3.3 Multiplanet systems

Multiplanet systems, consisting of two or more planets orbiting a common star, offer a window into diverse planetary architectures beyond our Solar System. These systems provide crucial insights into planet formation, system stability, and the potential for habitable worlds in various stellar environments.

Detection methods like transit timing variations and radial velocity measurements have revealed a wide range of system configurations, from compact super-Earth clusters to extended systems with giant planets. Studying the dynamics, formation, and evolution of these systems enhances our understanding of planetary system development and the conditions that may support life.

Definition of multiplanet systems

• Multiplanet systems consist of two or more planets orbiting a common host star, expanding our understanding of planetary system architectures beyond the Solar System
• These systems provide valuable insights into planet formation processes, system stability, and the potential for habitable worlds in diverse stellar environments

Criteria for classification

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• Confirmed planetary status requires multiple detection methods or repeated observations
• Minimum mass or radius thresholds distinguish planets from brown dwarfs or other substellar objects
• Orbital stability constraints ensure long-term survival of the system
• Clear gravitational influence on the host star or other planets in the system

Historical context

• First multiplanet system discovered around Upsilon Andromedae in 1999 using radial velocity method
• Kepler space telescope revolutionized multiplanet system detections, identifying thousands of candidates
• Ground-based surveys (HARPS, ESPRESSO) complemented space-based missions in confirming multiplanet systems
• Technological advancements in spectrographs and photometers enabled detection of smaller, Earth-sized planets in multiplanet configurations

Detection methods

• Multiplanet system detection requires sophisticated techniques to identify subtle signals from multiple orbiting bodies
• Combination of different detection methods increases confidence in system characterization and reduces false positives

Transit timing variations

• Measures deviations in expected transit times caused by gravitational interactions between planets
• Allows detection of non-transiting planets through their influence on transiting companions
• Provides constraints on planet masses and orbital parameters
• Particularly effective for detecting planets near mean motion resonances
• Requires long-term monitoring of transiting systems to accumulate sufficient data for analysis

Radial velocity measurements

• Detects periodic Doppler shifts in stellar spectra caused by orbiting planets
• Measures minimum planet masses ($M \sin i$) and orbital periods
• Enables detection of non-transiting planets in multiplanet systems
• Requires high-precision spectrographs (HARPS, ESPRESSO) to detect small-mass planets
• Challenges include disentangling signals from multiple planets and stellar activity noise

Direct imaging techniques

• Captures actual images of planets orbiting their host star
• Most effective for young, massive planets at wide separations from their host star
• Employs adaptive optics and coronagraphs to suppress stellar light
• Allows spectroscopic characterization of planetary atmospheres
• Examples include HR 8799 system with four directly imaged giant planets

Dynamics of multiplanet systems

• Multiplanet systems exhibit complex gravitational interactions that shape their long-term evolution and stability
• Understanding these dynamics informs theories of planet formation and system architecture development

Orbital stability

• Long-term stability requires sufficient separation between planets to avoid close encounters
• Hill stability criterion determines minimum separation for two-planet systems
• Chaos indicators (Lyapunov exponents) assess stability of more complex systems
• Stable systems often exhibit hierarchical architectures with well-separated orbital periods
• Unstable configurations can lead to planet ejections or collisions over long timescales

Mean motion resonances

• Occur when orbital periods of two planets form a simple integer ratio (2:1, 3:2, etc.)
• Enhance gravitational interactions and can promote long-term stability
• Examples include Jupiter-Saturn 5:2 resonance and Neptune-Pluto 3:2 resonance
• Resonant chains observed in systems like TRAPPIST-1 (7 planets in near-resonant orbits)
• Formation theories include convergent migration in protoplanetary disks

Secular interactions

• Long-term, periodic exchange of angular momentum between planets
• Causes oscillations in eccentricities and inclinations over timescales much longer than orbital periods
• Laplace-Lagrange theory describes secular dynamics in the low eccentricity, low inclination regime
• Can lead to apsidal alignment or anti-alignment of orbits in some systems
• Secular resonances occur when precession frequencies of two planets match, potentially destabilizing the system

Formation and evolution

• Multiplanet systems provide crucial insights into the processes that shape planetary systems from their birth in protoplanetary disks to their mature configurations
• Studying diverse system architectures informs our understanding of planet formation mechanisms and their relative importance

Protoplanetary disk processes

• Core accretion model explains formation of rocky planets and gas giant cores
• Pebble accretion accelerates growth of planetary embryos
• Gravitational instability may form massive planets directly from disk fragmentation
• Dust traps at pressure bumps concentrate solid material, promoting planet formation
• Disk lifetimes (typically 1-10 million years) constrain timescales for gas giant formation

Planet migration scenarios

• Type I migration affects low-mass planets through disk torques
• Type II migration occurs when massive planets open gaps in the disk
• Migration can explain the existence of hot Jupiters and compact systems of super-Earths
• Resonant chains form through convergent migration of multiple planets
• Late-stage migration may result from planet-planet scattering after disk dispersal

System architecture development

• Final system configurations result from interplay of formation, migration, and dynamical evolution
• In situ formation vs migration scenarios debated for compact super-Earth systems
• Giant planet migration influences distribution of smaller planets and planetesimal belts
• Planet traps at disk transitions can halt migration and shape system architecture
• Late-stage instabilities may explain eccentric giant planets and reduced multiplicity in some systems

Diversity of multiplanet systems

• Exoplanet surveys have revealed a wide range of system architectures, challenging our Solar System-centric views
• This diversity provides a rich dataset for testing planet formation and evolution theories

Compact vs extended systems

• Compact systems feature multiple planets with short orbital periods (days to months)
  • Often composed of super-Earths and mini-Neptunes
  • Examples include Kepler-11 (6 planets within 0.5 AU) and TOI-178 (6 planets in resonant chain)
• Extended systems have planets spread over a wider range of orbital distances
  • May include both inner rocky planets and outer gas giants
  • Solar System is an example of an extended system
• Formation theories suggest different disk conditions or migration histories for compact vs extended systems
• Stability considerations limit how tightly packed planets can be in both configurations

Hot Jupiters in multiplanet systems

• Hot Jupiters rarely found in multiplanet systems, suggesting disruptive formation or migration processes
• When present, companion planets tend to be much smaller and on wider orbits
• WASP-47 system is a rare exception with a hot Jupiter, two inner super-Earths, and an outer Neptune-sized planet
• Theories include:
  • Planet-planet scattering followed by tidal circularization
  • Secular chaos driving inner planet to very short orbital period
  • In situ formation in extremely massive disks (less favored)

Super-Earth dominated systems

• Systems with multiple planets between Earth and Neptune in size (1-4 Earth radii)
• Often found in compact configurations with short orbital periods
• Examples include Kepler-20 (5 super-Earths) and HD 40307 (6 super-Earths)
• Challenge traditional formation models based on Solar System
• Theories include:
  • Formation beyond snow line followed by inward migration
  • In situ formation from pebble accretion in high solid-to-gas ratio disks
  • Importance of water content in determining final planet sizes and compositions

Notable multiplanet systems

• Certain multiplanet systems have garnered significant attention due to their unique characteristics or potential for habitability
• These systems serve as important case studies for understanding planet formation, system dynamics, and the potential for life beyond Earth

TRAPPIST-1 system

• Seven Earth-sized planets orbiting an ultra-cool dwarf star
• All planets in or near the habitable zone
• Complex resonant chain configuration (periods form near-integer ratios)
• Planets likely have similar compositions, possibly water-rich
• Intense stellar activity poses challenges for potential habitability
• Serves as prime target for atmospheric characterization with JWST

Kepler-11 system

• Six planets with masses between those of Earth and Neptune
• Extremely compact system with five inner planets within 0.3 AU
• Planets have very low densities, suggesting substantial hydrogen-helium envelopes
• Challenges traditional formation models due to high mass concentration in inner system
• Provides insights into formation and evolution of compact super-Earth systems

HR 8799 system

• Four massive planets directly imaged orbiting a young A-type star
• Planets range from 5-10 Jupiter masses at wide separations (15-70 AU)
• System includes two debris disks (inner and outer) in addition to the planets
• Planets likely formed via gravitational instability rather than core accretion
• Serves as laboratory for studying young, massive planets and their atmospheres

Habitability in multiplanet systems

• Multiplanet systems offer diverse environments for potential habitability, with complex interactions between planets influencing conditions for life
• Understanding habitability factors in these systems is crucial for identifying promising targets in the search for extraterrestrial life

Habitable zones

• Region around a star where liquid water can exist on a planet's surface
• Depends on stellar properties (temperature, luminosity) and planetary characteristics (atmosphere, albedo)
• Multiplanet systems can have multiple planets within the habitable zone (TRAPPIST-1)
• Habitable zone boundaries evolve over time as stars change luminosity
• Concept of dynamical habitability considers orbital variations due to planet-planet interactions

Tidal effects

• Tidal forces between planets and host star can influence rotational and orbital properties
• Tidal locking can lead to permanent day and night sides on close-in planets
• Tidal heating can maintain subsurface oceans on otherwise frozen worlds (Europa, Enceladus)
• In multiplanet systems, planet-planet tides can enhance habitability of outer planets
• Tidal evolution can drive planets into or out of resonant configurations over time

Atmospheric retention

• Crucial for maintaining surface conditions suitable for life
• Influenced by planet mass, composition, and proximity to host star
• Stellar wind and radiation can strip atmospheres, especially for planets orbiting M-dwarfs
• Magnetic fields generated by planetary cores help protect atmospheres
• In multiplanet systems, giant planets may shield inner planets from impacts, preserving atmospheres

Comparative planetology

• Studying multiplanet systems allows for comparison between different planetary systems and our own Solar System
• This comparative approach enhances our understanding of planet formation and evolution processes

Solar system vs exoplanet systems

• Solar System lacks hot Jupiters and super-Earths, common in many exoplanet systems
• Compact systems of sub-Neptune planets more prevalent than Solar System-like architectures
• Solar System's distinct terrestrial and giant planet regions not universal
• Dynamical history of Solar System (e.g., Nice model) may explain its current architecture
• Exoplanet systems show greater diversity in planet sizes, compositions, and orbital configurations

System-wide trends

• Mass-radius relationships reveal composition classes (rocky, water-rich, gas-rich)
• Intra-system uniformity in planet sizes and spacings observed in many multiplanet systems
• Kepler dichotomy: apparent excess of single-transit systems compared to multis
• Correlation between stellar metallicity and giant planet occurrence
• Trends in planet occurrence rates with stellar mass and age inform formation theories

Observational challenges

• Detecting and characterizing multiplanet systems presents unique challenges that impact our understanding of their prevalence and properties
• Recognizing these challenges is crucial for interpreting exoplanet survey results and planning future observations

Bias in detection methods

• Transit method favors detection of large planets close to their stars
• Radial velocity sensitivity decreases for longer orbital periods and smaller planet masses
• Direct imaging biased towards young, massive planets at wide separations
• Microlensing events are rare and non-repeatable, limiting follow-up studies
• Astrometry requires long-term precision measurements, currently limited to nearby stars

Limits of current technology

• Atmospheric characterization limited to handful of exoplanets due to signal-to-noise constraints
• Difficulty in detecting Earth-sized planets in habitable zones of Sun-like stars
• Limited ability to measure planet masses for most transiting planets discovered by Kepler and TESS
• Stellar activity noise complicates detection of low-mass planets via radial velocity
• Direct imaging cannot yet resolve Earth-like planets around nearby stars

Future prospects

• Upcoming missions and technological advancements promise to revolutionize our understanding of multiplanet systems
• These future developments will address current observational limitations and open new avenues for exoplanet research

Upcoming space missions

• PLATO (ESA) will search for transiting planets around bright stars, including habitable zone Earth-analogs
• ARIEL (ESA) will conduct atmospheric surveys of a large, diverse sample of exoplanets
• Roman Space Telescope (NASA) will use microlensing to detect cold planets at wide orbital separations
• LUVOIR or HabEx concepts (NASA) could directly image and characterize Earth-like exoplanets

Ground-based observatories

• Extremely Large Telescopes (ELT, TMT, GMT) will enable direct imaging and spectroscopy of smaller, cooler exoplanets
• ESPRESSO and next-generation high-resolution spectrographs will push radial velocity precision to cm/s level
• Large survey telescopes (LSST) will enable detection of long-period transiting planets and exoplanet microlensing events
• Advances in adaptive optics and coronagraphy will improve direct imaging capabilities from the ground

Theoretical modeling advancements

• Improved N-body simulations will better constrain long-term stability of complex multiplanet systems
• Machine learning techniques will enhance signal extraction from noisy datasets
• Sophisticated atmospheric models will aid interpretation of exoplanet spectra
• Population synthesis models will connect planet formation theories with observed system architectures
• Coupling of dynamical, thermal, and chemical evolution models will provide holistic view of planet habitability