🪐Exoplanetary Science Unit 3 – Exoplanet System Structures

Key Concepts and Definitions

• Exoplanets are planets that orbit stars other than our Sun located outside of our solar system
• Exoplanetary systems consist of one or more exoplanets orbiting a host star or stars
• Transit method detects exoplanets by measuring the dimming of a star's light as a planet passes in front of it (transits)
• Radial velocity method detects exoplanets by measuring the wobble of a star caused by the gravitational pull of an orbiting planet
  • Also known as the Doppler method
• Circumstellar habitable zone (CHZ) is the range of distances from a star where liquid water could exist on a planet's surface
• Protoplanetary disk is a rotating disk of gas and dust surrounding a young star from which planets can form
• Kepler's laws of planetary motion describe the motion 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 its star sweeps out equal areas in equal times
  • Third law relates a planet's orbital period to its distance from the star

Types of Exoplanetary Systems

• Hot Jupiters are gas giant planets orbiting very close to their host stars (0.015 to 0.5 AU)
  • Typically have orbital periods of less than 10 days
  • Examples include 51 Pegasi b and HD 209458 b
• Super-Earths are rocky planets with masses between 1 and 10 Earth masses
  • May have diverse compositions and atmospheres
  • Examples include Gliese 581c and Kepler-10b
• Mini-Neptunes are planets with masses between Super-Earths and Neptune (10-40 Earth masses)
  • Likely have thick hydrogen-helium atmospheres
  • Examples include Kepler-11f and GJ 1214b
• Circumbinary planets orbit around binary star systems (two stars orbiting each other)
  • Planets must have stable orbits around both stars
  • Examples include Kepler-16b and Kepler-1647b
• Rogue planets are planetary-mass objects not gravitationally bound to any star
  • May have been ejected from their original planetary systems
• Multi-planet systems contain two or more planets orbiting a single star
  • Planets can interact gravitationally and influence each other's orbits
  • Examples include the TRAPPIST-1 system with seven Earth-sized planets

Detection Methods and Technologies

• Transit photometry measures the slight dimming of a star's light as a planet passes in front of it
  • Provides information on a planet's size, orbital period, and sometimes atmosphere
  • Kepler Space Telescope used transit method to discover over 2,600 confirmed exoplanets
• Radial velocity spectroscopy measures the gravitational wobble of a star caused by an orbiting planet
  • Provides information on a planet's mass and orbital period
  • Requires high-precision spectrographs to measure tiny Doppler shifts in starlight
• Direct imaging captures light from an exoplanet itself rather than inferring its presence
  • Requires advanced techniques to separate planet light from much brighter starlight
  • Provides information on a planet's temperature, atmosphere, and sometimes mass
• Gravitational microlensing occurs when a foreground star and planet align with a background star
  • The foreground star and planet act as a lens, magnifying the light of the background star
  • Sensitive to planets at wider separations from their host stars
• Astrometry measures the tiny back-and-forth motion of a star caused by an orbiting planet
  • Requires extremely precise measurements of a star's position over time
  • Gaia space observatory is expected to discover thousands of exoplanets using astrometry

Planetary Formation and Evolution

• Planets form from the gas and dust in a protoplanetary disk surrounding a young star
• Dust grains collide and stick together, growing into larger particles and eventually planetesimals
• Planetesimals continue to grow through collisions, forming planetary embryos and eventually planets
  • Terrestrial planets form from rocky/metallic material close to the star
  • Gas giants form from the accretion of gas onto solid cores farther from the star
• Planetary migration can occur due to interactions between planets and the protoplanetary disk
  • Planets can spiral inward or outward from their original orbits
  • Hot Jupiters may have formed farther out and migrated inward
• Giant impacts between planets or planetesimals can significantly alter a planet's composition and atmosphere
  • Earth's Moon likely formed from a Mars-sized impactor colliding with the early Earth
• Atmospheric escape can lead to the loss of a planet's primordial atmosphere over time
  • Factors include stellar wind, high-energy radiation, and low planetary mass
• Tidal heating can occur in planets orbiting close to their host stars or in multi-planet systems
  • The gravitational pull of the star or other planets can flex the planet, generating internal heat
  • Tidal heating may support subsurface oceans or geological activity on some planets

Exoplanet Atmospheres and Compositions

• Atmospheric composition depends on factors such as planetary mass, temperature, and formation history
• Transmission spectroscopy measures the wavelength-dependent absorption of starlight through a planet's atmosphere during transit
  • Can detect the presence of atoms, molecules, and clouds in the atmosphere
  • Examples of detected compounds include water vapor, carbon monoxide, and methane
• Emission spectroscopy measures the thermal emission from a planet's atmosphere
  • Can provide information on the temperature structure and composition of the atmosphere
  • Requires separating the planet's emission from the much brighter star
• Phase curves measure the variation in a planet's reflected light and thermal emission as it orbits its star
  • Can provide information on the planet's day-night temperature contrast and reflectivity (albedo)
• Biosignatures are atmospheric gases or features that could indicate the presence of life
  • Examples include oxygen (produced by photosynthesis), methane (produced by some microbes), and certain combinations of gases out of chemical equilibrium
  • False positives can occur from abiotic processes, so multiple lines of evidence are needed
• Clouds and hazes can significantly impact the appearance and evolution of a planet's atmosphere
  • Can obscure absorption features and affect the temperature structure of the atmosphere
  • Examples include water clouds on Earth, sulfuric acid clouds on Venus, and photochemical hazes on Titan

Orbital Dynamics and Stability

• Planets orbit their host stars in elliptical paths, with the star at one focus of the ellipse
• Orbital elements describe a planet's orbit, including semi-major axis, eccentricity, inclination, and argument of periastron
• Mean motion resonances occur when the orbital periods of two or more planets are integer ratios of each other
  • Can lead to stable orbits or chaotic interactions depending on the specific resonance
  • Examples include the 2:1 resonance between Neptune and Pluto, and the Laplace resonance among Jupiter's moons Io, Europa, and Ganymede
• Secular resonances involve the precession of orbital elements (e.g., periastron or ascending node) rather than orbital periods
  • Can drive long-term evolution of orbits and lead to instabilities
  • Example: The ν6 secular resonance in the asteroid belt, which shapes the inner edge of the belt
• Tidal forces can circularize orbits over time, particularly for planets orbiting close to their host stars
  • Tidal dissipation within the planet converts orbital energy into heat, causing the orbit to shrink and circularize
  • Hot Jupiters likely migrated inward and had their orbits circularized by tidal forces
• Kozai-Lidov mechanism can cause oscillations in a planet's orbital eccentricity and inclination due to the gravitational influence of a distant companion (star or planet)
  • Can lead to highly eccentric orbits and even planet-star collisions
  • May explain the formation of some hot Jupiters through inward migration and tidal circularization

Habitability and Potential for Life

• Habitability depends on various factors, including a planet's distance from its star, mass, composition, and atmospheric properties
• Circumstellar habitable zone (CHZ) is the range of distances from a star where liquid water could exist on a planet's surface
  • Depends on the star's luminosity and the planet's atmospheric composition
  • Inner edge is determined by the onset of a runaway greenhouse effect (e.g., Venus)
  • Outer edge is determined by the condensation of CO2 and the formation of permanent ice caps (e.g., Mars)
• Tidal heating can potentially support habitable conditions outside the traditional CHZ
  • Example: Jupiter's moon Europa, which may have a subsurface ocean maintained by tidal heating
• Plate tectonics may be important for long-term habitability by recycling carbon and regulating the atmosphere
  • Depends on factors such as planetary mass, composition, and internal heat generation
• Magnetic fields can protect a planet's atmosphere from erosion by stellar wind and high-energy particles
  • Requires a rotating, electrically conductive core (e.g., Earth's liquid outer core)
• Biosignatures are features that could indicate the presence of life, such as atmospheric gases, surface pigments, or seasonal variations
  • Must be distinguishable from abiotic processes that could produce similar features
  • Examples include oxygen (from photosynthesis), methane (from methanogenic microbes), and the "red edge" of vegetation
• Extremophiles are organisms that thrive in extreme environments on Earth, such as high temperatures, acidity, or salinity
  • Provide insight into the potential for life to adapt to diverse conditions on other planets
  • Examples include thermophiles in hot springs, acidophiles in acid mine drainage, and halophiles in salt lakes

Current Research and Future Directions

• James Webb Space Telescope (JWST) will provide unprecedented infrared observations of exoplanet atmospheres
  • Will search for biosignatures and characterize the compositions of potentially habitable planets
• Extremely Large Telescopes (ELTs) with mirror diameters of 30-40 meters will enable direct imaging and spectroscopy of exoplanets
  • Examples include the European Extremely Large Telescope (E-ELT), Thirty Meter Telescope (TMT), and Giant Magellan Telescope (GMT)
• Plato (PLAnetary Transits and Oscillations of stars) is a European Space Agency mission planned for launch in the 2020s
  • Will search for Earth-sized planets around Sun-like stars and characterize their bulk properties
• Atmospheric characterization will continue to be a major focus, with the goal of detecting biosignatures and understanding the diversity of exoplanet atmospheres
  • Requires development of advanced instrumentation and analysis techniques
• Comparative planetology will study the similarities and differences among exoplanets and solar system planets
  • Can provide insight into the processes that shape planetary formation, evolution, and habitability
• Astrobiology research will investigate the potential for life to emerge and thrive on different types of exoplanets
  • Combines expertise from fields such as biology, chemistry, geology, and astronomy
• Citizen science projects (e.g., Planet Hunters) engage the public in the search for and characterization of exoplanets
  • Harnesses the power of human pattern recognition to identify transits and other features in large datasets
• Machine learning techniques are being developed to automatically detect and classify exoplanets in large datasets
  • Can help prioritize targets for follow-up observations and identify patterns in exoplanet populations