) is the distance from the star beyond which volatile compounds (water, methane, ammonia) can condense into solid ice grains
  • Location depends on the temperature profile of the disk and the condensation temperatures of different compounds
  • Affects the composition of planets forming at different distances from the star
• Photoevaporation is the process by which high-energy radiation from the central star or nearby massive stars disperses the gas in the protoplanetary disk
  • Sets a time limit for gas giant formation and can affect the final masses and orbits of planets

Stages of Planet Growth

• Dust growth occurs as small dust grains collide and stick together through electrostatic forces and chemical bonding, forming larger aggregates
• Planetesimal formation happens when dust aggregates reach kilometer size through gravitational collapse or streaming instability
  • Planetesimals are the building blocks of planets and range in size from 1 to 100 km
• Runaway growth is a phase where the largest planetesimals grow much faster than smaller ones due to gravitational focusing and increased collision cross-section
  • Leads to the formation of planetary embryos (Moon to Mars-sized objects)
• Oligarchic growth is a slower phase where planetary embryos grow by accreting smaller planetesimals in their feeding zones
  • Embryos become isolated from each other as they deplete their feeding zones
• Giant impacts occur when planetary embryos collide with each other, leading to mergers and the formation of terrestrial planets
  • Thought to explain the origin of the Moon (collision between proto-Earth and a Mars-sized body)
• Core accretion is the main theory for gas giant formation, involving the growth of a solid core (~10 Earth masses) followed by rapid gas accretion
  • Requires formation of the core before the gas in the protoplanetary disk dissipates
• Disk instability is an alternative theory for gas giant formation, proposing that massive disks can fragment directly into self-gravitating clumps
  • May explain the formation of gas giants at large orbital distances or in low-metallicity environments

Composition and Structure

• Terrestrial planets are primarily composed of rock and metal, with dense iron cores and silicate mantles and crusts
  • Examples in the Solar System: Mercury, Venus, Earth, Mars
• Gas giants are massive planets mainly composed of hydrogen and helium, with small rocky cores
  • Examples: Jupiter, Saturn
  • May have metallic hydrogen layers in their interiors due to high pressures
• Ice giants are similar to gas giants but have a higher proportion of heavier elements (oxygen, carbon, nitrogen) in the form of ices and rocks
  • Examples: Uranus, Neptune
  • Likely have mantles of water, ammonia, and methane ices
• Bulk composition of a planet depends on the composition of the protoplanetary disk at its formation location and the accretion history of the planet
• Internal structure is determined by the planet's mass, composition, and thermal evolution
  • Affects properties such as density, gravity, magnetic field, and heat flow
• Differentiation leads to the formation of distinct layers (core, mantle, crust) based on density and chemical affinity
  • Driven by heat from accretion, radioactive decay, and gravitational compression
• Core size and composition have implications for planetary magnetic fields, tectonics, and habitability

Planetary Atmospheres and Surfaces

• Primary atmospheres form from the gas captured from the protoplanetary disk during planet formation
  • Dominant in gas giants and ice giants
• Secondary atmospheres are generated by outgassing from the planet's interior, impact delivery, and surface processes (volcanism, evaporation)
  • Important for terrestrial planets and can evolve over time
• Atmospheric escape occurs when molecules in the upper atmosphere gain enough energy to overcome the planet's gravity
  • Mechanisms include thermal escape, hydrodynamic escape, and non-thermal processes (sputtering, photochemical escape)
  • More significant for low-mass planets and close-in orbits
• Greenhouse effect is the warming of a planet's surface caused by atmospheric gases that absorb and re-emit infrared radiation
  • Important for maintaining habitable surface temperatures (Earth) or causing runaway heating (Venus)
• Surface processes shape the appearance and evolution of a planet's surface
  • Examples: tectonics, erosion, cratering, volcanism, weathering
• Habitability depends on factors such as surface temperature, presence of liquid water, atmospheric composition, and geological activity
  • Influenced by the planet's mass, orbit, and host star properties

Orbital Dynamics and Interactions

• Orbital elements describe the shape, size, and orientation of a planet's orbit
  • Include semi-major axis, eccentricity, inclination, longitude of ascending node, argument of periapsis, and true anomaly
• Mean motion is the average angular velocity of a planet in its orbit, related to its orbital period and semi-major axis
• Resonances occur when two orbiting bodies have orbital periods that are integer ratios of each other
  • Examples: 2:1 resonance between Jupiter and Saturn, 3:2 resonance between Pluto and Neptune
  • Can lead to orbital stability or chaos, depending on the configuration
• Secular perturbations are long-term, gradual changes in a planet's orbit caused by the gravitational influence of other planets
  • Affect eccentricity and inclination on timescales much longer than the orbital period
• Tidal interactions occur when a planet and its host star or moons exert gravitational forces on each other, leading to orbital and rotational changes
  • Examples: tidal locking (synchronous rotation), orbital circularization, tidal heating
• Kozai-Lidov mechanism is a type of orbital perturbation that can cause oscillations in a planet's eccentricity and inclination due to the gravitational influence of a distant companion (star or planet)
  • Can explain the existence of highly eccentric or misaligned orbits in some exoplanetary systems
• Planetary migration is the process by which planets change their orbital distances due to interactions with the protoplanetary disk or other planets
  • Type I migration affects low-mass planets embedded in the disk and is driven by torques from density waves
  • Type II migration affects massive planets that open gaps in the disk and is driven by viscous evolution of the disk

Detection and Observation Methods

• Radial velocity (Doppler) method measures the wobble of a star caused by the gravitational pull of an orbiting planet
  • Sensitive to massive planets in close orbits
• Transit method detects the periodic dimming of a star's light as a planet passes in front of it
  • Provides information on the planet's radius, orbital period, and inclination
  • Enables the study of planetary atmospheres through transmission spectroscopy
• Direct imaging captures the light emitted or reflected by a planet directly
  • Challenging due to the high contrast and small angular separation between the planet and its host star
  • Favors young, massive planets in wide orbits
• Gravitational microlensing occurs when a foreground star and its planets act as a gravitational lens, amplifying the light of a background star
  • Sensitive to low-mass planets at intermediate orbital distances
• Astrometry measures the tiny apparent motion of a star on the sky caused by the gravitational influence of orbiting planets
  • Requires extremely precise position measurements and favors massive planets in wide orbits
• Pulsar timing detects the variations in the regular radio pulses from a pulsar caused by the presence of orbiting planets
  • First method used to discover exoplanets (around pulsar PSR B1257+12)
• Disk kinematics uses the motion of gas and dust in a protoplanetary disk to infer the presence of embedded planets
  • Examples: gaps, spiral arms, velocity kinks

Current Research and Future Directions

• Exoplanet population statistics aim to understand the diversity and distribution of planets across different stellar types, metallicities, and environments
  • Inform models of planet formation and evolution
• Atmospheric characterization studies the composition, structure, and dynamics of exoplanetary atmospheres
  • Uses techniques such as transmission spectroscopy, emission spectroscopy, and phase curves
  • Searches for biosignatures (e.g., oxygen, methane) that could indicate the presence of life
• Habitability assessments evaluate the potential for exoplanets to support life based on factors such as surface temperature, presence of liquid water, and atmospheric composition
  • Consider the effects of stellar activity, planetary mass, and orbital parameters
• Formation and evolution models simulate the growth and development of planets from protoplanetary disks
  • Incorporate physical processes such as accretion, migration, and disk evolution
  • Aim to reproduce the observed diversity of exoplanetary systems
• Comparative planetology studies the similarities and differences between exoplanets and the planets in our Solar System
  • Provides insights into the fundamental processes shaping planetary systems
• Future missions and instrumentation aim to advance our understanding of exoplanets
  • Examples: James Webb Space Telescope (JWST), European Extremely Large Telescope (E-ELT), Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL)
  • Will enable more detailed characterization of exoplanetary atmospheres and the discovery of smaller, potentially habitable planets
• Interdisciplinary collaborations between astronomy, planetary science, geology, and biology are crucial for a comprehensive understanding of exoplanets and the search for extraterrestrial life